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8051 C Programming

Chapter 8

The 8051 Microcontroller by

Scott Mc Kenzie

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The RS-232 Standard

Most widely used serial I/O interfacing standard Input and output voltage levels are not TTL

compatible

1 bit is represented by -3 to -25 V

0 bit is +3 to +25 V -3 to +3 is undefined

To connect RS232 to a microcontroller system must

use voltage converters such as MAX232 to convert

the TTL logic levels to the RS232 voltage levels, andvice versa

MAX232 IC chips are commonly referred to as line drivers

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The RS-232 DB-25 Connector

RS232 Connector DB-25

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The RS-232 DB-9 Connector

DB-9 9-Pin Connector

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The Line Driver- Max 232

Max-232 Converts from RS232 voltage levels to TTL voltage levels

Uses a +5 V power source

MAX232 has two sets of line drivers for transferring and

receiving data Line drivers used for TxD are called T1 and T2

Line drivers for RxD are designated as R1 and R2

T1 and R1 are used together for TxD and RxD of the

8051 Second set is left unused

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Inside Max-232

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Why C51? Offers the benefits of high-level, structured programming languages

such as C. Writing subroutines with ease

Sequence of operations is simple to trace, facilitating debugging

Finite number of structures with standardized terminology

Structures lend themselves easily to building subroutines.

The set of structures is complete that is all programs can be written using

three structures Statements

Assignment, call to a subroutine, etc

Loops The while/do statement, Repeat/Until statements

Choice If-then-else statement, the case statement, the goto statement

Structured programming results in increased programming productivity

Relieves the programmer of the hardware details

Easier to write especially for large and complex programs

Produces more readable program source codes.

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The downside of C51

Generally generates larger machine codes May execute more slowly

Require relatively large amount of memory

Programmer has less control and less ability to

directly interact with the hardware Some problems can be difficult to solve using only

the three structures discussed earlier

Nested structures can be difficult to follow

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C51 Data types

Data Type Bits Bytes Value Range

bit 1 - 0 to 1

signed char 8 1 -128 to +127

unsigned char 8 1 0 to 255

signed short 16 2 -32768 to +32767

unsigned short 16 2 0 to 65535

signed int 16 2 -32768 to +32767

unsigned int 16 2 0 to 65535

signed long 32 4 -2147483648 to +2147483647

unsigned long 32 4 0 to 4294967295float 32 4 +-1.175494E-38 to +-3.402823E+38

sbit 1 - 0 to 1

sfr and sfr16 8 & 16 1 & 2 0 to 255 & 0 to 65535

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C-51 Data types

Data type bitcan be used to declare variables that reside in the 8051s bit-addressable locations

bit flag = 0;

Declares a bitvariable called flag and initializes it to 0.

Data type sbit is similar to the bitdata type, except that it is normally used to

declare 1-bit variables that reside in SFRs, for example:sbit P = 0xD0;

Declares the sbitP and specifies that it refers to bit address D0H, which is

really the LSB of PSW SFR.

In case ofsbitdeclarations the assignment operator indicates what address

the sbit resides in, while in bitdeclarations it is used to specify the initial

value of the bitvariable. We can use a previously defined sfrvariable as

base address and assign oursbitvariable to refer to a certain bit within that

sfr.sfr PSW = 0xD0;

sbit P = PSW^0;

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REG51.H

REG51.H contains the declarations for: All the special function registers in 8051

All the individual flags, status and control bits in the bit

addressable SFRs

REG51.H is an important include in our C51programs.

It enables us to refer to the SFRs and all the flags and

control bit etc using their symbols i.e. PSW, ACC, B, SP,

TR0, TR1 etc.

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Memory TypesMemory Type Description (Size)

code Code memory (64 Kbytes). Objects

declared in this segment must be initialized

at compile time.

data Directly addressable internal data memory

(128 bytes)

idata Indirectly addressable internal datamemory (256 bytes)

bdata Bit-addressable internal data memory (16

bytes)

xdata External data memory (64 Kbytes)

pdata Paged external data memory (256 bytes)

MOVX A, @Ri, MOVX @Ri, A

char code errormsg[] = An error occurred;

signed int data num1;

bit bdata numbit;

unsigned int xdata num2;

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Accessing External Data and Code Memory

Space

13

The higher order address

byte comes from P2, the

lower order comes from Ri,so before using this

instruction you have to

initialize P2

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Memory Models

Memory Model DescriptionSmall Variables default to the internal data memory (data)

Compact Variables default to the first 256 bytes of external data

memory (pdata)

Large Variables default to external data memory (xdata)

If you wish all the variables to be assigned a default memory type without

having to specify them one by one then

Use memory models by choosing anyone from the above.

The default model is Small.Use the

#pragmadirective to explicitly select any other memory model

than the default Smallmodel.

#pragma small

#pragma compact

#pragma large

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C51 Functions

return_type function_name(arguments) [memory] [reentrant][interrupt] [using]

return_type: refers to data type of the return value.function_name: is any name that you wish to call thefunction as.

arguments: is the list of the type and number of inputvalues.

memory: refers to an explicit memory model (small,compact or large)

reentrant: refers to whether the function is reentrant(recursive)

interrupt: indicates that the function is actually an ISRusing: explicitly specifies which register bank to use

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Parameter Passing

Parameters to functions are passed through registersand memory

Passing through registers is the faster and default way

The registers used and their purpose are as follows.

Since there are only 8 registers in available, any extra

parameters may be passed using fixed memory locations.

Number ofArguments

Char/1-bytepointer

Int/2-bytepointer

Long/Float GenericPointer

1 R7 R6 & R7 R4-R7 R1-R3

2 R5 R4 & R5 R0-R3

3 R3 R2 & R3

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Returning Values

Unlike parameters output values must be returnedfrom functions via registers only.

Return Type Register Description

bit Carry Flag (C)

char/unsigned char/1-

byte pointer

R7

int/unsigned int/2-byte

pointer

R6 and R7 MSB in R6 and LSB in R7

long/unsigned long R4-R7 MSB in R4 and LSB in R7

float R4-R7 32-bit IEEE format

generic pointer R1-R3 Memory type in R3, MSB in R2,

LSB in R1