L5- 7-Segment Display & ADC Control(1)

EEE3410 Microcontroller Applications

Laboratory Experiment 5 1 Vocational Training Council, Hong Kong

LABORATORY Experiment 5

7-segment Display & ADC Controls with 8051

Name Date

Class Class No.

Marks

7-segment Display & ADC Controls

Objectives

To understand the connection of 7-segment display to 8051 To know how to write 8051 assembly program to control 7-segment display To understand the connection of an ADC to 8051 To know how to write 8051 assembly program to control an ADC to acquire data

Equipment/Components Required

PC compatible computer with Windows XP MedWin 8051 simulation software. ADC & 7-segment Display training board with power supply and connection cable HT1000 89C51 Writer floppy disk/USB flash disk

Ver Author(s) Date Remark 1.0 KK SIN 9/2006

2 Experiment 5 Laboratory Vocational Training Council, Hong Kong

Part A: Background Information

A1. The 7-segment LED numeric display

A 7-segment LED numeric display contains 8 LEDs, of which 7 segments are arranged in the shape of the number 8 to display the digits 0-9, and the eighth one is the decimal point (DP). (Ignore the decimal point in this Laboratory.) Basically, it has one common lead (either anode or cathode) which is connected to the power supply, and the remaining 7 individual leads are for each segment.

Segment displays are driven by connecting each segment to a port bit, or they can be driven by decoder/driver IC designed for the purpose. A decoder/driver chip will accept a parallel input from a 8051 port (binary or ASCII) and drive the display to show the corresponding character. The table shows the corresponding connection of the 8 output port bits to the 8 input terminals of display segments.

Output Port bit 7 6 5 4 3 2 1 0 Display segment Dp g f e d c b a

There are two common types of configurations; namely, the common-anode configuration and the common-cathode configuration.

Common-anode configuration Segment will light up if the output port bit is set LOW. Common-cathode configuration - Segment will light up if the output port bit is set HIGH.

Normally a resistor of 330 is connected between each segment terminal and output port bit in order to protect the LED segment.

In this laboratory, you will appreciate how to program the 8051 to control 7-segment LED numeric displays.

a

b

c

d

e

f g

Segment Pattern

Dp

h

a b c d e f g Dp Common

Common-cathode configuration

a b c d e f g Dp Common

Common-anode configuration



A2. Analog-to-digital conversion (A/D)

Most real time data (e.g. temperature, voltage, velocity) are analog signal, i.e. the signal is continuous varying with time. Microcontroller is a digital device. It can only read and manipulate data in digital form. If this kind of real time analog data is being controlled by microcontroller, some means must be used first to convert the analog data to digital data before manipulating by microcontroller. The common process to get these analog data is to use a device called a transducer (also called sensor) to measure their values. The transducer normally converts measured value into electrical signal (which is either voltage or current) for further conversion to digital form. An example of transducer is a thermistor. Since its resistance varies with the ambient temperature, so the current passing through it will also vary. As a result, current value depends on the temperature being measured. Measure the current will then be the same as to measure the temperature. Yet, the output from a transducer (voltage/current) is still analog; a device is needed to convert them in digital signals (digitization). Such a device is called an analog-to-digital converter (ADC), and this process is called analog-to-digital (A/D) conversion. Once the signal is digitized, it can be sent to the microcontroller for further processing.

A2.1 The ADC0804 IC

The ADC0804 IC is one of the most popular analog-to-digital converters. It is in the family of the ADC0800 series. It works with a power supply of +5V, and has a resolution (i.e. output) of 8 bits.

Pin description of the ADC0804:

Chip select ( CS ) Pin CS is used to activate the ADC0804 IC. The IC is activated and ready to use if a LOW signal is set to this pin.

Read ( RD ) (Data output enable) Pin RD is also called output enable. It is an active-low output signal. It is used to get the converted data out of the ADC0804 IC. With CS =0, if a high-to-low pulse is applied to RD , the 8-bit digital is then output to the data pins D0-D7 for pick-up. Pin diagram ADC0804

D0 D1 D2 D3 D4 D5 D6 D7

VI+

VREF/2

VI-

CLK INTR

WR RD

CS CLKR

AGND

VCC

DGND

20 19 18 17 16 15 14 13 12 11

1 2 3 4 5 6 7 8 9 10


Write ( WR ) (Start conversion) WR is also called start conversion. It is an active-low input signal. It is used to start the ADC0804 IC for data conversion. With CS =0, if a low-to-high pulse is applied to pin WR , the ADC0804 IC will start converting the analog input value at pin VI+ to an 8-bit digital number. Once data conversion is complete, pin INTR will be set to LOW.

Interrupt ( INTR ) (End of conversion) INTR is also called end of conversion. It is an active-low output signal. It is usually high but once data conversion is complete, it set to LOW to indicate that the converted data is ready for pick-up.

VCC

VCC is the +5V power supply to the ADC0804 IC. It can be also used as a reference voltage (VREF) if the pin VREF/2 is open.

VREF/2

VREF/2 is an input used for reference voltage other than 0-5V range. But if VREF/2 is left open, the analog input voltage will be in the range of 0-5V.

Data pins D0-D7

DB0-DB7 are the digital data output pins, with D7 as the most significant bit (MSB) and D0 as the least significant bit (LSB). They can only be accessed when both CS =0 and RD is LOW.

A2.2 Digital output of the ADC804 IC

In this Laboratory, the VREF/2 pin is left open, i.e. the analog input voltage (VIN) is in the range 0-5V. Because the ADC0804 IC has a resolution of 8 bits, the range is divided into 28=256 steps. Or, we can say that there are 255 quantization levels (0V inclusive).

The step size, or the resolution of the ADC804 IC, is:

255V

122

V2sizestep REF8

REF

=

=

The digital data value (DOUT) of the ADC804 IC is:

sizestepVD INOUT =



e.g. : A 5V is set to the VREF pin of a ADC0804, find the digital value output from the ADC if it converts an analog input voltage of 3.5V.

Solution: V0196.0V2555

sizestep == ;

179or1780196.0

5.3DOUT == (depends on the accuracy of the ADC)

A2.3. Control of ADC0804 IC to carry out analog-to-digital conversion

Normally ADC0804 IC is interfaced with microcontroller to form a data acquisition system, circuit shown in fig. A2.1. The microcontroller will send out a fixed sequence of control signals to direct the ADC to complete the A/D conversion process and output the converted value.

Fig. A2.2 shows a flowchart gives the steps for signal time issued by a 8051 to control the ADC0804IC

Start

Set CS to LOW

Does INTR change to

LOW?

Set CS to LOW Set RD to LOW

Read the ready ADC Data by a 8051 input

port

N

Y

Set WR to LOW Next return WR to HIGH Then return CS to HIGH

Next return RD to HIGH Then return CS to HIGH

Repea

Fig. A2.2 Flowchart of timing control of ADC0804

DB0 DB1 DB2 DB3 DB4 DB5 DB6 DB7

VI+

VREF

VI-

CLK

INTR WR RD CS

CLKR

AGND

ADC0804

VCC

8051 Microcontroller

Input bits

Output Control bits

Input analog voltage

Fig. A2.1 Interface of ADC0804 to 8051 microcontroller


(Fig. A2.3 The read and write signal timing diagram for ADC804)

Sequence of signal to control the ADC0804

1. Set CS to LOW. (Select the ADC IC) 2. Send a low-to-high pulse to WR , so as to start data conversion. Return CS to

High again to de-select the ADC 3. Keep monitoring INTR . If INTR is LOW, that means the conversion is finished.

If INTR is still HIGH, keep polling until it becomes LOW. 4. Set CS to LOW to select the ADC 5. Send a high-to-low pulse to RD so that data read by 8051 input port can be done 6. Return RD to High again to indicate port reading completed 7. Return CS to High again to de-select the ADC 8. Repeat steps for another conversion

Read it

End conversion Start conversion

Data out

CS

WR

D0 D7

INTR

Note: CS is set to low for both RD and WR pulses



A3. ADC & 7-segment Display training board

Figure A3.1 shows the circuit diagram of the ADC & 7-segment Display training board. Port 2, Port bit 3.6 and Port bit 3.2 are connected to the 8-bit output, pin WR and pin

INTR of ADC0804 respectively. Port 0 is connected to a common-anode 7-segment display. 2-digit 7-segment numeric display is connected to the upper & lower nibbles of Port 1

via a two BCD to 7-segment decoders. Port bits 3.0, 3.1, 3.3 & 3.4 0 are connected to 4 push buttons (S1 to S4) respectively.


Part B: Procedures

B1. Controlling a 7-segment numeric display

A source program shown in table B1.1 controls the 7-segment display at Port 0 to show the digits 0-F in counting up manner.

B1.1 Complete the lines 29 31 in table B1.1. B2.2 Use the 8051 simulation software to check the correct execution of the program. B2.3 Use the HT1000 89C51 writer to write the HEX file of the program into a blank 89C51

chip. B2.4 Insert the 89C51 chip to the ADC & 7-segment Display training board and observe the

result of execution.

Table B1.1 Line Label Mnemonics Operands Remarks 1

ORG 00h 2

MOV R3, #0 ;reset the counter 3

MOV DPTR, #TABLE 4

MOV P0, #01000000b ;display 0 5

;subroutine to count from 0-9 6

COUNT_UP: INC R3 ;increment the counter 7

CJNE R3, #16, CONT ;if R310, continue counting 8

MOV R3, #0 ;otherwise count again from 0 9

CONT: MOV A, R3 10

MOVC A, @A+DPTR ;display the value of R3 in 11

MOV P0, A ;7-segment format 12

ACALL DELAY 13

AJMP COUNT_UP 14

15

DELAY: MOV R5, #5 ;sets the delay time 16

LOOP1: MOV R6, #250 17

LOOP2: MOV R7, #200 18

LOOP3: DJNZ R7, LOOP3 ;inner loop 19

DJNZ R6, LOOP2 ;middle loop 20

DJNZ R5, LOOP1 ;outer loop 21

RET ;return to main program 22

23

;lookup table to convert from hex to a corresponding digit 24

TABLE: DB 01000000b, 01111001b ;0, 1 25

DB 00100100b, 00110000b ;2 ,3 26

DB 00011001b, 00010010b ;4, 5 27

DB 00000010b, 01111000b ;6, 7 28

DB 00000000b, 00010000b ;8, 9 29

DB _________b,_________b ;A, b 30

DB _________b,_________b ;C, d 31

DB _________b,_________b ;E, F 32

END



B2. Controlling 7-segment numeric displays through BCD decoder/driver

A source program shown in table B2.1 is expected to control the 2-digit 7-segment display at Port 1 to show the values from 00 to 99 in counting up manner. But it cannot correctly display the values ONE after ONE in sequence. You are required to correct the program in table B2.1 to give the right result. B2.1 By using the 8051 simulation software, correct the program as required. B2.2 Write the HEX file of the program into a blank 89C51 chip. B2.3 Check the program by the ADC & 7-segment Display training board.

Table B2.1 Line Label Mnemonics Operands Remarks 1

ORG 00h 2

MOV A, #0 ;reset the counter 3

4

COUNT_UP: MOV P1, A ;display the value of A 5

ACALL DELAY 6

INC A ; A = A + 1 7

CJNE A, #100, CONT ;if A100, continue counting 8

MOV A, #0 ;otherwise count again from 0 9

CONT: AJMP COUNT_UP 10

; 11


LOOP1: MOV R6, #250 13

LOOP2: MOV R7, #200 14





END 19

20

21

B3. Controlling ADC

A source program is given in table B3.1 which controls an 8-bit ADC0804 IC repetitively to acquire analog voltage signals from a potentiometer, and convert the signals to 8-bit digital values and finally to output the values to the Port 2 of 8051. After the 8-bit digital value is inputted, the 8051 has to convert the 8-bit value to an equivalent decimal value and display it by the 2-digital 7-segment display. When you turn the potentiometer, the analog voltage input to the ADC will change and similarly, the digital values displayed will also change. Problem: it is found that the analog voltage input, measure by a DMM, to the ADC cannot be correctly displayed by the 2-digital 7-segment display.


B2.1 By using the 8051 simulation software, correct the program as required. B2.2 Write the HEX file of the program into a blank 89C51 chip. B2.3 Check the program by the ADC & 7-segment Display training board.

Table B3.1

Line Label Mnemonics Operands Remarks 1

ORG 00h 2

MOV P1, #0 ;Display 00 3

MOV P2, #0FFh ;set port 2 as input port 4

MOV P3, #0FFh ;set port 3 output to High 5

;

6

STARTCON:

CLR P3.6 ;Set port bit 3.6 to Low,

7

NOP ;start ADC conversion 8

SETB P3.6 ;reset port bit 3.6 to High 9

NOP 10

CHK_END: JB P3.2, CHK_END ;check for end of conversion 11

NOP 12

READ: MOV A, P2 ;input from ADC 13

; 14

MOV P1, A ;output to port 1 15

ACALL DELAY 16

AJMP STARTCON 17

18

19

20

21

22

; 23


LOOP1: MOV R6, #250 25

LOOP2: MOV R7, #200 26





END

Note: As the pins of CS and RD are directly connected to the ground in the training board, they are always at LOW input. That means the ADC is always selected and ready for output. So the source program in the above table has not necessary to include any instructions for chip selection and read action of the ADC.



B2.4 After the program is corrected and executed by a 89C51 chip, complete the table below by first turning the potentiometer knob at the end of counter clockwise direction, record the voltage input to the ADC by a DMM and the value displayed by the 2-digit 7-segment display. Then slowly turn the knob in clockwise direction until at the end position. Record down the values displayed at any four intermediate positions. Compare with the voltage values measured by DMM.

Position of knob Value indicated by the

2-digital 7-segment display

Analog voltage inputted to the ADC

(Volts) At Full counter clockwise 0 0 0 V At Intermediate position 1

At Intermediate position 2



At Full clockwise


Part C: Exercise

C1. Assume that pin Vref/2 is connected to a reference voltage of 2V. Determine the following.

(a) Step Size:

(b) Maximum range for Vin

(c) D7 D0 value if Vin = 1.2 V

(d) Vin if D7 D0 = 1111 1111

(e) Vin if D7 D0 = 1101 1011

C2. What is the value for Vref/2 to set the ADC0804 has a step size of 8 mV.

- End -

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L5- 7-Segment Display & ADC Control(1)

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