Embedded systems-and-robotics-by vivekanand goud j

2013

Beginners Guide To

Embedded Systems & Robotics

Vivekanand goud j

www.myembeddedrobotics.blogspot.in

[email protected]

http://www.myembeddedrobotics.blogspot.in/

Contribution

mazidi

Abhijeet Gupta

Ankur Agrawal

Tanmay Gangwani

Acknowledgement

Robocon Team, IIT Kanpur

Robotics Club, IIT Kanpur

Electronics Club, IIT Kanpur

Centre for Mechatronics, IIT Kanpur

2 | P a g e Designed by vivekanand goud j| my embedded robotics

Contents

Section A: Digital Electronics

Chapter 1 Basics ....................................................................................... 8

1.1 What is a Digital system? .................................................................................................... 8

1.2 Assigning States .................................................................................................................. 8

1.3 Number Systems in digital electronics................................................................................ 8

1.4 Types of Digital Circuits....................................................................................................... 8

1.5 Clock: Building block of a sequential circuit ....................................................................... 9

1.6 Logic Gates: Building block of a combinatorial circuitry..................................................... 9

1.7 Practical Circuiting Elements ............................................................................................ 10

1.7.1 Resistor: .................................................................................................................... 10

1.7.2 Capacitor: .................................................................................................................. 10

1.7.3 Breadboard: .............................................................................................................. 11

1.7.4 Integrated Circuits (IC) .............................................................................................. 11

1.7.5 LED ............................................................................................................................ 12

Chapter 2 Some Integrated Circuits and Implementation ....................... 13

2.1 555 .................................................................................................................................... 13

2.1.1 Monostable mode..................................................................................................... 13

2.1.2 Astable mode ............................................................................................................ 14

2.2 4029 counter..................................................................................................................... 15

2.2.1 Pin Description .......................................................................................................... 15

2.3 7447: BCD to 7 segment display decoder ......................................................................... 16

2.3.1 Pin Description .......................................................................................................... 16

2.4 LDR (Light Dependent Resistor) ........................................................................................ 17

2.5 Operational Amplifier (Opamp) ........................................................................................ 17

2.5.1 Opamp as a comparator ........................................................................................... 18

2.6 7805 Voltage Regulator .................................................................................................... 18


Section B: Embedded Systems

Chapter 3 Introduction to Microcontrollers ........................................... 20

3.1 Compiler / IDE (Integrated Development Environment) .................................................. 22

3.2 Programmer ...................................................................................................................... 22

Chapter 4 Code Vision AVR (CVAVR) ......................................................

23

4.1 CHIP:.................................................................................................................................. 24

4.2 PORT: ................................................................................................................................ 25

Chapter 5 Introduction to Atmega 16 Microcontroller ........................... 26

5.a Introduction to Atmega 16 Microcontroller 5.1 Features ............................................................................................................................ 26

5.2 Pin Configuration .............................................................................................................. 26

5.3 Block Diagram ................................................................................................................... 27

5.4 Pin Descriptions ................................................................................................................ 28

5.5 Digital Input Output Port .................................................................................................. 29

5.6 Registers............................................................................................................................ 29

5.b Introduction to 8051 Microcontroller

• 5.1 What is 8051 Standard?

• 5.2 8051 Microcontroller's pins

• 5.3 Input/Output Ports (I/O Ports)

• 5.4 8051 Microcontroller Memory Organisation

• 5.5 SFRs (Special Function Registers)

• 5.6 Counters and Timers

• 5.7 UART (Universal Asynchronous Receiver and Transmitter)

• 5.8 8051 Microcontroller Interrupts

5.9 8051 Microcontroller Power Consumption Control

Chapter 6 I/O Ports: ............................................................................... 30

6.1 DDRX (Data Direction Register) ........................................................................................ 30

6.2 PORTX (PORTX Data Register)...........................................................................................

31

6.2.1 Output Pin................................................................................................................. 31

6.2.2 Input Pin.................................................................................................................... 31

6.3 PINX (Data Read Register)................................................................................................. 32


Chapter 7 LCD Interfacing ...................................................................... 33

7.1 Circuit Connection ............................................................................................................ 33

7.2 Setting up in Microcontroller............................................................................................ 33

7.3 Printing Functions ............................................................................................................. 34

7.3.1 lcd_clear() ................................................................................................................. 34

7.3.2 lcd_gotoxy(x,y).......................................................................................................... 34

7.3.3 lcd_putchar(char c) ................................................................................................... 35

7.3.4 lcd_putsf(constant string)......................................................................................... 35


7.3.5 lcd_puts(char arr) ..................................................................................................... 35

7.3.6 itoa(int val, char arr[])............................................................................................... 35

7.3.7 ftoa(float val, char decimal_places, char arr[])......................................................... 35

Chapter 8 UART Communication ............................................................ 36

8.1 UART: Theory of Operation .............................................................................................. 36

8.2 Serial Port of Computer .................................................................................................... 37

8.3 Setting up UART in microcontroller .................................................................................. 38

8.3.1 putchar() ................................................................................................................... 39

8.3.2 getchar().................................................................................................................... 39

8.3.3 putsf()........................................................................................................................ 39

8.4 Docklight ........................................................................................................................... 40

Chapter 9 Timers .................................................................................... 41

9.1 Basic Theory ...................................................................................................................... 41

9.2 Fast PWM Mode ............................................................................................................... 42

9.3 CTC Mode.......................................................................................................................... 43

Chapter 10 SPI: Serial Peripheral Interface ............................................. 44

10.1 Theory of Operation ..................................................................................................... 44

10.2 Setting up SPI in Microcontroller .................................................................................. 45

10.2.1 Master Microcontroller ........................................................................................ 45

10.2.2 Slave Microcontroller............................................................................................ 45

10.3 Data Functions .............................................................................................................. 45

10.3.1 Transmit Data........................................................................................................ 45

10.3.2 Receive Data ......................................................................................................... 45

Chapter 11 ADC: Analog to Digital Converter .........................................

46

11.1 Theory of operation ...................................................................................................... 46

11.2 Setting up Microcontroller............................................................................................ 46

11.3 Function for getting ADC .............................................................................................. 47

Chapter 12 Interrupts............................................................................. 48

Example .................................................................................................................................. 48


12.1 Polling ........................................................................................................................... 48

12.2 Hardware interrupt....................................................................................................... 48

12.3 Hardware Interrupt or polling?..................................................................................... 48

12.4 Setting up Hardware Interrupt in Microcontroller ....................................................... 49

12.5 Functions of Interrupt Service Routine......................................................................... 49

Section C: Robotics

Chapter 13 Introduction to Autonomous Robots ................................... 51

13.1 Robot Chassis Designing ............................................................................................... 51

13.1.1 Robot with steering wheel:................................................................................... 51

13.1.2 Robot with differential drive: ............................................................................... 52

Chapter 14 Motor Driver ........................................................................ 54

14.1 H- Bridge: ...................................................................................................................... 54

14.2 Motor Driver ICs: L293/L293D and L298.......................................................................

55

14.2.1 Difference between L293 and L298: ..................................................................... 56

14.2.2 Speed Control: ...................................................................................................... 56

Chapter 15 Sensors ................................................................................ 57

15.1 Analog Sensor ............................................................................................................... 57

15.2 Digital IR Sensor - TSOP Sensor.....................................................................................

58


Section A

Digital Electronics


Chapter 1 Basics

1.1 What is a Digital system?

In most general terms, this system’s behavior is sufficiently explained by using only two of its

states can be Voltage(more than x volts or less?),distance covered(more than 2.5 km or less?],true-

false or weight of an elephant(will my weighing machine withstand it?) )

Note that although in every case, the all the intermediate states ARE POSSIBLE AND DO

EXIST, our point of interest are such that we don’t require their explicit description. In electronic

systems we mostly deal with Voltage levels as digital entities.

1.2 Assigning States

There is no specific fixed definition of logic levels in electronics. Most commonly used level

designation is the one used in CMOS and TTL (transistor transistor logic) families:

Logic high –> designated as ‘1’

Logic low –> designated as ‘0’

Where high and low are actually ‘higher’ and ‘lower’ with respect to a reference voltage

level (ideally taken as 2.5V)

GOOGLY: Why assign ‘0’and ‘1’ and not ‘a ‘and ‘b’, ’x’ and ‘y ,’cat ‘and ‘dog’?

ANS: Computational ease!

1.3 Number Systems in digital electronics

1. Binary: Only ‘0’ and ‘1’.

2. Hexadecimal: 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

1.4 Types of Digital Circuits

Combinatorial Circuits: In these circuits, the past states are immaterial

and the output depends only upon the present state. Example logic gates

Sequential circuits: In these circuits, the next state is completely

determined by the past states. Hence these follow a predictable structure

and essentially require a timing device. Ex. counters, flip flops.


Figure 1

h h l 𝑇𝑙+𝑇ℎ

1.5 Clock: Building block of a sequential circuit

A clock is simply alternate high and low states of voltage with time i.e. essentially a square wave.

Important terms related to clock are its duty cycle and its frequency:

Duty cycle: It is the ratio of T and T +T 𝐷 = 𝑇ℎ

1.6 Logic Gates: Building block of a combinatorial circuitry

These are essentially combinatorial circuits used to implement logical Boolean operations like AND,

NAND, OR, XOR and NOT. NOT and NAND are called universal gates as any other gate can be

formed using either of them!

Figure 2: Table of Logic

Gates


1.7 Practical Circuiting Elements

1.7.1 Resistor:

A color scheme is followed to give the specifications of a resistor. The table for color code is

shown below:

The 1st

two bands specify the 2 digits of the resistor value whereas the

3rd

band specifies the multiplier in terms of the power to which 10 is

raised and multiplied to the 2 digits.

The tolerance tells the possible % variation of the resistor value about

the value indicated by bands.

Figure 3: Table of Resistance

1.7.2 Capacitor:

The 2 types of capacitors we frequently use in circuits are ceramic and electrolytic capacitors. While

ceramic capacitors do not have a fixed polarity; electrolytic capacitors should be connected in their

specified polarities only else they might blow off! This polarity is usually provided on the side of the

capacitors ‘corresponding leg.

Figure 5: Electrolytic cap with –ve polarity leg seen Figure 4: Ceramic cap with value 15x104 pF


1.7.3 Breadboard:

This is the base used for setting up the circuit. This has embedded metal strips in it that form a grid

of connections inside its body. This allows us to take multiple connections from a single point

without any need of soldering/disordering as in PCBs. It is always a good habit to test the circuit on

breadboard before making it on a PCB.

Figure 6: Top view showing the connecting holes. Bottom view shows the contact metal strips

1.7.4 Integrated Circuits (IC)

ICs or Integrated Circuits are packaged circuits designed for some

fixed purpose. An IC has its fixed IC name/number that can be used

to get catalog of its functions and pin configuration. ICs come in

various sizes and packages depending upon the purpose.

NOTE: Numbering scheme of IC pins will be explained in the

lab session. Different ICs may have different number of pins.

Figure 7: IC


1.7.5 LED

LED (Light Emitting Diode) is frequently used to display the outputs at various stages of the circuit. It is essentially a Diode with the energy released in the form of photons due to electron transitions

falling in the visible region. Hence normal diode properties apply to it.

It glows only in fwd bias mode i.e. with p junction connected to +ve voltage and n junction to

negative.

Diodes are essentially low power devices. The current through the LED should be less than

20mA.Hence always put a 220 ohm resistor in series with the LED.

Never forget that LEDs consume a significant amount of power of the outputs of the ICs (CMOS

based).Hence it is advisable to only use them for checking the voltage level (high or low) and then

remove them.

Figure 8:

LEDs


Chapter 2 Some Integrated Circuits and Implementation

2.1 555

555 is an IC used to generate a clock .The

two attributes of a clock are

Frequency

Duty cycle.

Both of these can be changed using this IC,

however the duty cycle is always <50%.

There are two modes in which 555 can run.

2.1.1 Monostable mode

Figure 9: Pin Configuration of 555

As the name suggests; in this mode the output is stable in only one (mono) state i.e. ‘off’ state.

Thus it can stay only for a finite time, if triggered, to the other state i.e. ‘on’ state. This time can be

set choosing appropriate values of resistances in the formula:

T = 1.1 x R1 x C1

Figure 10: 555 in monostable mode


F =

2.1.2 Astable mode

In this mode; the output is stable neither in ‘high’ state nor in ‘low ’ state. Hence it oscillates from one

state to another giving us a square wave or clock. We can set the clock frequency and Duty cycle D by

the formulae:

.𝟒𝟒 𝑹 + 𝑹 𝑪

(𝑹 +𝑹 ) D =

(𝑹 + 𝑹 )

Figure 11: 555 in astable mode

NOTE: Capacitor C2 is just to filter the noise and its value can be suitably chosen to be 0.01µF. It

can also be neglected.


2.2 4029 counter

With the clock made, we are ready to count the number of pulses passed into the circuit. Note that

any kind of counting requires a memory (you got to know that you have just counted ‘3’ to go to

‘4’!). Hence 4029 can also be used as a memory element that remembers its immediate previous

state.

Figure 12: 4029 pin

configuration

2.2.1 Pin Description

PIN No. Name Pin function Connection reqd.

1 Parallel load If given high; loads the value of

Parallel bit into the o/p bits

Gnd

2 Output Bit 3 Most significant bit of o/p To pin 6 of 7447

3 Parallel input bit 3 Most significant bit of parallel i/p Input

4 Parallel input bit 0 Least significant bit of parallel i/p Input

5 Clock enable bar Low on this pin enables counting as

per the clock received

Gnd

6 Output Bit 0 Least significant bit of parallel o/p To pin 7 of 7447

7 TC bar Output bit that gives a low when the

count is complete. Can be used to signal

the end of counting.

None if you don’t

want to use it

8 Gnd Needed for powering Gnd

9 Binary/Hex bar To choose b/w binary and

hexadecimal modes

10 Up/Down bar count To choose b/w up counting and

down counting modes

low for count 0-15

high for count 0-9

Low for down count

High for up count

11 Output bit 1 2nd

bit of o/p To pin 1 of 7447

12 Parallel input bit 1 2nd

bit of i/p Input

13 Parallel input bit 2 3rd

bit of i/p Input

14 Output bit 2 3rd

bit of o/p To pin 2 of 7447

15 Clock pulse Clock pulse is given here Clock from 555

16 Vdd Needed for powering +5 V


2.3 7447: BCD to 7 segment display decoder

For displaying the number in the counter output on a seven segment display (i.e. 7 LEDs making up a

figure of ‘8’ as in a general calculator. See fig. ) we need to decode the 4 bits and match them to the 7

pins for lighting the LEDs corresponding to the number. This work is done by 7447.

Figure 13: 7447 pin configuration

2.3.1 Pin Description

PIN

no.

Name Function Connection reqd.

1 i/p B 2nd

bit(O1) of 4029’s o/p To O1 of 4029

2 i/p C 3rd bit(O2) of 4029’s o/p To O2 of 4029

3 Lamp Test

bar

Used to check that all LEDs of 7

seg are working.

High for normal fn

Low to glow all

LEDs

4 BI /RBI Kept high to allow normal function Kept high

5 RBI Blanks ‘0’ from being displayed Kept high

6 i/p D Most significant bit(O3) of 4029’s

o/p

To O2 of 4029

7 i/p A 3rd bit(O2) of 4029’s o/p To O2 of 4029

8 Gnd For power Connected to gnd

9-15 a-g as per the

fig

The o/p pins to 7segment display To 7 seg display

16 Vcc For power Connected to +5 V

NOTE:

The COM pins are to be connected to Vcc via 220 ohm resistor. Why resistor is required??

The dot pin is just for display of decimal point and essentially only makes the upper and

lower sides distinguishable from each other for a single display. without the asymmetry

produced by dot how will we be able to see which side is upper and which is lower?


2.4 LDR (Light Dependent Resistor)

Light Dependent Resistor (LDR) or photo resistor is a device that acts like a resistance and its

resistance varies with the intensity of light incident on it. In this device, if photons of sufficient

energy fall on it, the resistance drops drastically as the electrons in the semiconductor are able to

jump from the valence band to the conduction band and are available for conduction. The LDRs used

are mostly responsive to visible light. The resistance might drop from as high as 1MΩ in the dark to 1

k Ω in bright light.

Figure 14: LDRs. The coiled portion is responsive to light

2.5 Operational Amplifier (Opamp)

Opamp is a very important device used in everyday electronics .It is essentially a differential

amplifier with a very high gain of the order of 105!By differential amplifier I mean that it amplifies

the difference of 2 signals and gives the output.

Opamp equation:

Vout= A (V+ - V- ) where A is the gain of the order 105.

Ironically, this high gain in open loop makes it impossible to use it as a general purpose

differential amplifier directly.

GOOGLY: If (V+-V-) = 0.005V; Vss = 12V what will be the output??


2.5.1 Opamp as a comparator

Simplest use of Opamp is as a comparator. It can be used to convert an analog signal to a digital

signal defined by a fixed threshold. Set V- as the threshold voltage say 2.5 V and apply the analog

signal to be digitized at V+ .What will be the output? Well if you have worked out the googly; this

should be a piece of cake!

2.6 7805 Voltage Regulator

7805 voltage regulator is used to get +5 V output out of a higher voltage supply (7.5V-20V).We use adapter’s supply to generate +5V here. Connect the gnd and +12V of adapter to the pins as shown

and get +5V directly as an output out of the 3rd

pin. Current up to 0.5 A can be obtained from this regulator without any significant fall in voltage level.

NOTE: Use 2 capacitors of value say 0.1µF to filter the noise in the input and output of regulator’s

supply as shown.


Section B

Embedded Systems


Chapter 3 Introduction to Microcontrollers

You must be knowing about Digital Integrated Circuits (ICs) right ? For example:

7404: Hex Inverter

7408: Quad 2-input AND gate

7410: Triple 3-input NAND Gate

7432: Quad 2-input OR Gate

7457: 60:1 Frequency divider

There are AND, XOR, NAND, NOR, OR logic gate ICs, Counters, Timers, Seven Segment

Display Drivers and much more. Just check out 7400 Series and 4000 Series of Integrated

Circuits.

Now let’s take Quad 2 input AND gate IC. It has 4 AND gates, each having 2 pins for input

and 1 pin for output. The truth table or the function table of each gate is fixed. This is as

follows,

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 0

1 1 1

Similarly all the Integrated circuits have their function tables and input and output pins fixed.

You cannot change the function and no input pin act as output and vice versa. So whenever

you want to design some circuit you first have to get the output as a function of inputs and

then design it using gates or whatever the requirement is.

So once a circuit is built we cannot change its function ! Even if you want to make some

changes again you have to consider all the gates and components involved. Now if you are

designing any circuit which involves change of the function table every now and then you are

in trouble ! For example if I want to design an Autonomous Robot which should perform

various tasks and I don’t just want to fix the task. Suppose I make it to move in a path then I

want to change the path ! How to do that ?

Here comes the use of Microcontrollers ! Now if I give you an Integrated Circuit with 20 pins

and tell you that you can make any pin as output or input also you can change the function

table by programming the IC using your computer ! Then your reactions will be wow ! that’s

nice :-) That’s what the most basic function of a microcontroller is. It has set of pins called as

PORT and you can make any pin either as output or input. After configuring pins you can

program it to perform according to any function table you want. You can change the

configuration or the function table as many times you wants.

There are many Semiconductor Companies which manufactures

microcontrollers. Some of them are:

Intel

Atmel

Microchip

Motorola


http://en.wikipedia.org/wiki/Integrated_circuit

http://en.wikipedia.org/wiki/Inverter_%28logic_gate%29

http://en.wikipedia.org/wiki/AND_gate

http://en.wikipedia.org/wiki/OR_gate

http://en.wikipedia.org/wiki/List_of_7400_series_integrated_circuits

http://en.wikipedia.org/wiki/List_of_4000_series_integrated_circuits

http://en.wikipedia.org/wiki/Truth_table

We will discuss about Atmel Microcontrollers commonly known as AVR in this

section.

Question: How a microcontroller works !

Answer: Well I cannot go into lot of details about the working because it is a vast topic in

itself. I can just give an overview.

Microcontroller consists of an Microprocessor (CPU that is Central processing Unit) which is

interfaced to RAM (Random Access Memory) and Flash Memory (one your pen drive has !).

You feed your program in the Flash Memory on the microcontroller. Now when you turn on

the microcontroller, CPU accesses the instruction from RAM which access your code from

Flash. It sets the configuration of pins and start performing according to your program.

Question: How to make the code ?

Answer: You basically write the program on your computer in any of the high level

languages like C, C++, JAVA etc. Then you compile the code to generate the machine file.

Now you will ask what this machine file is ? All the machines understand only one language,

0 & 1 that is on and off. Now this 0 & 1 both corresponds to 2 different voltage levels for

example 0 volt for 0 logic and +5 volt for 1 logic. Actually the code has to be written in this

0, 1 language and then saved in the memory of the microcontroller. But this will be very

difficult for us ! So we write the code in the language we understand (C) and then compile

and make the machine file (.hex). After we make this machine file we feed this to the

memory of the microcontroller.

Question: How to feed the code in the flash of Microcontroller ?

Answer: Assuming you have the machine file (.hex) ready and now you want to feed that to

the flash of the microcontroller. Basically you want to make communication between your

computer and microcontroller. Now computer has many communication ports such as Serial

Port, Parallel Port and USB (Universal Serial Bus).

Lets take Serial Port, it has its own definition that is voltage level to define 0 & 1 (yeah all

the data communication is a just collection of 0 & 1 ) Serial Port's protocol is called as UART

(Universal Asynchronous Receiver & Transmitter) Its voltage levels are :-12 volt for 0 logic

and +12 volt for 1 logic.

Now the voltage levels of our microcontroller are based on CMOS (Complementary Metal

Oxide Semiconductor) technology which has 0 volt for 0 logic and +5 volt for 1 logic.

Two different machines with 2 different ways to define 0 & 1 and we want to exchange

information between them. Consider microcontroller as a French and Computer's Serial Port

as an Indian person (obviously no common language in between !) If they want to exchange

information they basically need a mediator who knows both the language. He will listen one

person and then translate to other person. Similarly we need a circuit which converts CMOS

(microcontroller) to UART (serial port) and vice versa. This circuit is called as programmer.

Using this circuit we can connect computer to the microcontroller and feed the machine file

to the flash.


http://en.wikipedia.org/wiki/Serial_port

http://en.wikipedia.org/wiki/Serial_port

http://en.wikipedia.org/wiki/Parallel_port

http://en.wikipedia.org/wiki/Universal_Serial_Bus

http://en.wikipedia.org/wiki/UART

http://en.wikipedia.org/wiki/CMOS

3.1 Compiler / IDE (Integrated Development Environment)

Atmel Microcontrollers are very famous as they are very easy to use. There are many

development tools available for them. First of all we need an easy IDE for developing code. I

suggest beginners to use CVAVR (Code Vision AVR) Evaluation version is available for free

download from the website. It has limitation of code size. It works on computers with

Windows platform that is Windows XP & Vista.

Some famous compilers/development tools supporting Windows for Atmel Microcontrollers

are:

WINAVR (AVRGCC for Windows)

Code Vision AVR (CVAVR)

AVR Studio (Atmel's free developing tool)

AVRGCC is a very nice open source compiler used by most of the people.

3.2 Programmer

Programmer basically consists of two parts:

Software (to open .hex file on your computer)

Hardware (to connect microcontroller)

Hardware depends on the communication port you are using on the computer (Serial, Parallel

or USB). I suggest beginners to use Serial Programmer as it is very easy to build. Software

for that is Pony Prog. Some famous Windows (XP, Vista) programmers are:

Pony Prog (Serial, Parallel)

AVRdude (supports many hardwares)

AVRStudio (supports Atmel's hardware)

ATProg (Serial)

USB-ASP (USB)


http://www.atmel.com/

http://www.hpinfotech.ro/html/cvavr.htm

http://winavr.sourceforge.net/


http://www.atmel.com/dyn/Products/tools_card.asp?tool_id=2725

http://www.avrfreaks.net/index.php?module=FreaksTools&func=viewItem&item_id=145

http://www.lancos.com/prog.html

http://www.bsdhome.com/avrdude/

http://www.atmel.com/dyn/Products/tools_card.asp?tool_id=2725

http://www.speedy-bl.de/avr-prog-e.htm

http://www.fischl.de/usbasp/

Chapter 4 Code Vision AVR (CVAVR)

An IDE has following functions:

Preprocessing

Compilation

Assembly

Linking

Object Translation

Text Editor

If we just use compiler and linker independently we still need to get a text editor. So

combining everything will actually mess things up. So the best way is to get Software which

has it all. That’s called an Integrated Development Environment, in short IDE.

I consider Code-Vision-AVR to be the best IDE for getting started with AVR

programming on Windows XP, Vista. It has a very good Code Wizard which generate codes

automatically ! You need not mess with the assembly words. So in all my tutorials I will be

using CVAVR. You can download evaluation version for free which has code size limitation

but good enough for our purpose.

For all my examples I will be using Atmega-16 as default microcontroller because it very

easily available and is powerful enough with sufficient number of pins and peripherals we

use. You can have a look on the datasheet of Atmega-16 in the datasheet section.

Let’s take a look on the software. The main window looks like following,



Now click on File ---> New --->Project

A pop up window will come asking whether you want to use Code Wizard AVR,

obviously select yes because that is the reason we are using CVAVR !

Now have a look on this Wizard. It has many tabs where we can configure PORTS,

TIMERS, LCD, ADC etc. I am explaining some of them

4.1 CHIP:

Select the chip for which you are going to write the program. Then select the frequency at

which Chip is running. By default all chips are set on Internal Oscillator of 1 MHz so select 1

MHz if that is the case. If you want to change the running clock frequency of the chip then

you have to change its fuse bits (I will talk more about this in fuse bits section).


4.2 PORT:

PORT is usually a collection of 8 pins.

From this tab you can select which pin you want to configure as output and which as

input. It basically writes the DDR and PORT register through this setting. Registers are

basically RAM locations which configure various peripherals of microcontroller and by

changing value of these registers we can change the function it is performing. I will talk more

about registers later. All the details are provided in the datasheet.

So you can configure any pin as output or input by clicking the box.

For Atmega-16 which has 4 Ports we can see 4 tabs each corresponding to one Port. You

can also set initial value of the Pins you want to assign. or if you are using a pin as input then

whether you want to make it as pull-up or tristated, again I will talk in details about these

functions later.

Similarly using this code wizard you can very easily configure all the peripherals on the

Atmega.

Now for generating code just go to File ----> Generate, Save and Exit (of the code

wizard)

Now it will ask you name and location for saving three files. Two being project files and

one being the .C file which is your program. try to keep same names of all three files to avoid

confusion. By default these files are generated in C:\CVAVR\bin

The generated program will open in the text editor. Have a look it has some declarations

like PORT, DDR, TCCR0 and many more. These are all registers which configures various

functions of Atmega and by changing these value we make different functions. All the details

about the registers are commented just below them. Now go down and find following infinite

while loop there. We can start writing our part of program just before the while loop. And as

for most of the applications we want microcontroller to perform the same task forever we put

our part of code in the infinite while loop provided by the code wizard !

while (1)

// Place your code here

;

See how friendly this code wizard is, all the work (configuring registers) automatically

done and we don’t even need to understand and go to the details about registers too !

Now we want to generate the hex file, so first compile the program. Either press F9 or go

to Project ---> Compile.

It will show compilation errors if any. If program is error free we can proceed to making

of hex file. So either press Shift+F9 or go to Project ----> Make. A pop up window will

come with information about code size and flash usage etc.

So the machine file is ready now ! It is in the same folder where we saved those 3 files.


Chapter 5 Introduction to Atmega 16 Microcontroller

5.1 Features

Advanced RISC Architecture

Up to 16 MIPS Throughput at 16 MHz

16K Bytes of In-System Self-Programmable Flash

512 Bytes EEPROM

1K Byte Internal SRAM

32 Programmable I/O Lines

In-System Programming by On-chip Boot Program

8-channel, 10-bit ADC

Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes

One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture

Four PWM Channels

Programmable Serial USART

Master/Slave SPI Serial Interface

Byte-oriented Two-wire Serial Interface

Programmable Watchdog Timer with Separate On-chip Oscillator

External and Internal Interrupt Sources

5.2 Pin Configuration


5.3 Block Diagram


5.4 Pin Descriptions

VCC: Digital supply voltage. (+5V)

GND: Ground. (0 V) Note there are 2 ground Pins.

Port A (PA7 - PA0)

Port A serves as the analog inputs to the A/D Converter. Port A also serves as an 8-bit bi- directional I/O port, if the A/D Converter is not used. When pins PA0 to PA7 are used as inputs and

are externally pulled low, they will source current if the internal pull-up resistors are activated. The

Port A pins are tri-stated when a reset condition becomes active, even if the clock is not running.

Port B (PB7 - PB0)

Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). Port B also serves the functions of various special features of the ATmega16 as listed on page 58 of

datasheet.

Port C (PC7 - PC0) Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). Port

C also serves the functions of the JTAG interface and other special features of the ATmega16 as listed on page 61 of datasheet. If the JTAG interface is enabled, the pull-up resistors on pins

PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset occurs.

Port D (PD7 - PD0) Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit).

Port D also serves the functions of various special features of the ATmega16 as listed on page 63 of datasheet.

RESET: Reset Input. A low level on this pin for longer than the minimum pulse length will

generate a reset, even if the clock is not running.

XTAL1: External oscillator pin 1

XTAL2: External oscillator pin 2

AVCC: AVCC is the supply voltage pin for Port A and the A/D Converter. It should be

externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected

to VCC through a low-pass filter.

AREF: AREF is the analog reference pin for the A/D Converter.


5.5 Digital Input Output Port

So let’s start with understanding the functioning of AVR. We will first discuss about I/O

Ports. Again I remind you that I will be using and writing about Atmega-16. Let’s first have

a look at the Pin configuration of Atmega-16. Image is attached, click to enlarge.

You can see it has 32 I/O (Input/Output) pins grouped as A, B, C & D with 8 pins in each

group. This group is called as PORT.

PA0 - PA7 (PORTA)

PB0 - PB7 (PORTB)

PC0 - PC7 (PORTC)

PD0 - PD7 (PORTD)

Notice that all these pins have some function written in bracket. These are additional function

that pin can perform other than I/O. Some of them are.

ADC (ADC0 - ADC7 on PORTA)

UART (Rx,Tx on PORTD)

TIMERS (OC0 - OC2)

SPI (MISO, MOSI, SCK on PORTB)

External Interrupts (INT0 - INT2)

5.6 Registers

All the configurations in microcontroller is set through 8 bit (1 byte) locations in RAM

(RAM is a bank of memory bytes) of the microcontroller called as Registers. All the

functions are mapped to its locations in RAM and the value we set at that location that is at

that Register configures the functioning of microcontroller. There are total 32 x 8bit registers

in Atmega-16. As Register size of this microcontroller is 8 bit, it called as 8 bit

microcontroller.

5. B 8051 Microcontroller Architecture

5.1 What is 8051 Standard?

5.2 8051 Microcontroller's pins

5.3 Input/Output Ports (I/O Ports)

5.4 8051 Microcontroller Memory Organisation

5.5 SFRs (Special Function Registers)

5.6 Counters and Timers

5.7 UART (Universal Asynchronous Receiver and Transmitter)

5.8 8051 Microcontroller Interrupts


http://www.mikroe.com/en/books/8051book/ch2/#ch2.1









2.1 What is 8051 Standard?

Microcontrollers’ producers have been struggling for a long time for attracting more and more choosy

customers. Every couple of days a new chip with a higher operating frequency, more memory and more

high-quality A/D converters comes on the market.

Nevertheless, by analyzing their structure it is concluded that most of them have the same (or at least very

similar) architecture known in the product catalogs as “8051 compatible”. What is all this about?

The whole story began in the far 80s when Intel launched its series of the microcontrollers labelled with

MCS 051. Although, several circuits belonging to this series had quite modest features in comparison to the

new ones, they took over the world very fast and became a standard for what nowadays is ment by a word

microcontroller.

The reason for success and such a big popularity is a skillfully chosen configuration which satisfies needs of

a great number of the users allowing at the same time stable expanding ( refers to the new types of the

microcontrollers ). Besides, since a great deal of software has been developed in the meantime, it simply was

not profitable to change anything in the microcontroller’s basic core. That is the reason for having a great

number of various microcontrollers which actually are solely upgraded versions of the 8051 family. What is

it what makes this microcontroller so special and universal so that almost all the world producers

manufacture it today under different name ?

As shown on the previous picture, the 8051 microcontroller has nothing impressive at first sight:

4 Kb program memory is not much at all.

128Kb RAM (including SFRs as well) satisfies basic needs, but it is not imposing amount.

4 ports having in total of 32 input/output lines are mostly enough to make connection to peripheral

environment and are not luxury at all.

As it is shown on the previous picture, the 8051 microcontroller have nothing impressive at first sight:

The whole configuration is obviously envisaged as such to satisfy the needs of most programmers who work

on development of automation devices. One of advantages of this microcontroller is that nothing is missing

and nothing is too much. In other words, it is created exactly in accordance to the average user‘s taste and

needs. The other advantage is the way RAM is organized, the way Central Processor Unit (CPU) operates

and ports which maximally use all recourses and enable further upgrading.

2.2 8051 Microcontroller's pins

Pins 1-8: Port 1 Each of these pins can be configured as input or output.

Pin 9: RS Logical one on this pin stops microcontroller’s operating and erases the contents of most registers.

By applying logical zero to this pin, the program starts execution from the beginning. In other words, a

positive voltage pulse on this pin resets the microcontroller.

Pins10-17: Port 3 Similar to port 1, each of these pins can serve as universal input or output . Besides, all of

them have alternative functions:

Pin 10: RXD Serial asynchronous communication input or Serial synchronous communication output.

Pin 11: TXD Serial asynchronous communication output or Serial synchronous communication clock

output.

Pin 12: INT0 Interrupt 0 input

Pin 13: INT1 Interrupt 1 input

Pin 14: T0 Counter 0 clock input

Pin 15: T1 Counter 1 clock input

Pin 16: WR Signal for writing to external (additional) RAM

Pin 17: RD Signal for reading from external RAM

Pin 18, 19: X2, X1 Internal oscillator input and output. A quartz crystal which determines operating

frequency is usually connected to these pins. Instead of quartz crystal, the miniature ceramics resonators can

be also used for frequency stabilization. Later versions of the microcontrollers operate at a frequency of 0 Hz

up to over 50 Hz.

Pin 20: GND Ground

Pin 21-28: Port 2 If there is no intention to use external memory then these port pins are configured as

universal inputs/outputs. In case external memory is used then the higher address byte, i.e. addresses A8-A15

will appear on this port. It is important to know that even memory with capacity of 64Kb is not used ( i.e.

note all bits on port are used for memory addressing) the rest of bits are not available as inputs or outputs.

Pin 29: PSEN If external ROM is used for storing program then it has a logic-0 value every time the

microcontroller reads a byte from memory.

Pin 30: ALE Prior to each reading from external memory, the microcontroller will set the lower address byte

(A0-A7) on P0 and immediately after that activates the output ALE. Upon receiving signal from the ALE

pin, the external register (74HCT373 or 74HCT375 circuit is usually embedded ) memorizes the state of P0

and uses it as an address for memory chip. In the second part of the microcontroller’s machine cycle, a signal

on this pin stops being emitted and P0 is used now for data transmission (Data Bus). In this way, by means of

only one additional (and cheap) integrated circuit, data multiplexing from the port is performed. This port at

the same time used for data and address transmission.

Pin 31: EA By applying logic zero to this pin, P2 and P3 are used for data and address transmission with no

regard to whether there is internal memory or not. That means that even there is a program written to the

microcontroller, it will not be executed, the program written to external ROM will be used instead.

Otherwise, by applying logic one to the EA pin, the microcontroller will use both memories, first internal and

afterwards external (if it exists), up to end of address space.

Pin 32-39: Port 0 Similar to port 2, if external memory is not used, these pins can be used as universal inputs

or outputs. Otherwise, P0 is configured as address output (A0-A7) when the ALE pin is at high level (1) and

as data output (Data Bus), when logic zero (0) is applied to the ALE pin.

Pin 40: VCC Power supply +5V

2.3 Input/Output Ports (I/O Ports)

All 8051 microcontrollers have 4 I/O ports, each consisting of 8 bits which can be configured as inputs or

outputs. This means that the user has on disposal in total of 32 input/output lines connecting the

microcontroller to peripheral devices.

A logic state on a pin determines whether it is configured as input or output: 0=output, 1=input. If a pin on

the microcontroller needs to be configured as output, then a logic zero (0) should be applied to the

appropriate bit on I/O port. In this way, a voltage level on the appropriate pin will be 0.

Similar to that, if a pin needs to be configured as input, then a logic one (1) should be applied to the

appropriate port. In this way, as a side effect a voltage level on the appropriate pin will be 5V (as it is case

with any TTL input). This may sound a bit confusing but everything becomes clear after studying a

simplified electronic circuit connected to one I/O pin.

Input/Output (I/O) pin This is a simplified overview of what is connected to a pin inside the microcontroller. It concerns all pins

except those included in P0 which do not have embedded pullup resistor.

Output pin A logic zero (0) is applied to a bit in the P register. By turning output FE transistor on, the appropriate pin is

directly connected to ground.

Input pin A logic one (1) is applied to a bit in the P register. Output FE transistor is turned off. The appropriate pin

remains connected to voltage power supply through a pull-up resistor of high resistance.

A logic state (voltage) on any pin can be changed or read at any moment. A logic zero (0) and logic one (1)

are not equal. A logic one (0) represents almost short circuit to ground. Such a pin is configured as output.

A logic one (1) is “loosely” connected to voltage power supply through resistors of high resistance. Since

this voltage can be easily “pulled down” by an external signal, such a pin is configured as input.

Port 0

It is specific to this port to have a double purpose. If external memory is used then the lower address byte

(addresses A0-A7) is applied on it. Otherwise, all bits on this port are configured as inputs or outputs.

Another characteristic is expressed when it is configured as output. Namely, unlike other ports consisting of

pins with embedded pull-up resistor ( connected by its end to 5 V power supply ), this resistor is left out here.

This, apparently little change has its consequences:

If any pin on this port is configured as input then it performs as if it “floats”. Such input has unlimited input

resistance and has no voltage coming from “inside”.

When the pin is configured as output, it performs as “open drain”, meaning that by writing 0 to some port’s

bit, the appropriate pin will be connected to ground (0V). By writing 1, the external output will keep on

“floating”. In order to apply 1 (5V) on this output, an external pull-up resistor must be embedded.

Only in case P0 is used for addressing external memory ( only in that case), the microcontroller will provide internal

power supply source in order to establish logical ones on pins. There is no need to add external pullup resistors.

Port 1

This is a true I/O port, because there are no role assigning as it is the case with P0. Since it has embedded

pull-up resistors it is completely compatible with TTL circuits.

Port 2

Similar to P0, when using external memory, lines on this port occupy addresses intended for external

memory chip. This time it is the higher address byte with addresses A8-A15. When there is no additional

memory, this port can be used as universal input-output port similar by its features to the port 1.

Port 3

Even though all pins on this port can be used as universal I/O port, they also have an alternative function.

Since each of these functions use inputs, then the appropriate pins have to be configured like that. In other

words, prior to using some of reserve port functions, a logical one (1) must be written to the appropriate bit

in the P3 register. From hardware’s perspective , this port is also similar to P0, with the difference that its

outputs have a pull-up resistor embedded.

Current limitations on pins

When configured as outputs ( logic zero (0) ), single port pins can "receive" current of 10mA. If all 8 bits on

a port are active, total current must be limited to 15mA (port P0: 26mA). If all ports (32 bits) are active, total

maximal current must be limited to 71mA. When configured as inputs (logic 1), embedded pull-up resistor

provides very weak current, but strong enough to activate up to 4 TTL inputs from LS series.

It may be seen from description of some ports, that even though all pins have more or less similar internal

structure, it is necessary to pay attention to which of them will be used for what and how.

For example: If they are used as outputs with high voltage level (5V), then port 0 should be avoided because

its pins do not have added resistor for connection to +5V. Only low logic level can be obtained therefore, if

another port is used for the same purpose, one should have in mind that pull-up resistors have a relatively

high resistance. Consequentaly it can be counted on only several hundreds microamperes of current coming

out of a pin.

2.4 8051 Microcontroller Memory Organisation

The microcontroller memory is divided into Program Memory and Data Memory. Program Memory (ROM)

is used for permanent saving program being executed, while Data Memory (RAM) is used for temporarily

storing and keeping intermediate results and variables. Depending on the model in use ( still referring to the

whole 8051 microcontroller family) at most a few Kb of ROM and 128 or 256 bytes of RAM can be used.

However…

All 8051 microcontrollers have 16-bit addressing bus and can address 64 kb memory. It is neither a mistake

nor a big ambition of engineers who were working on basic core development. It is a matter of very clever

memory organization which makes these controllers a real “ programmers’ tidbit“ .

Program Memory

The oldest models of the 8051 microcontroller family did not have internal program memory . It was added

from outside as a separate chip. These models are recognizable by their label beginning with 803 ( for ex.

8031 or 8032). All later models have a few Kbytes ROM embedded, Even though it is enough for writing

most of the programs, there are situations when additional memory is necessary. A typical example of it is

the use of so called lookup tables. They are used in cases when something is too complicated or when there

is no time for solving equations describing some process. The example of it can be totally exotic (an estimate

of self-guided rockets’ meeting point) or totally common( measuring of temperature using non-linear thermo

element or asynchronous motor speed control). In those cases all needed estimates and approximates are

executed in advance and the final results are put in the tables ( similar to logarithmic tables ).

How does the microcontroller handle external memory depends on the pin EA logic state:

EA=0 In this case, internal program memory is completely ignored, only a program stored in external

memory is to be executed.

EA=1 In this case, a program from builtin ROM is to be executed first ( to the last location). Afterwards, the

execution is continued by reading additional memory.

in both cases, P0 and P2 are not available to the user because they are used for data nd address transmission.

Besides, the pins ALE and PSEN are used too.

Data Memory

As already mentioned, Data Memory is used for temporarily storing and keeping data and intermediate

results created and used during microcontroller’s operating. Besides, this microcontroller family includes

many other registers such as: hardware counters and timers, input/output ports, serial data buffers etc. The

previous versions have the total memory size of 256 locations, while for later models this number is

incremented by additional 128 available registers. In both cases, these first 256 memory locations (addresses

0-FFh) are the base of the memory. Common to all types of the 8051 microcontrollers. Locations available to

the user occupy memory space with addresses from 0 to 7Fh. First 128 registers and this part of RAM is

divided in several blocks.

The first block consists of 4 banks each including 8 registers designated as R0 to R7. Prior to access them, a

bank containing that register must be selected. Next memory block ( in the range of 20h to 2Fh) is bit-

addressable, which means that each bit being there has its own address from 0 to 7Fh. Since there are 16 such

registers, this block contains in total of 128 bits with separate addresses (The 0th bit of the 20h byte has the

bit address 0 and the 7th bit of th 2Fh byte has the bit address 7Fh). The third group of registers occupy

addresses 2Fh-7Fh ( in total of 80 locations) and does not have any special purpose or feature.

Additional Memory Block of Data Memory

In order to satisfy the programmers’ permanent hunger for Data Memory, producers have embedded an

additional memory block of 128 locations into the latest versions of the 8051 microcontrollers. Naturally, it’s

not so simple…The problem is that electronics performing addressing has 1 byte (8 bits) on disposal and due

to that it can reach only the first 256 locations. In order to keep already existing 8-bit architecture and

compatibility with other existing models a little trick has been used.

Using trick in this case means that additional memory block shares the same addresses with existing

locations intended for the SFRs (80h- FFh). In order to differentiate between these two physically separated

memory spaces, different ways of addressing are used. A direct addressing is used for all locations in the

SFRs, while the locations from additional RAM are accessible using indirect addressing.

How to extend memory?

In case on-chip memory is not enough, it is possible to add two external memory chips with capacity of

64Kb each. I/O ports P2 and P3 are used for their addressing and data transmission.

From the users’ perspective, everything functions quite simple if properly connected because the most

operations are performed by the microcontroller itself. The 8051 microcontroller has two separate reading

signals RD#(P3.7) and PSEN#. The first one is activated byte from external data memory (RAM) should be

read, while another one is activated to read byte from external program memory (ROM). These both signals

are active at logical zero (0) level. A typical example of such memory extension using special chips for RAM

and ROM, is shown on the previous picture. It is called Hardward architecture.

Even though the additional memory is rarely used with the latest versions of the microcontrollers, it will be

described here in short what happens when memory chips are connected according to the previous schematic.

It is important to know that the whole process is performed automatically, i.e. with no intervention in the

program.

When the program during execution encounters the instruction which resides in exter nal memory (ROM), the

microcontroller will activate its control output ALE and set the first 8 bits of address (A0-A7) on P0. In this

way, IC circuit 74HCT573 which "lets in" the first 8 bits to memory address pins is activated.

A signal on the pin ALE closes the IC circuit 74HCT573 and immediately afterwards 8 higher bits of address

(A8-A15) appear on the port. In this way, a desired location in addtional program memory is completely

addressed. The only thing left over is to read its content.

Pins on P0 are configured as inputs, the pin PSEN is activated and the microcon troller reads content from

memory chip. The same connections are used both for data and lower address byte.

Similar occurs when it is a needed to read some location from external Data Memory. Now, addressing is

performed in the same way, while reading or writing is performed via signals which appear on the control

outputs RD or WR .

Addressing

While operating, processor processes data according to the program instructions. Each instruction consists of

two parts. One part describes what should be done and another part indicates what to use to do it. This later

part can be data (binary number) or address where the data is stored. All 8051 microcontrollers use two ways

of addressing depending on which part of memory should be accessed:

Direct Addressing

On direct addressing, a value is obtained from a memory location while the address of that location is

specified in instruction. Only after that, the instruction can process data (howdepends on the type of

instruction: addition, subtraction, copy…). Obviously, a number being changed during operating a variable

can reside at that specified address. For example:

Since the address is only one byte in size ( the greatest number is 255), this is how only the first 255

locations in RAM can be accessed in this case the first half of the basic RAM is intended to be used freely,

while another half is reserved for the SFRs.

MOV A,33h; Means: move a number from address 33 hex. to accumulator

Indirect Addressing

On indirect addressing, a register which contains address of another register is specified in the instruction. A

value used in operating process resides in that another register. For example:

Only RAM locations available for use are accessed by indirect addressing (never in the SFRs). For all latest

versions of the microcontrollers with additional memory block ( those 128 locations in Data Memory), this is

the only way of accessing them. Simply, when during operating, the instruction including “@” sign is

encountered and if the specified address is higher than 128 ( 7F hex.), the processor knows that indirect

addressing is used and jumps over memory space reserved for the SFRs.

MOV A,@R0; Means: Store the value from the register whose address is in the R0 register

into accumulator

On indirect addressing, the registers R0, R1 or Stack Pointer are used for specifying 8-bit addresses. Since

only 8 bits are avilable, it is possible to access only registers of internal RAM in this way (128 locations in

former or 256 locations in latest versions of the microcontrollers). If memory extension in form of additional

memory chip is used then the 16-bit DPTR Register (consisting of the registers DPTRL and DPTRH) is used

for specifying addresses. In this way it is possible to access any location in the range of 64K.

2.5 SFRs (Special Function Registers)

SFRs are a kind of control table used for running and monitoring microcontroller’s operating. Each of these

registers, even each bit they include, has its name, address in the scope of RAM and clearly defined purpose (

for example: timer control, interrupt, serial connection etc.). Even though there are 128 free memory

locations intended for their storage, the basic core, shared by all types of 8051 controllers, has only 21 such

registers. Rest of locations are intensionally left free in order to enable the producers to further improved

models keeping at the same time compatibility with the previous versions. It also enables the use of programs

written a long time ago for the microcontrollers which are out of production now.

A Register (Accumulator)

This is a general-purpose register which serves for storing intermediate results during operating. A number

(an operand) should be added to the accumulator prior to execute an instruction upon it. Once an arithmetical

operation is preformed by the ALU, the result is placed into the accumulator. If a data should be transferred

from one register to another, it must go through accumulator. For such universal purpose, this is the most

commonly used register that none microcontroller can be imagined without (more than a half 8051

microcontroller's instructions used use the accumulator in some way).

B Register

B register is used during multiply and divide operations which can be performed only upon numbers stored

in the A and B registers. All other instructions in the program can use this register as a spare accumulator

(A).

During programming, each of registers is called by name so that their exact address is not so important for the user.

During compiling into machine code (series of hexadecimal numbers recognized as instructions by the

microcontroller), PC will automatically, instead of registers’ name, write necessary addresses into the microcontroller.

R Registers (R0-R7)

This is a common name for the total 8 generalpurpose registers (R0, R1, R2 ...R7). Even they are not true

SFRs, they deserve to be discussed here because of their purpose. The bank is active when the R registers it

includes are in use. Similar to the accumulator, they are used for temporary storing variables and

intermediate results. Which of the banks will be active depends on two bits included in the PSW Register.

These registers are stored in four banks in the scope of RAM.

The following example best illustrates the useful purpose of these registers. Suppose that mathematical

operations on numbers previously stored in the R registers should be performed: (R1+R2) - (R3+R4).

Obviously, a register for temporary storing results of addition is needed. Everything is quite simple and the

program is as follows :

MOV A,R3; Means: move number from R3 into accumulator

ADD A,R4; Means: add number from R4 to accumulator (result remains in accumulator)

MOV R5,A; Means: temporarily move the result from accumulator into R5

MOV A,R1; Means: move number from R1 into accumulator

ADD A,R2; Means: add number from R2 to accumulator

SUBB A,R5; Means: subtract number from R5 ( there are R3+R4 )

PSW Register (Program Status Word)

This is one of the most important SFRs. The Program Status Word (PSW) contains several status bits that

reflect the current state of the CPU. This register contains: Carry bit, Auxiliary Carry, two register bank

select bits, Overflow flag, parity bit, and user-definable status flag. The ALU automatically changes some of

register’s bits, which is usually used in regulation of the program performing.

P - Parity bit. If a number in accumulator is even then this bit will be automatically set (1), otherwise it will

be cleared (0). It is mainly used during data transmission and receiving via serial communication.

- Bit 1. This bit is intended for the future versions of the microcontrollers, so it is not supposed to be here.

OV Overflow occurs when the result of arithmetical operation is greater than 255 (deci mal), so that it can

not be stored in one register. In that case, this bit will be set (1). If there is no overflow, this bit will be

cleared (0).

RS0, RS1 - Register bank select bits. These two bits are used to select one of the four register banks in

RAM. By writing zeroes and ones to these bits, a group of registers R0-R7 is stored in one of four banks in

RAM.

RS1 RS2 Space in RAM

0 0 Bank0 00h-07h

0 1 Bank1 08h-0Fh

1 0 Bank2 10h-17h

1 1 Bank3 18h-1Fh

F0 - Flag 0. This is a general-purpose bit available to the user.

AC - Auxiliary Carry Flag is used for BCD operations only.

CY - Carry Flag is the (ninth) auxiliary bit used for all arithmetical operations and shift instructions.

DPTR Register (Data Pointer)

These registers are not true ones because they do not physically exist. They consist of two separate registers:

DPH (Data Pointer High) and (Data Pointer Low). Their 16 bits are used for external memory addressing.

They may be handled as a 16-bit register or as two independet 8-bit registers.Besides, the DPTR Register is

usually used for storing data and intermediate results which have nothing to do with memory locations.

SP Register (Stack Pointer)

A value of the Stack Pointer ensures that the Stack Pointer will point to valid RAM and permits Stack

availability. By starting each subprogram, the value in the Stack Pointer is incremented by 1. In the same

manner, by ending subprogram, this value is decremented by 1. After any reset, the value 7 is written to the

Stack Pointer, which means that the space of RAM reserved for the Stack starts from this location. If another

value is written to this register then the entire Stack is moved to a new location in the memory.

P0, P1, P2, P3 - Input/Output Registers

In case that external memory and serial communication system are not in use then, 4 ports with in total of 32

input-output lines are available to the user for connection to peripheral environment. Each bit inside these

ports coresponds to the appropriate pin on the microcontroller. This means that logic state written to these

ports appears as a voltage on the pin ( 0 or 5 V). Naturally, while reading, the opposite occurs – voltage on

some input pins is reflected in the appropriate port bit.

The state of a port bit, besides being reflected in the pin, determines at the same time whether it will be

configured as input or output. If a bit is cleared (0), the pin will be configured as output. In the same manner,

if a bit is set to 1 the pin will be configured as input. After reset, as well as when turning the microcontroller

on , all bits on these ports are set to one (1). This means that the appropriate pins will be configured as

inputs.

Conditionally said, I/O ports are directly connected to the microcontroller’s pins. This means that a logic

state of these registers can be checked by voltmeter and vice versa-voltage on the pins can be checked by

testing their bits!

2.6 Counters and Timers

As explained in the previous chapter, the main oscillator of the microcontroller uses quartz crystal for its

operating. As the frequency of this oscillator is precisely defined and very stable, these pulses are the most

suitable for time measuring (such oscillators are used in quartz clocks as well). In order to measure time

between two events it is only needed to count up pulses from this oscillator. That is exactly what the timer is

doing. Namely, if the timer is properly programmed, the value written to the timer register will be

incremented or decremented after each coming pulse, i.e. once per each machine cycle cycle. Taking into

account that one instruction lasts 12 quartz oscillator periods (one machine cycle), by embedding quartz with

oscillator frequency of 12MHz, a number in the timer register will be changed million times per second, i.e.

each microsecond.

The 8051 microcontrollers have 2 timer counters called T0 and T1. As their names tell, their main purpose is

to measure time and count external events. Besides, they can be used for generating clock pulses used in

serial communication, i.e. Baud Rate.

Timer T0

As it is shown in the picture below, this timer consists of two registers – TH0 and TL0. The numbers these

registers include represent a lower and a higher byte of one 16-digit binary number.

This means that if the content of the timer 0 is equal to 0 (T0=0) then both registers it includes will include 0.

If the same timer contains for example number 1000 (decimal) then the register TH0 (higher byte) will

contain number 3, while TL0 (lower byte) will contain decimal number 232.

Formula used to calculate values in registers is very simple:

TH0 × 256 + TL0 = T

Matching the previous example it would be as follows :

3 × 256 + 232 = 1000

Since the timers are virtually 16-bit registers, the greatest value that could be written to them is 65 535. In

case of exceeding this value, the timer will be automatically reset and afterwords that counting starts from 0.

It is called overflow. Two registers TMOD and TCON are closely connected to this timer and control how it

operates.

TMOD Register (Timer Mode)

This register selects mode of the timers T0 and T1. As illustrated in the following picture, the lower 4 bits

(bit0 - bit3) refer to the timer 0, while the higher 4 bits (bit4 - bit7) refer to the timer 1. There are in total of 4

modes and each of them is described here in this book.

Bits of this register have the following purpose:

GATE1 starts and stops Timer 1 by means of a signal provided to the pin INT1 (P3.3):

o 1 - Timer 1 operates only if the bit INT1 is set

o 0 - Timer 1 operates regardless of the state of the bit INT 1

C/T1 selects which pulses are to be counted up by the timer/counter 1:

o 1 - Timer counts pulses provided to the pin T1 (P3.5)

o 0 - Timer counts pulses from internal oscillator

T1M1,T1M0 These two bits selects the Timer 1 operating mode.

T1M1 T1M0 Mode Description

0 0 0 13-bit timer

0 1 1 16-bit timer

1 0 2 8-bit auto-

reload

1 1 3 Split mode

GATE0 starts and stops Timer 1, using a signal provided to the pin INT0 (P3.2):

o 1 - Timer 0 operates only if the bit INT0 is set

o 0 - Timer 0 operates regardless of the state of the bit INT0

C/T0 selects which pulses are to be counted up by the timer/counter 0:

o 1 - Timer counts pulses provided to the pin T0(P3.4)

o 0 - Timer counts pulses from internal oscillator

T0M1,T0M0 These two bits select the Timer 0 operating mode.

T0M1 T0M0 Mode Description

0 0 0 13-bit timer

0 1 1 16-bit timer

1 0 2 8-bit auto-

reload

1 1 3 Split mode

Timer 0 in mode 0 (13-bit timer)

This is one of the rarities being kept only for compatibility with the previuos versions of the

microcontrollers. When using this mode, the higher byte TH0 and only the first 5 bits of the lower byte TL0

are in use. Being configured in this way, the Timer 0 uses only 13 of all 16 bits. How does it operate? With

each new pulse coming, the state of the lower register (that one with 5 bits) is changed. After 32 pulses

received it becomes full and automatically is reset, while the higher byte TH0 is incremented by 1. This

action will be repeated until registers count up 8192 pulses. After that, both registers are reset and counting

starts from 0.

Timer 0 in mode 1 (16-bit timer)

All bits from the registers TH0 and TL0 are used in this mode. That is why for this mode is being more

commonly used. Counting is performed in the same way as in mode 0, with difference that the timer counts

up to 65 536, i.e. as far as the use of 16 bits allows.

Timer 0 in mode 2 (Auto-Reload Timer)

What does auto-reload mean? Simply, it means that such timer uses only one 8-bit register for counting, but

it never counts from 0 but from an arbitrary chosen value (0- 255) saved in another register.

The advantages of this way of counting are described in the following example: suppose that for any reason

it is continuously needed to count up 55 pulses at a time from the clock generator.

When using mode 1 or mode 0, It is needed to write number 200 to the timer registers and check constantly

afterwards whether overflow occured, i.e. whether the value 255 is reached by counting . When it has

occurred, it is needed to rewrite number 200 and repeat the whole procedure. The microcontroller performs

the same procedure in mode 2 automatically. Namely, in this mode it is only register TL0 operating as a

timer ( normally 8-bit), while the value from which counting should start is saved in the TH0 register.

Referring to the previous example, in order to register each 55th pulse, it is needed to write the number 200

to the register and configure the timer to operate in mode 2.

Timer 0 in Mode 3 (Split Timer)

By configuring Timer 0 to operate in Mode 3, the 16-bit counter consisting of two registers TH0 and TL0 is

split into two independent 8-bit timers. In addition, all control bits which belonged to the initial Timer 1

(consisting of the registers TH1 and TL1), now control newly created Timer 1. This means that even though

the initial Timer 1 still can be configured to operate in any mode ( mode 1, 2 or 3 ), it is no longer able to

stop, simply because there is no bit to do that. Therefore, in this mode, it will uninterruptedly “operate in the

background “.

The only application of this mode is in case two independent 'quick' timers are used and the initial Timer 1

whose operating is out of control is used as baud rate generator.

TCON - Timer Control Register

This is also one of the registers whose bits directly control timer operating.

Only 4 of all 8 bits this register has are used for timer control, while others are used for interrupt control

which will be discussed later.

TF1 This bit is automatically set with the Timer 1 overflow

TR1 This bit turns the Timer 1 on

o 1 - Timer 1 is turned on

o 0 - Timer 1 is turned off

TF0 This bit is automatically set with the Timer 0 overflow.

TR0 This bit turns the timer 0 on

o 1 - Timer 0 is turned on

o 0 - Timer 0 is turned off

How to start Timer 0 ?

Normally, first this timer and afterwards its mode should be selected. Bits which control that are resided in

the register TMOD:

This means that timer 0 operates in mode 1 and counts pulses from internal source whose frequency is equal

to 1/12 the quartz frequency.

In order to enable the timer, turn it on:

Immediately upon the bit TR0 is set, the timer starts operating. Assuming that a quartz crystal with frequency

of 12MHz is embedded, a number it contains will be incremented every microsecond. By counting up to

65.536 microseconds, the both registers that timer consists of will be set. The microcontroller automatically

reset them and the timer keeps on repeating counting from the beginning as far as the bit’s value is logic one

(1).

How to 'read' a timer ?

Depending on the timer’s application, it is needed to read a number in the timer registers or to register a

moment they have been reset.

- Everything is extremely simple when it is needed to read a value of the timer which uses only one register

for counting (mode 2 or Mode 3) . It is sufficient to read its state at any moment and it is it!

- It is a bit complicated to read a timer’s value when it operates in mode 2. Assuming that the state of the

lower byte is read first (TL0) and the state of the higher byte (TH0) afterwards, the result is:

TH0 = 15 TL0 = 255

Everything seems to be in order at first sight, but the current state of register at the moment of reading was:

TH0 = 14 TL0 = 255

In case of negligence, this error in counting ( 255 pulses ) may occur for not so obvious but quite logical

reason. Reading the lower byte is correct ( 255 ), but at the same time the program counter “ was taking “ a

new instruction for the TH0 state reading, an overflow occurred and both registers have changed their

contents ( TH0: 14→15, TL0: 255→0). The problem has simple solution: the state of the higher byte should

be read first, then the state of the lower byte and once again the state of the higher byte. If the number stored

in the higher byte is not the same both times it has been read then this sequence should be repeated ( this is a

mini- loop consisting of only 3 instructions in a program).

There is another solution too. It is sufficient to simply turn timer off while reading ( the bit TR0 in the

register TCON should be 0), and turn it on after that.

Detecting Timer 0 Overflow

Usually, there is no need to continuously read timer registers’ contents. It is sufficient to register the moment

they are reset, i.e. when counting starts from 0. It is called overflow. When this has occurred, the bit TF0

from the register TCON will be automatically set. The microcontroller is waiting for that moment in a way

that program will constantly check the state of this bit. Furthermore, an interrupt to stop the main program

execution can be enabled. Assuming that it is needed to provide a program pause ( time the program

appeared to be stopped) in duration of for example 0.05 seconds ( 50 000 machine cycles ):

First, it is needed to calculate a number that should be written to the timer registers:

This number should be written to the timer registers TH0 and TL0:

Once the timer is started it will continue counting from the written number. Program instruction checks if the

bit TF0 is set, which happens at the moment of overflow, i.e. after exactly 50.000 machine cycles and 0.05

seconds respectively.

How to measure pulses?

Suppose it is needed to measure the duration of an event, for example how long some device has been turned

on? Look at the picture of the timer and pay attention to the purpose of the bit GATE0 ( which resides in the

TMOD register ). If this bit is cleared then the state on the pin P3.2 does not affect the timer operating. If

GATE0 = 1 the timer will operate as far as the pin P3.2 has logic one (1) value. If this pin is supplied with

5V through some external switch at the moment the device is being turned on, the timer will measure

duration of its operating, which actually was the aim.

How to count up pulses?

This time, the answer lies in the register TCON, and bit C/T0 respectively. Similar to the previous example,

this bit brings into an external signal. If the bit is cleared everything occurs in the same way as in the

previous examples and the timer counts pulses from oscillator of defined frequency, i.e. measures the time

that went by. If the bit is set, the timer input is provided with pulses from the pin P3.4 (T0). Since these

pulses do not have some definite time or order, it is not possible to measure time by counting them. For that

reason, this timer is turned into the counter. The highest frequency that could be measured by such a counter

is 1/24 frequency of used quartz-crystal.

Timer 1

Referring to its characteristics, this timer is “ a twin brother “ to the Timer 0. This means that they have the

same purpose, their operating is controlled by the same registers TMOD and TCON and both of them can

operate in one of 4 different modes.

2.7 UART (Universal Asynchronous Receiver and Transmitter)

One of the features that makes this microcontroller so powerful is an integrated UART, better known as a

serial port. It is a duplex port, which means that it can transmit and receive data simultaneously. Without it,

serial data sending and receiving would be endlessly complicated part of the program where the pin state

continuously is being changed and checked according to strictly determined rhythm. Naturally, it does not

happen here because the UART resolves it in a very elegant manner. All the programmer needs to do is to

simply select serial port mode and baud rate. When the programmer is such configured, serial data sending is

done by writing to the register SBUF while data receiving is done by reading the same register. The

microcontroller takes care of all issues necessary for not making any error during data exchange.

Serial port should be configured prior to being used. That determines how many bits one serial “word”

contains, what the baud rate is and what the pulse source for synchronization is. All bits controlling this are

stored in the SFR Register SCON (Serial Control).

SCON Register (Serial Port Control Register)

SM0 - bit selects mode

SM1 - bit selects mode

SM2 - bit is used in case that several microcontrollers share the same interface. In normal circumstances this

bit must be cleared in order to enable connection to function normally.

REN - bit enables data receiving via serial communication and must be set in order to enable it.

TB8 - Since all registers in microcontroller are 8-bit registers, this bit solves the problem of sending the 9th bit

in modes 2 and 3. Simply, bits content is sent as the 9th bit.

RB8 - bit has the same purpose as the bit TB8 but this time on the receiver side. This means that on receiving

data in 9-bit format , the value of the last ( ninth) appears on its location.

TI - bit is automatically set at the moment the last bit of one byte is sent when the USART operates as a

transmitter. In that way processor “knows” that the line is available for sending a new byte. Bit must be clear

from within the program!

RI - bit is automatically set once one byte has been received. Everything functions in the similar way as in the

previous case but on the receive side. This is line a “doorbell” which announces that a byte has been received

via serial communication. It should be read quickly prior to a new data takes its place. This bit must also be

also cleared from within the program!

As seen, serial port mode is selected by combining the bits SM0 and SM2 :

SM0 SM1 Mode Description Baud Rate

0 0 0 8-bit Shift Register 1/12 the quartz frequency

0 1 1 8-bit UART Determined by the timer 1

1 0 2 9-bit UART 1/32 the quartz frequency (1/64

the quartz frequency)

1 1 3 9-bit UART Determined by the timer 1

In mode 0, the data are transferred through the RXD pin, while clock pulses appear on the TXD pin. The

bout rate is fixed at 1/12 the quartz oscillator frequency. On transmit, the least significant bit (LSB bit) is

being sent/received first. (received).

TRANSMIT - Data transmission in form of pulse train automatically starts on the pin RXD at the moment

the data has been written to the SBUF register.In fact, this process starts after any instruction being

performed on this register. Upon all 8 bits have been sent, the bit TI in the SCON register is automatically

set.

RECEIVE - Starts data receiving through the pin RXD once two necessary conditions are met: bit REN=1

and RI=0 (both bits reside in the SCON register). Upon 8 bits have been received, the bit RI (register SCON)

is automatically set, which indicates that one byte is received.

Since, there are no START and STOP bits or any other bit except data from the SBUF register, this mode is

mainly used on shorter distance where the noise level is minimal and where operating rate is important. A

typical example for this is I/O port extension by adding cheap IC circuit ( shift registers 74HC595, 74HC597

and similar).

Mode 1

In Mode1 10 bits are transmitted through TXD or received through RXD in the following manner: a START

bit (always 0), 8 data bits (LSB first) and a STOP bit (always 1) last. The START bit is not registered in this

pulse train. Its purpose is to start data receiving mechanism. On receive the STOP bit is automatically written

to the RB8 bit in the SCON register.

TRANSMIT - A sequence for data transmission via serial communication is automatically started upon the

data has been written to the SBUF register. End of 1 byte transmission is indicated by setting the TI bit in the

SCON register.

RECEIVE - Receiving starts as soon as the START bit (logic zero (0)) appears on the pin RXD. The

condition is that bit REN=1and bit RI=0. Both of them are stored in the SCON register. The RI bit is

automatically set upon receiving has been completed.

The Baud rate in this mode is determined by the timer 1 overflow time.

Mode 2

In mode 2, 11 bits are sent through TXD or received through RXD: a START bit (always 0), 8 data bits

(LSB first), additional 9th data bit and a STOP bit (always 1) last. On transmit, the 9th data bit is actually the

TB8 bit from the SCON register. This bit commonly has the purpose of parity bit. Upon transmission, the 9th

data bit is copied to the RB8 bit in the same register ( SCON).The baud rate is either 1/32 or 1/64 the quartz

oscillator frequency.

TRANSMIT - A sequence for data transmission via serial communication is automatically started upon the

data has been written to the SBUF register. End of 1 byte transmission is indicated by setting the TI bit in the

SCON register.

RECEIVE - Receiving starts as soon as the START bit (logic zero (0)) appears on the pin RXD. The

condition is that bit REN=1and bit RI=0. Both of them are stored in the SCON register. The RI bit is

automatically set upon receiving has been completed.

Mode 3

Mode 3 is the same as Mode 2 except the baud rate. In Mode 3 is variable and can be selected.

The parity bit is the bit P in the PSW register. The simplest way to check correctness of the received byte is

to add this parity bit to the transmit side as additional bit. Simply, immediately before transmit, the message

is stored in the accumulator and the bit P goes into the TB8 bit in order to be “a part of the message”. On the

receive side is the opposite : received byte is stored in the accumulator and the bit P is compared with the bit

RB8 ( additional bit in the message). If they are the same- everything is OK!

Baud Rate

Baud Rate is defined as a number of send/received bits per second. In case the UART is used, baud rate

depends on: selected mode, oscillator frequency and in some cases on the state of the bit SMOD stored in the

SCON register. All necessary formulas are specified in the table :

Baud Rate BitSMOD

Mode 0 Fosc. / 12

Mode 1

1 Fosc.

16 12 (256-

TH1)

BitSMOD

Mode 2 Fosc. / 32

Fosc. / 64

1

0

Mode 3

1 Fosc.

16 12 (256-

TH1)

Timer 1 as a baud rate generator

Timer 1 is usually used as a baud rate generator because it is easy to adjust various baud rate by the means of

this timer. The whole procedure is simple:

First, Timer 1 overflow interrupt should be disabled

Timer T1 should be set in auto-reload mode

Depending on necessary baud rate, in order to obtain some of the standard values one of the numbers from the

table should be selected. That number should be written to the TH1 register. That's all.

Baud Rate

Fosc. (MHz)

Bit SMOD

11.0592 12 14.7456 16 20

150 40 h 30 h 00 h 0

300 A0 h 98 h 80 h 75 h 52 h 0

600 D0 h CC h C0 h BB h A9 h 0

1200 E8 h E6 h E0 h DE h D5 h 0

2400 F4 h F3 h F0 h EF h EA h 0

4800 F3 h EF h EF h 1

4800 FA h F8 h F5 h 0

9600 FD h FC h 0

9600 F5 h 1

19200 FD h FC h 1

38400 FE h 1

76800 FF h 1

Multiprocessor Communication

As described in the previous text, modes 2 and 3 enable the additional 9th data bit to be part of message. It

can be used for checking data via parity bit. Another useful application of this bit is in communication

between two microcontrollers, i.e. multiprocessor communication. This feature is enabled by setting the SM2

bit in the SCON register. The consequence is the following: when the STOP bit is ready, indicating end of

message, the serial port interrupt will be requested only in case the bit RB8 = 1 (the 9th bit).

The whole procedure will be performed as follows:

Suppose that there are several connected microcontrollers having to exchange data. That means that each of

them must have its address. The point is that each address sent via serial communication has the 9th bit set

(1), while data has it cleared (0). If the microcontroller A should send data to the microcontroller C then it at

will place first send address of C and the 9th bit set to 1. That will generate interrupt and all microcontrollers

will check whether they are called.

Of course, only one of them will recognize this address and immediately clear the bit SM2 in the SCON

register. All following data will be normally received by that microcontroller and ignored by other

microcontrollers.

2.8 8051 Microcontroller Interrupts

There are five interrupt sources for the 8051, which means that they can recognize 5 different event that can

interrupt regular program execution. Each interrupt can be enabled or disabled by setting bits in the IE

register. Also, as seen from the picture below the whole interrupt system can be disabled by clearing bit EA

from the same register.

Now, one detail should be explained which is not completely obvious but refers to external interrupts- INT0

and INT1. Namely, if the bits IT0 and IT1 stored in the TCON register are set, program interrupt will occur

on changing logic state from 1 to 0, (only at the moment). If these bits are cleared, the same signal will

generate interrupt request and it will be continuously executed as far as the pins are held low.

IE Register (Interrupt Enable)

EA - bit enables or disables all other interrupt sources (globally)

o 0 - (when cleared) any interrupt request is ignored (even if it is enabled)

o 1 - (when set to 1) enables all interrupts requests which are individually enabled

ES - bit enables or disables serial communication interrupt (UART)

o 0 - UART System can not generate interrupt

o 1 - UART System enables interrupt

ET1 - bit enables or disables Timer 1 interrupt

o 0 - Timer 1 can not generate interrupt

o 1 - Timer 1 enables interrupt

EX1 - bit enables or disables INT 0 pin external interrupt

o 0 - change of the pin INT0 logic state can not generate interrupt

o 1 - enables external interrupt at the moment of changing the pin INT0 state

ET0 - bit enables or disables timer 0 interrupt

o 0 - Timer 0 can not generate interrupt

o 1 - enables timer 0 interrupt

EX0 - bit enables or disables INT1 pin external interrupt

o 0 - change of the INT1 pin logic state can not cause interrupt

o 1 - enables external interrupt at the moment of changing the pin INT1 state

Interrupt Priorities

It is not possible to predict when an interrupt will be required. For that reason, if several interrupts are

enabled. It can easily occur that while one of them is in progress, another one is requested. In such situation,

there is a priority list making the microcontroller know whether to continue operating or meet a new interrupt

request.

The priority list cosists of 3 levels:

1. Reset! The apsolute master of the situation. If an request for Reset omits, everything is stopped and the

microcontroller starts operating from the beginning.

2. Interrupt priority 1 can be stopped by Reset only.

3. Interrupt priority 0 can be stopped by both Reset and interrupt priority 1.

Which one of these existing interrupt sources have higher and which one has lower priority is defined in the

IP Register ( Interrupt Priority Register). It is usually done at the beginning of the program. According to

that, there are several possibilities:

Once an interrupt service begins. It cannot be interrupted by another inter rupt at the same or lower priority

level, but only by a higher priority interrupt.

If two interrupt requests, at different priority levels, arrive at the same time then the higher priority interrupt is

serviced first.

If the both interrupt requests, at the same priority level, occur one after another , the one who came later has to

wait until routine being in progress ends.

If two interrupts of equal priority requests arrive at the same time then the interrupt to be serviced is selected

according to the following priority list :

1. External interrupt INT0

2. Timer 0 interrupt

3. External Interrupt INT1

4. Timer 1 interrupt

5. Serial Communication Interrupt

IP Register (Interrupt Priority)

The IP register bits specify the priority level of each interrupt (high or low priority).

PS - Serial Port Interrupt priority bit

o Priority 0

o Priority 1

PT1 - Timer 1 interrupt priority

o Priority 0

o Priority 1

PX1 - External Interrupt INT1 priority

o Priority 0

o Priority 1

PT0 - Timer 0 Interrupt Priority

o Priority 0

o Priority 1

PX0 - External Interrupt INT0 Priority

o Priority 0

o Priority 1

Handling Interrupt

Once some of interrupt requests arrives, everything occurs according to the following order:

1. Instruction in progress is ended

2. The address of the next instruction to execute is pushed on the stack

3. Depending on which interrupt is requested, one of 5 vectors (addresses) is written to the program counter in

accordance to the following table:

4.

Interrupt Source Vector (address)

IE0 3 h

TF0 B h

TF1 1B h

RI, TI 23 h

All addresses are in hexadecimal format

5. The appropriate subroutines processing interrupts should be located at these addresses. Instead of

them, there are usually jump instructions indicating the location where the subroutines reside.

6. When interrupt routine is executed, the address of the next instruction to execute is poped from the stack to the

program counter and interrupted program continues operating from where it left off.

From the moment an interrupt is enabled, the microcontroller is on alert all the time. When interrupt request

arrives, the program execution is interrupted, electronics recognizes the cause and the program “jumps” to

the appropriate address (see the table above ). Usually, there is a jump instruction already prepared

subroutine prepared in advance. The subroutine is executed which exactly the aim- to do something when

something else has happened. After that, the program continues operating from where it left off…

Reset

Reset occurs when the RS pin is supplied with a positive pulse in duration of at least 2 machine cycles ( 24

clock cycles of crystal oscillator). After that, the microcontroller generates internal reset signal during which

all SFRs, excluding SBUF registers, Stack Pointer and ports are reset ( the state of the first two ports is

indefinite while FF value is being written to the ports configuring all pins as inputs). Depending on device

purpose and environment it is in, on power-on reset it is usually push button or circuit or both connected to

the RS pin. One of the most simple circuit providing secure reset at the moment of turning power on is

shown on the picture.

Everything functions rather simply: upon the power is on, electrical condenser is being charged for several

milliseconds through resistor connected to the ground and during this process the pin voltage supply is on.

When the condenser is charged, power supply voltage is stable and the pin keeps being connected to the

ground providing normal operating in that way. If later on, during the operation, manual reset button is

pushed, the condenser is being temporarily discharged and the microcontroller is being reset. Upon the

button release, the whole process is repeated…

Through the program- step by step...

The microcontrollers normally operate at very high speed. The use of 12 Mhz quartz crystal enables

1.000.000 instructions per second to be executed! In principle, there is no need for higher operating rate. In

case it is needed, it is easy to built-in crystal for high frequency. The problem comes up when it is necessary

to slow down. For example, when during testing in real operating environment, several instructions should be

executed step by step in order to check for logic state of I/O pins.

Interrupt system applied on the 8051 microcontrollers practically stops operating and enables instructions to

be executed one at a time by pushing button. Two interrupt features enable that:

Interrupt request is ignored if an interrupt of the same priority level is being in progress.

Upon interrupt routine has been executed, a new interrupt is not executed until at least one instruction from the

main program is executed.

In order to apply this in practice, the following steps should be done:

1. External interrupt sensitive to the signal level should be enabled (for example INT0).

2. Three following instructions should be entered into the program (start from address 03hex.):

What is going on? Once the pin P3.2 is set to “0” (for example, by pushing button), the microcontroller will

interrupt program execution jump to the address 03hex, will be executed a mini-interrupt routine consisting

of 3 instructions is located at that address.

The first instruction is being executed until the push button is pressed ( logic one (1) on the pin P3.2). The

second instruction is being executed until the push button is released. Immediately after that, the instruction

RETI is executed and processor continues executing the main program. After each executed instruction, the

interrupt INT0 is generated and the whole procedure is repeated ( push button is still pressed). Button Press =

One Instruction.


Conditionally said microcontroller is the most part of its “lifetime” is inactive for some external signal in

order to takes its role in a show. It can make a great problem in case batteries are used for power supply. In

extremely cases, the only solution is to put the whole electronics to sleep in order to reduce consumption to

the minimum. A typical example of this is remote TV controller: it can be out of use for months but when

used again it takes less than a second to send a command to TV receiver. While normally operating, the

AT89S53 uses current of approximately 25mA, which shows that it is not too sparing microcontroller.

Anyway, it doesn’t have to be always like this, it can easily switch the operation mode in order to reduce its

total consumption to approximately 40uA. Actually, there are two power-saving modes of operation: Idle and

Power Down.

Idle

mode

Immediately upon instruction which sets the bit IDL in the PCON register, the microcontroller turns off the

greatest power consumer- CPU unit while peripheral units serial port, timers and interrupt system continue

operating normally consuming 6.5mA. In Idle mode, the state of all registers and I/O ports is remains

unchanged.

In order to terminate the Idle mode and make the microcontroller operate normally, it is necessary to enable

and execute any interrupt or reset.Then, the IDL bit is automatically cleared and the program continues

executing from instruction following that instruction which has set the IDL bit. It is recommended that three

first following one which set NOP instructions. They do not perform any operation but keep the

microcontroller from undesired changes on the I/O ports.

Power Down mode

When the bit PD in the register PCON is set from within the program, the microcontroller is set to

Powerdown mode. It and turns off its internal oscillator reducing drastically consumption in that way. In

powerdown mode the microcontroller can operate using only 2V power supply while the total power

consumption is less than 40uA. The only way to get the microcontroller back to normal mode is reset.

During Power Down mode, the state of all SFR registers and I/O ports remains unchanged, and after the

microcontroller is put get into the normal mode, the content of the SFR register is lost, but the content of

internal RAM is saved. Reset signal must be long enough approximately 10mS in order to stabilize quartz

oscillator operating.

PCON register

The purpose of the Register PCON bits :

SMOD By setting this bit baud rate is doubled.

GF1 General-purpose bit (available for use).



PD By setting this bit the microcontroller is set into Power Down mode.

IDL By setting this bit the microcontroller is set into Idle mode.


Chapter 6 I/O Ports:

Input Output functions are set by Three Registers for each PORT.

DDRX ----> Sets whether a pin is Input or Output of PORTX.

PORTX ---> Sets the Output Value of PORTX.

PINX -----> Reads the Value of PORTX.

Go to the page 50 in the datasheet or you can also see the I/O Ports tab in the Bookmarks.

6.1 DDRX (Data Direction Register)

First of all we need to set whether we want a pin to act as output or input. DDRX register sets

this. Every bit corresponds to one pin of PORTX. Let’s have a look on DDRA register.

Bit 7 6 5 4 3 2 1 0

PIN PA7 PA6 PA5 PA4 PA3 PA2 PA1 PA0

Now to make a pin act as I/O we set its corresponding bit in its DDR register.

To make Input set bit 0

To make Output set bit 1

If I write DDRA = 0xFF (0x for Hexadecimal number system) that is setting all the bits of

DDRA to be 1, will make all the pins of PORTA as Output.

Similarly by writing DDRD = 0x00 that is setting all the bits of DDRD to be 0, will make all

the pins of PORTD as Input.

Now let’s take another example. Consider I want to set the pins of PORTB as shown in table,

PORT-B PB7 PB6 PB5 PB4 PB3 PB2 PB1 PB0

Function Output Output Input Output Input Input Input Output

DDRB 1 1 0 1 0 0 0 1

For this configuration we have to set DDRB as 11010001 which in hexadecimal is D1. So we

will write DDRB=0xD1

Summary

DDRX -----> to set PORTX as input/output with a byte.

DDRX.y ---> to set yth pin of PORTX as input/output with a bit (works only with CVAVR).


6.2 PORTX (PORTX Data Register)

This register sets the value to the corresponding PORT. Now a pin can be Output or Input. So

let’s discuss both the cases.

6.2.1 Output Pin

If a pin is set to be output, then by setting bit 1 we make output High that is +5V and by

setting bit 0 we make output Low that is 0V.

Let’s take an example. Consider I have set DDRA=0xFF, that is all the pins to be Output.

Now I want to set Outputs as shown in table,

PORT-A PA7 PA6 PA5 PA4 PA3 PA2 PA1 PA0

Value High(+5V

) High(+5V

) Low(0V) Low(0V) Low(0V) High(+5V

) High(+5V

) Low(0V)

PORTA 1 1 0 0 0 1 1 0

For this configuration we have to set PORTA as 11000110 which in hexadecimal is C6. So

we will write PORTA=0xC6;

6.2.2 Input Pin

If a pin is set to be input, then by setting its corresponding bit in PORTX register will make it

as follows,

Set bit 0 ---> Tri-Stated

Set bit 1 ---> Pull Up

Tristated means the input will hang (no specific value) if no input voltage is specified on that

pin.

Pull Up means input will go to +5V if no input voltage is given on that pin. It is basically

connecting PIN to +5V through a 10K Ohm resistance.

Summary

PORTX ----> to set value of PORTX with a byte.

PORTX.y --> to set value of yth

pin of PORTX with a bit (works only with CVAVR).


6.3 PINX (Data Read Register)

This register is used to read the value of a PORT. If a pin is set as input then corresponding

bit on PIN register is,

0 for Low Input that is V < 2.5V

1 for High Input that is V > 2.5V (Ideally, but actually 0.8 V - 2.8 V is error zone !)

For an example consider I have connected a sensor on PC4 and configured it as an input pin

through DDR register. Now I want to read the value of PC4 whether it is Low or High. So I

will just check 4th bit of PINC register.

We can only read bits of the PINX register; can never write on that as it is meant for reading

the value of PORT.

Summary

PINX ----> Read complete value of PORTX as a byte.

PINX.y --> Read yth

pin of PORTX as a bit (works only with CVAVR).

I hope you must have got basic idea about the functioning of I/O Ports. For detailed reading

you can always refer to datasheet of Atmega.


http://home.iitk.ac.in/~sankule/robotics/datsheets/Atmega16.pdf

Chapter 7 LCD Interfacing

Now we need to interface an LCD to our microcontroller so that we can display messages, outputs,

etc. Sometimes using an LCD becomes almost inevitable for debugging and calibrating the sensors

(discussed later). We will use the 16x2 LCD, which means it has two rows of 16 characters each.

Hence in total we can display 32 characters.

7.1 Circuit Connection

There are 16 pins in an LCD; See reverse side of the LCD for the PIN

configuration. The connections have to be made as shown below:

Figure 15: LCD connections

7.2 Setting up in Microcontroller

When we connect an LCD to Atmega16, one full PORT is dedicated to it, denoted by PORT-X in

the figure. To enable LCD interfacing in the microcontroller, just click on the LCD tab in the Code

Wizard and select the PORT at which you want to connect the LCD. We will select PORTC. Also

select the number of characters per line in your LCD. This is 16 in our case. Code Wizard now shows

you

the complete list of connections which you will have to make in order to interface the LCD. These are

nothing but the same as in the above figure for general PORT-X.


Figure 16: LCD settings on CVAVR wizard window.

As you can see, there are some special connections other than those to uC, Vcc and gnd. These

are general LCD settings. Pin 3 (VO) is for the LCD contrast, ground it through a <1kΩ resistance/

potentiometer for optimum contrast. Pin 15 & 16 (LEDA and LEDK) are for LCD backlight, give

them permanent +5V and GND respectively as we need to glow it continuously.

7.3 Printing Functions

Now once the connections have been made, we are ready to display something on our screen.

Displaying our name would be great to start with. Some of the general LCD functions which you must

know are:

7.3.1 lcd_clear()

Clears the lcd. Remember! Call this function before the while(1) loop, otherwise you won’t be

able to see anything!

7.3.2 lcd_gotoxy(x,y)

Place the cursor at coordinates (x,y) and start writing from there. The first coordinate is (0,0).

Hence, x ranges from 0 to 15 and y from 0 to 1 in our LCD. Suppose you want to display something

starting from the 5th

character in second line, then the function would be

lcd_gotoxy(5,1);


7.3.3 lcd_putchar(char c)

To display a single character. E.g.,

lcd_putchar(‘H’);

7.3.4 lcd_putsf(constant string)

To display a constant string. Eg,

lcd_putsf(“IIT Kanpur”);

7.3.5 lcd_puts(char arr)

To display a variable string, which is nothing but an array of characters (data type char) in C

language . e.g., You have an array char c[10] which keeps on changing. Then to display it, the

function would be called as

lcd_puts(c);

Now we have seen that only characters or strings (constant or variable) can be displayed on the

LCD. But quite often we have to display values of numeric variables, which is not possible directly.

Hence we need to first convert that numeric value to a string and then display it. For e.g., if we have a

variable of type integer, say int k, and we need to display the value of k (which changes every now

and then, 200 now and 250 after a second... and so on). For this, we use the C functions itoa() and

ftoa(), but remember to include the header file stdlib.h to use these C functions.

7.3.6 itoa(int val, char arr[])

It stores the value of integer val in the character array arr. E.g., we have already defined int i and

char c[20], then

itoa(i,c);

lcd_puts(c);

Similarly we have

7.3.7 ftoa(float val, char decimal_places, char arr[])

It stores the value of floating variable f in the character array arr with the number of decimal places

as specified by second parameter. E.g., we have already defined float f and char c[20], then

ftoa(f,4,c); // till 4 decimal places

lcd_puts(c);

Now we are ready to display anything we want on our LCD. Just try out something which you

would like to see glowing on it!


Chapter 8 UART Communication

UART (Universal Asynchronous Receiver Transmitter) is a way of communication between the

microcontroller and the computer system or another microcontroller. There are always two parts to

any mode of communication-a Receiver and a Transmitter. Hence, our Atmega can receive data as

well as send data to other microcontroller, computer or any other device.

8.1 UART: Theory of Operation

Figure 16 illustrates a basic UART data packet. While no data is being transmitted, logic 1 must be placed in

the Tx line. A data packet is composed of 1 start bit, which is always a logic 0, followed by a programmable

number of data bits (typically between 6 to 8), an optional parity bit, and a programmable number of stop bits

(typically 1). The stop bit must always be logic 1.

Most UART uses 8bits for data, no parity and 1 stop bit. Thus, it takes 10 bits to transmit a byte of data.

Figure 17: Basic UART packet format: 1 start bit, 8 data bits, 1 parity bit, 1 stop bit.

BAUD Rate: This parameter specifies the desired baud rate (bits per second) of the UART. Most typical

standard baud rates are: 300, 1200, 2400, 9600, 19200, etc. However, any baud rate can be used. This

parameter affects both the receiver and the transmitter. The default is 2400 (bauds).

In the UART protocol, the transmitter and the receiver do not share a clock signal. That is, a clock signal does

not emanate from one UART transmitter to the other UART receiver. Due to this reason the protocol is said to

be asynchronous.

Since no common clock is shared, a known data transfer rate (baud rate) must be agreed upon prior to data

transmission. That is, the receiving UART needs to know the transmitting UART’s baud rate (and conversely

the transmitter needs to know the receiver’s baud rate, if any). In almost all cases the receiving and transmitting

baud rates are the same. The transmitter shifts out the data starting with the LSB first.

Once the baud rate has been established (prior to initial communication), both the transmitter and the receiver’s

internal clock is set to the same frequency (though not the same phase). The receiver "synchronizes" its internal

clock to that of the transmitter’s at the beginning of every data packet received. This allows the receiver to

sample the data bit at the bit-cell center.

A key concept in UART design is that UART’s internal clock runs at much faster rate than the baud rate. For

example, the popular 16450 UART controller runs its internal clock at 16 times the baud rate. This allows the

UART receiver to sample the incoming data with granularity of 1/16 the baud-rate period. This "oversampling"

is critical since the receiver adds about 2 clock-ticks in the input data synchronizer uncertainty. The incoming

data is not sampled directly by the receiver, but goes through a synchronizer which translates the clock

domain from the transmitter’s to that of the receiver. Additionally, the greater the granularity, the receiver

has greater immunity with the baud rate error.


The receiver detects the start bit by detecting the transition from logic 1 to logic 0 (note that while the data line

is idle, the logic level is high). In the case of 16450 UART, once the start-bit is detected, the next data bit’s

"center" can be assured to be 24 ticks minus 2 (worse case synchronizer uncertainty) later. From then on, every

next data bit center is 16 clock ticks later. Figure 2 illustrates this point.

Once the start bit is detected, the subsequent data bits are assembled in a de-serializer. Error condition

maybe generated if the parity/stop bits are incorrect or missing.

Figure 18: Data sampling points by the UART receiver.

8.2 Serial Port of Computer

We will be using Serial Port for

communication between the uC and the

computer. A serial port has 9 pins as shown. If

you have a laptop, then most probably there

won’t be a serial port. Then you can use a USB

to serial Converter.

If you have to transmit one byte of data,

the serial port will transmit 8 bits as one bit at

a time. The advantage is that a serial port needs

only one wire to transmit the 8 bits.

Figure 19: A Serial Port


Pin 3 is the Transmit (TX) pin, pin 2 is the Receive (RX) pin and pin 5 is Ground pin. Other pins

are used for controlling data communication in case of a modem. For the purpose of data transmission,

only the pins 3 and 5 are required.

The standard used for serial communication is RS-232 (Recommended Standard 232). The RS-

232 standard defines the voltage levels that correspond to logical one and logical zero levels. Valid

signals are plus or minus 3 to 15 volts. The range near zero volts is not a valid RS-232 level; logic one

is defined as a negative voltage, the signal condition is called marking, and has the functional

significance of OFF. Logic zero is positive; the signal condition is spacing, and has the function ON.

Now we know that this is not the voltage level at which our microcontroller works. Hence, we

need a device which can convert this voltage level to that of CMOS, i.e., logic 1 = +5V and logic 0

=

0V. This task is carried out by an IC MAX 232, which is always used with four 10uF capacitors.

The circuit is shown:

The Rx and Tx shown in above figure (pins 11 and 12 of MAX232) are the Rx and Tx of

Atmega

16 (PD0 and PD1 respectively).

8.3 Setting up UART in microcontroller

Once our electronic circuit is complete, we can start with coding. To enable UART mode in

Atmega16, click on the USART tab in Code Wizard. Now depending upon your requirement, you can

either check receiver, transmitter or both. You get a list of options to select from for the baud rate.

Baud Rate is the unit of data transfer, defined as bits per second. We will select 9600 as it is fair

enough for our purpose, and default setting in most of the applications (like Matlab, Docklight, etc.).

Keep the communication parameters as default, i.e., 8 data, 1 stop and no parity and mode

asynchronous. You also have option of enabling Rx and Tx Interrupt functions, but we won’t do at

this point.


Once you generate and save the code, all the register values are set by CVAVR and you only need

to know some of the C functions to transfer data. Some of them are:

8.3.1 putchar()

To send one character to the buffer, which will be received by the device (uC or computer)? E.g.,

putchar(‘A’); //sends character ‘A’ to the buffer

putchar(c); // sends a variable character c

8.3.2 getchar()

To receive one character from the buffer, which might have been sent by the other uC or the

computer. E.g., if we have already defined a variable char c, then

c = getchar(); // receives the character from buffer and save it in variable c

8.3.3 putsf()

To send a constant string. Eg,

putsf(“Robocon Team”);


8.4 Docklight

To communicate with the computer, you need a terminal where you can send data through keyboard

and the received data can be displayed on the screen. There are many softwares which provide such

terminal, but we will be using Docklight. Its evaluation version is free for download on internet,

which is sufficient for our purpose.

To start with, check the Terminal Settings in Docklight. Go to Tools -> Project Settings. Select the

Send/Receive communication channel, i.e., the name by which the serial port is known in your

computer (like COM1…). In the COM port settings, select the same values as you had set while

coding Atmega 16. So, we will select Baud Rate 9600, Data Bits 8, Stop Bits 1, Parity Bits none.

You can select ‘none’ in Parity Error Character. Click OK.

We are now ready to send/receive data, so, select Run -> Start Communication, or, press F5. If your

uC is acting as a transmitter, then the characters it sends will appear in the Communication window of

Docklight. E.g.,

putchar(‘K’);

delay_ms(500); // Sends character K after every 500ms

Hence what you get on the screen is a KKKKKKKKKKKKKKKKKKKKKKKKK…….. one K increasing

every

500ms. To stop receiving characters, select Run -> Stop Communication, or press F6.

If the receiver option is also enabled in Atmega16, then whatever you type from keyboard will be

received by it. You can either display these received characters on an LCD, control motors depending

on what characters you send, etc. e.g.,

c = getchar() ; // receive character

lcd_putchar(c); // display it on LCD

if (c==’A’)

PORTA=255; // if that character is A, make all pins of PORTA high.

When you are done with sending data, select Run -> Stop Communication, or press F6.


Chapter 9 Timers

9.1 Basic Theory

Atmega 16 has following timers,

Timer 0, 8 bit

Timer 1, 16 bit consisting of two 8 bit parts, A and B

Timer 2, 8 bit

Now there are two clocks,

1. System Clock (fs): This is the clock frequency at which Atmega is running. By default it is 1

MHz which can be changed by setting fuse bits.

2. Timer Clock (ft): This is the clock frequency at which

timer module is running. Each timer module has different

clocks.

Now ft can be in ratios of fs. That is, ft = fs, fs/8, fs/64 ...

For example, if we keep fs = 8 MHz then available options for ft

are: 8 MHz, 1 MHz, 125 KHz ... (look in the image)

There are total 4 settings for Timers,

1. Clock Source: Source for timer clock, keep it as system

clock. You can also provide external clock. Read datasheet

for more information about external clock source.

2. Clock value: This is value of ft. Drop down for

available options of ft. Chose whichever is required

3. Mode: There are many modes of timers. We will be

discussing following 2 modes,

a. Fast PWM top = FFh

b. CTC top=OCRx (x=0, 1A, 2)

4. Output: Depending upon the mode we have chosen there

are options for output pulse. We will look in detail later.

Basically each Timer has a counter unit with size 8 bit for Timer 0, 2 and 16 bit for Timer 1. I will

be talking about Timer 0 and same will follow for other Timers.

Each counter has a register which increments by one on every rising edge of timer clock. After

counting to its full capacity, 255 for 8 bit, it again starts from 0. By using this register we can have

different modes.

Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers.

TOP: The counter reaches the TOP when it becomes equal to the highest value in the count

sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value stored in

the OCR0 Register. The assignment is dependent on the mode of operation.


9.2 Fast PWM Mode

PWM = Pulse Width Modulation.

This mode is used to generate pulse with

Fixed Frequency (F)

Variable Duty Cycle (D)

F = Ft / 256

D = OCR0 / 255 (non inverted)

D = (255-OCR0) / 255 (inverted)

By changing OCR0 value we can change the duty cycle of the

output pulse.

As OCR0 is an 8bit register it can vary from 0 o 255.

0 ≤ OCR0 ≤

255

Pins for Output pulse,

Timer 0 : OC0, pin

4

Timer 1A : OC1A, pin 19

Timer 1B : OC1B, pin 18

Timer 2 : OC2, pin 21

Figure 20: Fast PWM mode Timing

Diagram


9.3 CTC Mode

CTC = Clear Timer on Compare

Match. This mode is to generate pulse

with,

Fixed Duty Cycle (D = 0.5)

Variable Frequency (F)

𝐟𝐭 F = (𝑶𝑪𝑹+)

D = 0.5

By changing value of OCR0 we can change the value of

output pulse frequency. As OCR0 is an 8bit register it can

vary from 0 to 255.

0 ≤ OCR0 ≤ 255

Figure 21: CTC Mode timing

diagram


Chapter 10 SPI: Serial Peripheral Interface

10.1 Theory of Operation

The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link used to communicate

between two or more microcontroller and devices supporting SPI mode data transfer. Devices

communicate in master/slave mode where the master device initiates the data frame. Multiple slave

devices are allowed with individual slave select (chip select) lines.

This communication protocol consists of folliwng lines or pins,

1. MOSI : Master Out Slave In (Tx for Master and Rx for Slave)

2. MISO : Master In Slave Out (Rx for Master Tx for Slave)

3. SCK : Serial Clock (Clock line)

4. SS : Slave Select (To select Slave chip) (if given 0 device acts as slave)

Master: This device provides the serial clock to the other device for data transfer. As a clock is used

for the data transfer, this protocol is Synchronous in nature. SS for Master will be disconnected.

Slave: This device accepts the clock from master device. SS for this has to be made 0 externally.

Master

MOS

I

MIS

O

SCK

MOS

I

MIS

O

SCK

0 SS

Slave

Pin connections for SPI protocol

Figure 22: SPI Data Communication


10.2 Setting up SPI in Microcontroller

10.2.1 Master Microcontroller

Open Code Wizard and go to SPI tab. Enable SPI and choose

Master in SPI Type.

Select clock rate depending upon your data transfer speed

requirement.

Keep other settings as default.

10.2.2 Slave Microcontroller

Open Code Wizard and go to SPI tab. Enable SPI and choose

Slave in SPI Type.

Select clock rate depending upon your data transfer speed

requirement.

Keep other settings as kept in the master microcontroller.

10.3 Data Functions

You can transmit or receive 1 byte of data at a time.

10.3.1 Transmit Data

spi(1 byte data); Example: spi(‘A’);

10.3.2 Receive Data

c = spi(0); // c is 1 byte variable Example: char c = spi(0);


Chapter 11 ADC: Analog to Digital Converter

11.1 Theory of operation

What we have seen till now that the input given to uC was digital, i.e., either +5 V (logic 1) or 0V

(logic 0). But what if we have an analog input, i.e., value varies over a range, say 0V to +5V? Then

we require a tool that converts this analog voltage to discrete values. Analog to Digital Converter

(ADC) is such a tool.

ADC is available at PORTA of Atmega16. Thus we have 8 pins available where we can apply

analog voltage and get corresponding digital values. The ADC register is a 10 bit register, i.e., the

digital value ranges from 0 to 1023. But we can also use only 8 bit out of it (0 to 255) as too much

precision is not required.

Reference voltage is the voltage to which the ADC assigns the maximum value (255 in case of 8

bit and 1023 for 10 bit). Hence, the ADC of Atmega16 divides the input analog voltage range (0V to

Reference Voltage) into 1024 or 256 equal parts, depending upon whether 10 bit or 8 bit ADC is

used. For example, if the reference voltage is 5V and we use 10bit ADC, 0V has digital equivalent 0,

+5V is digitally 1023 and 2.5V is approximately equal to 512.

ADC =Vin x 255/Vref (8 bit)

ADC =Vin x 1023/Vref (10 bit)

11.2 Setting up Microcontroller

To enable ADC in Atmega16, click on the ADC tab in Code Wizard and enable the checkbox.

You can also check “use 8 bits” as that is sufficient for our purpose and 10 bit accuracy is not

required. If the input voltage ranges from 0 to less than +5V, then apply that voltage at AREF (pin 32)

and select the Volt. Ref. as AREF pin. But if it ranges from 0 to +5 V, you can select the Volt. Ref. as AVCC pin itself. Keep the clock at its default value of 125 kHz and select the Auto Trigger Source as

Free Running. You can also enable an interrupt function if you require.


11.3 Function for getting ADC

Now when you generate and save the code, all the register values are set automatically along

with a function:

unsigned char read_adc(unsigned char adc_input).

This function returns the digital value of analog input at that pin of PORTA whose number is

passed as parameter, e.g., if you want to know the digital value of voltage applied at PA3, and then

just call the function as,

read_adc(3);

If the ADC is 8 bit, it will return a value from 0 to 255. Most probably you will need to print it on

LCD. So, the code would be somewhat like

int a; char c[10]; // declare in the section of global variables

a=read_adc(3);

itoa(a,c);

lcd_puts(c);


Chapter 12 Interrupts

An interrupt is a signal that stops the current program forcing it to execute another program

immediately. The interrupt does this without waiting for the current program to finish. It is

unconditional and immediate which is why it is called an interrupt.

The whole point of an interrupt is that the main program can perform a task without worrying about an

external event.

Example

For example if you want to read a push button connected to

one pin of an input port you could read it in one of two ways

either by polling it or by using interrupts.

12.1 Polling

Polling is simply reading the button input regularly. Usually

you need to do some other tasks as well e.g. read an RS232

input so there will be a delay between reads of the button. As

long as the delays are small compared to the speed of the

input change then no button presses will be missed.

If however you had to do some long calculation then a keyboard press could be missed while the

processor is busy. With polling you have to be more aware of the processor activity so that you allow

enough time for each essential activity.

12.2 Hardware interrupt

By using a hardware interrupt driven button reader the calculation could proceed with all button

presses captured. With the interrupt running in the background you would not have to alter the

calculation function to give up processing time for button reading.

The interrupt routine obviously takes processor time – but you do not have to worry about it while

constructing the calculation function.

You do have to keep the interrupt routine small compared to the processing time of the other

functions - it’s no good putting tons of operations into the ISR. Interrupt routines should be kept

short and sweet so that the main part of the program executes correctly e.g. for lots of interrupts

you need the routine to finish quickly ready for the next one.

12.3 Hardware Interrupt or polling?

The benefit of the hardware interrupt is that processor time is used efficiently and not wasted

polling input ports. The benefit of polling is that it is easy to do.

Another benefit of using interrupts is that in some processors you can use a wake-from-sleep

interrupt. This lets the processor go into a low power mode, where only the interrupt hardware is

active, which is useful if the system is running on batteries.

Hardware interrupt Common terms

Terms you might hear associated with hardware interrupts are ISR, interrupt mask, non maskable

interrupt, an asynchronous event, and interrupt vector and context switching.


12.4 Setting up Hardware Interrupt in Microcontroller

There are 3 external interrupts in Atmega 16. They are

INT0 : PD2, Pin 16

INT1 : PD3, Pin 17

INT2 : PB2, Pin 3

There are 3 modes of external Interrupts,

1. Low Level: In this mode interrupt occurs whenever it

detects a ‘0’ logic at INT pin. To use this, you should put

an external pull up resistance to avoid interrupt every

time.

2. Falling Edge: In this mode interrupt occurs whenever it detects a falling edge that is ‘1’ to ‘0’

logic change at INT pin.

3. Rising Edge: In this mode interrupt occurs whenever it detects a falling edge that is ‘0’ to ‘1’ logic

change at INT pin.

12.5 Functions of Interrupt Service Routine

After generating the code, you can see the following function,

// External Interrupt 0 service routine

interrupt [EXT_INT0] void

ext_int0_isr(void)

// Place your code here

You can place your code in the function, and the code will be executed whenever interrupt occurs.


Section C

Robotics


Chapter 13 Introduction to Autonomous Robots

Any Autonomous Robot consists of following essential parts.

1. Robot Chassis and actuators

Includes wheeled or any type of chassis with all the necessary actuators fitted on the chassis to

achieve desired goal. We mostly use DC geared motors as actuators.

2. Electronics

Electronics includes Sensors, motion control circuits, power management system etc.

3. Power Source

Usually battery pack consisting of Lead acid, Nickel cadmium, Nickel metal hydride or

Lithium batteries is used.

4. Intelligence

This is the most important part of the autonomous robots. Usually intelligence is achieved by using

Microcontroller.

First step in making an autonomous robot is to chalk out what tasks we are expecting the robot to

perform. After gauging these we get a vague idea about the design and appearance of the robot.

13.1 Robot Chassis Designing

Selecting the Drive Mechanism for the wheeled motion:

13.1.1 Robot with steering wheel:

Power for motion is provided by back wheels and turning is achieved using front wheels.

This scheme is similar to that of cars.


Advantages:

1. When path to be followed is straight in nature with curved turns this configuration gives fastest

speed and graceful path following.

2. Don’t need to modify left or right wheels velocity to follow the path. This is very advantageous

when we want precision velocity control. In this case back wheels take care of velocity control

and front wheels take care of direction control.

Disadvantages:

1. It will not able to take very sharp turns. Hence it is difficult to move robot on the grid of lines.

2. Somewhat difficult and expensive to make.

3. Front wheels will need position feedback to control turning control.

13.1.2 Robot with differential drive:

A method of controlling a robot where the left and right wheels are powered independently.

The Three Wheel Differential drive uses two motors and a caster or an omni-directional wheel

easiest to design and program.

Figure 23: Ball bearing caster, wheel based swivel caster and omni directional wheel

The radius and centre of rotation can be varied by the varying the relative speed of rotation

between the two motors.


Rotating the wheels in different directions provides a sharp turn.

For a smooth turn, rotate the wheels in the same direction but with different speeds. Greater the

difference in speeds, smaller the radius of rotation.

Advantages:

1. Zero turning radius achievable.

2. Easy to move when path to be followed is contoured and zigzag in nature. E.g., navigating along

the maze of lines.

Disadvantages:

1. If we want to move along curved path we have to control left and right motor’s velocity

independently. Hence precision velocity control becomes difficult as actual velocity of the robot

will be average of the both wheels.


S1 S2 S3 S4 Result 1 0 0 1 Motor rotates in one

direction

0 1 1 0 Motor rotates in opposite direction

0 0 0 0 Motor free runs (coasts)

0 1 0 1 Motor brakes 1 0 1 0 Motor brakes

Chapter 14 Motor Driver

14.1 H- Bridge:

It is an electronic circuit which enables a voltage to be applied across a load in either

direction.

It allows a circuit full control over a standard electric DC motor. That is, with an H-bridge, a

microcontroller, logic chip, or remote control can electronically command the motor to go

forward, reverse, brake, and coast.

H-bridges are available as integrated circuits, or can be built from discrete components.

A "double pole double throw" relay can generally achieve the same electrical functionality as

an H-bridge, but an H-bridge would be preferable where a smaller physical size is needed,

high speed switching, low driving voltage, or where the wearing out of mechanical parts is

undesirable.

The term "H-bridge" is derived from the typical graphical representation of such a circuit,

which is built with four switches, either solid-state (eg, L293/ L298) or mechanical (eg,

relays).

Structure of an H-bridge (highlighted in red)

To power the motor, you turn on two switches that are diagonally opposed.

The two basic states of an H-bridge.


14.2 Motor Driver ICs: L293/L293D and L298

Figure 25: L293D Figure 24: L298

The current provided by the MCU is of the order of 5mA and that required by a motor is

~500mA. Hence, motor can’t be controlled directly by MCU and we need an interface

between the MCU and the motor.

A Motor Driver IC like L293D or L298 is used for this purpose which has two H-bridge drivers.

Hence, each IC can drive two motors.

Note that a motor driver does not amplify the current; it only acts as a switch (An H bridge is

nothing but 4 switches).

Drivers are enabled in pairs, with drivers 1 and 2 being enabled by the Enable pin. When an

enable input is high (logic 1 or +5V), the associated drivers are enabled and their outputs are

active and in phase with their inputs.

When the enable pin is low, the output is neither high nor low (disconnected), irrespective of

the input.

Direction of the motor is controlled by asserting one of the inputs to motor to be high (logic

1) and the other to be low (logic 0).

To move the motor in opposite direction just interchange the logic applied to the two inputs of

the motors.


Asserting both inputs to logic high or logic low will stop the motor.

Resistance of our motors is about 26 ohms i.e. its short circuit current will be around.

0.46Amp which is below the maximum current limit.

It is always better to use high capacitance (~1000µF) in the output line of a motor driver

which acts as a small battery at times of current surges and hence improves battery life.

Difference between L293 and L293D: Output current per channel = 1A for L293 and 600mA

for L293D.

14.2.1 Difference between L293 and L298:

L293 is quadruple half-H driver while L298 is dual full-H driver, i.e, in L293 all four input-

output lines are independent while in L298, a half H driver cannot be used independently,

only full H driver has to be used.

Output current per channel = 1A for L293 and 2A for L298. Hence, heat sink is provided in

L298.

Protective Diodes against back EMF are provided internally in L293D but must be provided

externally in L298.

14.2.2 Speed Control:

To control motor speed we can use pulse width modulation (PWM), applied to the enable

pins of L293 driver.

PWM is the scheme in which the duty cycle of a square wave output from the

microcontroller is varied to provide a varying average DC output.

What actually happens by applying a PWM pulse is that the motor is switched ON and OFF at

a given frequency. In this way, the motor reacts to the time average of the power supply.

Figure 26: Velocity control of motor using PWM


Chapter 15 Sensors

15.1 Analog Sensor

The IR analog sensor consists of:

Transmitter: An Infra Red emitting diode

Receiver: A Phototransistor (also referred as photodiode)

It is better to keep R2 as a variac to vary the sensitivity.

The output varies from 0V to 5V depending upon the amount of IR it receives, hence the name

analog.

The output can be taken to a microcontroller either to its ADC (Analog to Digital Converter) or LM

339 can be used as a comparator.


15.2 Digital IR Sensor - TSOP Sensor

TSOP 1738 Sensor is a digital IR Sensor; It is logic 1 (+5V) when IR below a threshold is

falling on it and logic 0 (0V) when it receives IR above threshold.

It does not respond to any stray IR, it only responds to IR falling on it at a pulse rate of 38

KHz. Hence we have a major advantage of high immunity against ambient light.

No comparator is required and the range of the sensor can be varied by varying the intensity of

the IR emitting diode (the variac in figure).


Chapter 16 References

[1] Atmega 16 Datasheet

[2] www.wikipedia.org

[3] http://www.cmosexod.com/micro_uart.htm

[4] http://www.best-microcontroller-projects.com/hardware-interrupt.html

[5] http://www.avrtutor.com/tutorial/interrupt/interrupts.php


http://www.wikipedia.org/

http://www.cmosexod.com/micro_uart.htm

http://www.best-microcontroller-projects.com/hardware-interrupt.html

http://www.avrtutor.com/tutorial/interrupt/interrupts.php

Date post:	07-May-2015
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