Introduction to Embedded C

OverviewOverview

C for microcontrollersC for microcontrollers Review of C basicsReview of C basics Compilation flow for SiLabs IDECompilation flow for SiLabs IDE C extensionsC extensions In-line assemblyIn-line assembly Interfacing with CInterfacing with C

ExamplesExamples Arrays and PointersArrays and Pointers I/O CircuitryI/O Circuitry Functions and Header FilesFunctions and Header Files Multitasking and multithreadingMultitasking and multithreading

C for MicrocontrollersC for Microcontrollers

Of higher level languages, C is the Of higher level languages, C is the closest to assembly languagesclosest to assembly languages bit manipulation instructionsbit manipulation instructions pointers (indirect addressing)pointers (indirect addressing)

Most microcontrollers have available C Most microcontrollers have available C compilers compilers

Writing in C simplifies code Writing in C simplifies code development for large projects.development for large projects.

Available C CompilersAvailable C Compilers

Keil C – integrated with the IDE we have Keil C – integrated with the IDE we have been using for labs.been using for labs.

Reads51 – available on web site (Reads51 – available on web site (http://www.rigelcorp.com/reads51.htmhttp://www.rigelcorp.com/reads51.htm))

Freeware: SDCC - Small Device C Freeware: SDCC - Small Device C Compiler (Compiler (http://sdcc.sourceforge.net/http://sdcc.sourceforge.net/))

Other freeware versions …Other freeware versions …

Compilation Process (Keil)Compilation Process (Keil)

program.c

program.OBJ

program.M51

compile

program.LST

build/make

no SRC option

Modular ProgrammingModular Programming

Like most high level languages, C is a Like most high level languages, C is a modular programming language (but modular programming language (but NOT an object oriented language)NOT an object oriented language)

Each task can be encapsulated as a Each task can be encapsulated as a function.function.

Entire program is encapsulated in Entire program is encapsulated in “main” function.“main” function.

Basic C Program StructureBasic C Program Structure

1.1. Compiler directives and include filesCompiler directives and include files

2.2. Declarations of global variables and constantsDeclarations of global variables and constants

3.3. Declaration of functionsDeclaration of functions

4.4. Main functionMain function

5.5. Sub-functionsSub-functions

6.6. Interrupt service routinesInterrupt service routines

Back to C BasicsBack to C Basics All C programs consists of:All C programs consists of:

VariablesVariables Functions (one must be “main”)Functions (one must be “main”)

StatementsStatements

To define the SFRs as To define the SFRs as variablesvariables::#include <reg51.h>#include <reg51.h>

VariablesVariables All variables must be declared at top of All variables must be declared at top of

program, before the first statement.program, before the first statement. Declaration includes Declaration includes type type and list of variables.and list of variables.

Example: void main (void) {Example: void main (void) { int var, tmp;int var, tmp;

Types:Types: intint (16-bits in our compiler) (16-bits in our compiler) charchar (8-bits) (8-bits) shortshort (16-bits) (16-bits) longlong (32-bits) (32-bits) sbitsbit (1-bit) (1-bit) others that we will discuss laterothers that we will discuss later

not standard C – an 8051 extension

must go HERE!

VariablesVariables The following variable types can be The following variable types can be

signed or unsigned:signed or unsigned:signed char (8 bits) –128 to +127signed short (16 bits) –32768 to +32767signed int (16 bits) –32768 to +32767signed long (32 bits) –2147483648 to +2147483648

unsigned char (8 bits) 0 to + 255unsigned short (16 bits) 0 to + 65535unsigned int (16 bits) 0 to + 65535unsigned long (32 bits) 0 to + 4294967295

NOTE: Default is signed – it is best to specify.

StatementsStatements Assignment statement:Assignment statement:

variable = variable = constantconstant or or expressionexpression or or variablevariable

examples: examples: upper = 60upper = 60;;

I = I + 5;I = I + 5;

J = I;J = I;

OperatorsOperators

Arithmetic: +, -, *, /Arithmetic: +, -, *, / Relational comparisons: >, >=, <, <=Relational comparisons: >, >=, <, <= Equality comparisons: ==, !=Equality comparisons: ==, != Logical operators: && (and), || (or)Logical operators: && (and), || (or) Increment and decrement: ++, --Increment and decrement: ++, -- Example:Example:

if (x != y) && (c == b)if (x != y) && (c == b){ {

a=c + d*b;a=c + d*b;a++;a++;

}}

Example – Adder program Example – Adder program (add 2 16-bit numbers)(add 2 16-bit numbers)

$INCLUDE (C8051F020.inc) $INCLUDE (C8051F020.inc) XL equ 0x78 XL equ 0x78 XH equ 0x79 XH equ 0x79 YL equ 0x7A YL equ 0x7A YH equ 0x7BYH equ 0x7B

cseg at 0 cseg at 0 ljmp Main ljmp Main

cseg at 100h cseg at 100h ; Disable watchdog timer; Disable watchdog timerMain: Main: mov 0xFF, #0DEhmov 0xFF, #0DEh mov 0xFF, #0ADhmov 0xFF, #0ADh

mov a, XLmov a, XLadd a, YLadd a, YLmov XL, amov XL, a mov mov

a, XHa, XHaddc a, YHaddc a, YHmov XH, amov XH, anopnop

endend

#include <reg51.h> #include <reg51.h>

void main (void)void main (void)

{{

int x, y, z; //16-bit variablesint x, y, z; //16-bit variables

// disable watchdog timer// disable watchdog timer

WDTCN = 0xde;WDTCN = 0xde;

WDTCN = 0xad;WDTCN = 0xad;

z = x + y;z = x + y;

}}

The C version

The assembly version

Compilation Process (Keil)Compilation Process (Keil)

adder.c

adder.OBJ

adder.M51

compile

adder.SRC

build/make

Use the #pragma CODE compiler directive to get assembly code generated in SRC file.

Map file shows where variables are stored. One map file is generated per project.

Symbol Table in M51 file:------ DO D:0008H SYMBOL x D:000AH SYMBOL y D:000CH SYMBOL z ------- ENDDO

look here in RAMwhen debugging

assemble

adder.SRCadder.SRC x?040: DS 2 y?041: DS 2 z?042: DS 2main:

; SOURCE LINE # 12; int x, y, z;; WDTCN = 0xde; // disable watchdog timer

; SOURCE LINE # 14MOV WDTCN,#0DEH

; WDTCN = 0xad;; SOURCE LINE # 15

MOV WDTCN,#0ADH ; z = x + y;

; SOURCE LINE # 17MOV A,x?040+01HADD A,y?041+01HMOV z?042+01H,AMOV A,x?040ADDC A,y?041MOV z?042,A

; } ; SOURCE LINE # 18RET

; END OF mainEND

Bitwise Logic InstructionsBitwise Logic Instructions

ANDAND OROR XORXOR left shiftleft shift right shiftright shift 1’s 1’s

complementcomplement

&|^

<<>>~

n = n & 0xF0;

n = n & (0xFF << 4)

n = n & ~(0xFF >> 4)

Examples:

Example – Logic in Assembly Example – Logic in Assembly and Cand C

Main: Main:

mov WDTCN, #0DEh mov WDTCN, #0DEh

mov WDTCN, #0ADh mov WDTCN, #0ADh

xrl a, #0xF0 ; invert bits 7-4 xrl a, #0xF0 ; invert bits 7-4

orl a, #0x0C ; set bits 3-2 orl a, #0x0C ; set bits 3-2

anl a, #0xFC ; reset bits 1-0anl a, #0xFC ; reset bits 1-0

mov P0, amov P0, a ; send to port0 ; send to port0

void main (void) {void main (void) {

char x;char x;

WDTCN = 0xDE;WDTCN = 0xDE;

WDTCN = 0xAD;WDTCN = 0xAD;

x = x ^ 0xF0;x = x ^ 0xF0;

x = x | 0x0C;x = x | 0x0C;

x = x & 0xFC;x = x & 0xFC;

P0 = x;P0 = x;

}}

Loop Statements - WhileLoop Statements - While

While loop:While loop:

while (condition) { statements }while (condition) { statements }

while condition is true, execute statementswhile condition is true, execute statements

if there is only one statement, we can lose the if there is only one statement, we can lose the {}{}

Example: Example: while (1) ;while (1) ; // loop forever// loop forever

Loop Statements - ForLoop Statements - For For statement:For statement:

for (initialization; condition; increment) {statements}for (initialization; condition; increment) {statements}

initializationinitialization done before statement is executed done before statement is executed

conditioncondition is tested, if true, execute statements is tested, if true, execute statementsdo do incrementincrement step and go back and test condition again step and go back and test condition again

repeat last two steps until condition is not truerepeat last two steps until condition is not true

Example: for loopExample: for loop

for (n = 0; n<1000; n++)for (n = 0; n<1000; n++)

n++ means n = n + 1n++ means n = n + 1

Be careful with signed integers!Be careful with signed integers!

for (i=0; i < 33000; i++) LED = ~LED;for (i=0; i < 33000; i++) LED = ~LED;

Why is this an infinite loop?Why is this an infinite loop?

Loops: do - whileLoops: do - while

dodo

statementsstatements

while (expression);while (expression);

Test made at the bottom of the loopTest made at the bottom of the loop

Decision – if statementDecision – if statement

if (condition1)if (condition1)

{statements1}{statements1}

else if (condition2)else if (condition2)

{statements2}{statements2}

……

elseelse

{statementsn}{statementsn}

Decision – switch statementDecision – switch statement

switch (expression) {switch (expression) {

case const-expr: statementscase const-expr: statements

case const-expr: statementscase const-expr: statements

default: statementsdefault: statements

}}

Example: switch Example: switch

switch (unibble) {switch (unibble) {

case 0x00 : return (0xC0); case 0x00 : return (0xC0);

case 0x01 : return (0xF9); case 0x01 : return (0xF9);

case 0x02 : return (0xA4); case 0x02 : return (0xA4);

case 0x03 : return (0xC0); case 0x03 : return (0xC0);

default : return (0xFF);default : return (0xFF);

}}

Need a statement like “return” or “break” or execution falls through to the next case (unlike VHDL)

C Extensions: Additional C Extensions: Additional KeywordsKeywords

Specify where variables goin memory

For accessing SFRs

Accessing Specific MemoryAccessing Specific Memory

C Access to 8051 MemoryC Access to 8051 Memorycode: program memory accessed by movc @a + dptr data

idata

bdata

xdata

C Extensions for 8051 C Extensions for 8051 (Cygnal)(Cygnal)

New data types:New data types:

Example:Example:

bitbit bit new_flag; //stored in 20-2Fbit new_flag; //stored in 20-2F

sbitsbit sbit LED = P1^6;sbit LED = P1^6;

sfrsfr sfr SP = 0x81;sfr SP = 0x81; //stack pointer//stack pointer

sfr16 sfr16 sfr16 DP = 0x82; // data pointersfr16 DP = 0x82; // data pointer

$INCLUDE ($INCLUDE (c8051F020.hc8051F020.h) )

C Data Types With C Data Types With ExtensionsExtensions

Declaring Variables in Declaring Variables in MemoryMemory

char data temp;char idata varx;int xdata array[100];char code text[] = “Enter data”;

Example: Accessing External Example: Accessing External MemoryMemory

Program defines two 256 element Program defines two 256 element arrays in external memoryarrays in external memory

First array is filled with values that First array is filled with values that increase by 2 each location.increase by 2 each location.

First array is copied to second array.First array is copied to second array. Similar to block move exercise done Similar to block move exercise done

in assembly.in assembly.

Interrupts – Original 8051Interrupts – Original 8051

void timer0 (void) interrupt 1 {

if (++interruptcnt == 4000) /* count to 4000 */{

second++; /* second counter */interruptcnt = 0; /* clear int counter */}

}

Specify register bank 2

In-line AssemblyIn-line Assembly

When it is more efficient, or easier, When it is more efficient, or easier, can insert assembly code in C can insert assembly code in C programs.programs.

#pragma asm#pragma asm

put your assembly code hereput your assembly code here

#pragma endasm#pragma endasm

Compilation Process (Keil)Compilation Process (Keil)program.c

program.OBJ

program.M51

compile

program.LST

build/make

program.SRC

.OBJ or .SRC canbe generated, not both

program.OBJ

rename file

program.asm

assemblebuild/make

no SRC option

with SRC option

Must use this path for C programs with in-line assemblyIt is also necessary to add #pragma SRC to code

Case study :: Temperature Case study :: Temperature DisplayDisplay

Temperature Sensor programTemperature Sensor program Configures the external oscillatorConfigures the external oscillator Configures the ADC0 for temp. sensorConfigures the ADC0 for temp. sensor Configures Port1 so LED can be usedConfigures Port1 so LED can be used Configures Timer3 to synch the ADC0Configures Timer3 to synch the ADC0 Uses ADC0 ISR to take temperature samples Uses ADC0 ISR to take temperature samples

and averages 256 of them and posts average and averages 256 of them and posts average to global variableto global variable

Main program compares average temp. to Main program compares average temp. to room temp. and lights LED if temp is warmer.room temp. and lights LED if temp is warmer.

THANK YOUTHANK YOU