How to combine C program with external ASM

documentation may be as follows:

• Code for AVR's that have no RAM;

• Very timing critical applications;

• Special tweaks that cannot be done in C.

As I mentioned all this can be also done using in-line assembler feature. For somebody in-line assembler may look too complicated because of special syntax requirements. Before starting to write assembler program first read (What Registers are used by the C compiler) to ensure you are using right registers in your assembler program and won't mess up the normal program flow by occupying registers that are storing values from other routines. In most cases push and pop may do the job.

OK lets have simple example to clear things out. Dont pay attention to fact that I am writing program which can be easily written in C. This is only for demonstration purposes how to relate C and ASM. Project consists of 3 parts:

• mainC.c – C program;

• extasm.S – ASM program;

• Makefile – makefileneeded to make a project.

Note, that assembler file must have capital extension .S, otherwise program wont compile and maybe cleared during make clear command.

Lets write program:

<<<<<<<<<<<<,mainC.c>>>>>>>>>>>>>>>

#include <avr/io.h>

volatile uint8_t pinbits; //define poer pins

extern void InitPort(void);//init port for output

extern void sendpinbits(void);//send to port

int main()

{

	pinbits=1;//set pin 1 as output

	InitPort();//call subroutine from assembler file

	while(1)//repeat for ever

	{

	pinbits=1;//Pin High

	sendpinbits();//call subroutine from assembler file

	pinbits=0;//Pin Low

	sendpinbits();//call subroutine from assembler file

	}

}

Then assembler program:

<<<<<<<<<<<<extasm.S>>>>>>>>>>>

#include <avr/io.h>				;required for register definitions

.extern pinbits					;external variable

.global InitPort				;make accesible globally

InitPort:					;function name  

	push r18				;save register value

	lds r18, pinbits			;load variable to r18

	out _SFR_IO_ADDR(DDRD), r18	;set pins as output

	pop r18				;restore register

	ret					;return from subroutine

.global sendpinbits				;make global

sendpinbits:					;function name

	push r18				;save register value

	lds r18, pinbits			;load variable to r18

	out _SFR_IO_ADDR(PORTD), r18	;pins to High

	pop r18				;restore register

	ret					;return from subroutine

Makefile is generated with Mfile which comes with WinAVR. You only have to change these parts:

<<<<<<Makefile>>>>>>>>

# MCU name

MCU = atmega8

# Processor frequency.

F_CPU = 8000000

# Output format. (can be srec, ihex, binary)

FORMAT = ihex

# Target file name (without extension).

TARGET = mainC

# List Assembler source files here.

ASRC = extasm.S

Other parts of Makefile leave as they are. Then call command make all (Programmers Notepad->Tools->Make All) and program should compile without errors. I have tested this example with AVRStudio debugger and it works properly.

Good luck.