Basic Microcontroller in C Programming.pdf

C8051F020 C Programming

6.0

Introduction

6.1

Code

Basic C program Structure

6.2

Programming Memory Models

Overriding the default memory model, Bit-valued data,

Special Function Registers, Locating Variables at absolute

addresses

6.3

C Language Control Structures

Relational Operators, Logical Operators, Bitwise Logical

Operators, Compound Operators, Making Choices (if..else,

switch .. case), Repetition(for loop, while loop), Waiting for

Events, Early Exits

6.4

Functions

Standard functions - Initializing System Clock, Memory Model

Used for a Function

6.5

Interrupt Functions

Timer 3 Interrupt Service Routine, Disabling Interrupts before

Initialization, Timer 3 Interrupt Initialization, Register Banks

6.6

Reentrant functions

6.7

Pointers

A Generic Pointer in Keil TM C, Memory Specific Pointers

6.8

Summary of Data Types

6.9

Tutorial Questions

Chapter 6 C8051F020 C Programming

6.0 Introduction

This chapter introduces the Keil TM C compiler for the Cygnal C8051F020

board. We assume some familiarity with the C programming language to

the level covered by most first courses in the C language.

Experienced C programmers who have little experience with the

C8051F020 architecture should become familiar with the system. The

differences in programming the C8051F020 in C compared to a standard

C program are almost all related to architectural issues. These

explanations will have little meaning to those without an understanding of

the C8051F020 chip.

The Keil TM C compiler provided with the Cygnal C8051F020 board does

not come with a floating point library and so the floating point variables

and functions should not be used. However if you require floating point

variables a full license for the Keil TM C compiler can be purchased.

6.1 Register Definitions, Initialization and Startup

Code

C is a high level programming language that is portable across many

hardware architectures. This means that architecture specific features

such as register definitions, initialization and start up code must be made

available to your program via the use of libraries and include files.

For the 8051 chip you need to include the file reg51.h or using the

Cygnal C8051F020-TB development board include the file c8051f020.h :

#include <reg51.h>

#include < c8051f020.h >

These files contain all the definitions of the C8051F020 registers. The

standard initialization and startup procedures for the C8051F020 are

contained in startup.a51 . This file is included in your project and will be

assembled together with the compiled output of your C program. For

custom applications, this start up file might need modification.

Chapter 6 C8051F020 C Programming

Basic C program structure

The following is the basic C program structure; all the programs you will

write will have this basic structure.

//-------------------------------------------------

// Basic blank C program that does nothing

// other than disable the watch dog timer

//-------------------------------------------------

// Includes

//-------------------------------------------------

#include <c8051f020.h> // SFR declarations

void main (void)

{

// disable watchdog timer

WDTCN = 0xde;

WDTCN = 0xad;

while(1); // Stops program terminating and

// restarting

}

//-------------------------------------------------

Note: All variables must be declared at the start of a code block, you

cannot declare variables among the program statements.

You can test this program in the Cygnal IDE connected to the

C8051F020 development board. You won’t see anything happening on

the board, but you can step through the program using the debugger.

6.2

Programming Memory Models

The C8051F020 processor has 126 Bytes of directly addressable internal

memory and up to 64 Kbytes of externally addressable space. The Keil TM

C compiler has two main C programming memory models, SMALL and

LARGE which are related to these two types of memory. In the SMALL

memory model the default storage location is the 126 Bytes of internal

memory while in the LARGE memory model the default storage location

is the externally addressed memory.

Chapter 6 C8051F020 C Programming

The default memory model required is selected using the pragma

compiler control directive :

#pragma small

int X;

Any variable declared in this file (such as the variable X above) will be

stored in the internal memory of the C8051F020.

The choice of which memory model to use depends on the program, the

anticipated stack size and the size of data. If the stack and the data

cannot fit in the 128 Bytes of internal memory then the default memory

model should be LARGE, otherwise SMALL should be used.

Yet another memory model is the COMPACT memory model. This

memory model is not discussed in this chapter. More information on the

compact model can be found in the document Cx51 Compiler User’s

Guide for Keil TM Software .

You can test the different memory models with the Cygnal IDE

connected to the C8051F020-TB development board. Look at the symbol

view after downloading your program and see in which memory

addresses the compiler has stored your variables.

Overriding the default memory model

The default memory model can be overridden with the use of Keil TM C

programming language extensions that tell the compiler to place the

variables in another location. The two main available language

extensions are data and xdata :

int data X;

char data Initial;

int xdata Y;

char data SInitial;

The integer variable X and character variable Initial are stored in the

internal memory while the integer variable Y and character variable

SInitial are stored in the external memory overriding any default memory

model.

Chapter 6 C8051F020 C Programming

Constant variables can be stored in the read-only code section of the

C8051F020 using the code language extension:

const char code CR=0xDE;

In general, access to the internal memory is the fastest, so frequently

used data should be stored here while less frequently used data should

be stored on the external memory.

The memory storage related language extensions, bdata, and

associated data types bit , sbit , sfr and sfr16 will be discussed in the

following sections. Additional memory storage language extensions

including, pdata and idata, are not discussed in this chapter; refer to the

document Cx51 Compiler User’s Guide for Keil TM Software for

information on this.

Bit-valued Data

Bit-valued data and bit-addressable data must be stored in the bit-

addressable memory space on the C8051F020 (0x20 to 0x2F). This

means that bit- valued data and bit-addressable data must be labelled as

such using the bit , sbit and bdata .

Bit-addressable data must be identified with the bdata language

extension:

int bdata X;

The integer variable X declared above is bit-addressable.

Any bit valued data must be given the bit data type, this is not a standard

C data type:

bit flag;

The bit-valued data flag is declared as above.

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