AVR Freaks

As you know your avr-gcc programs all begin:

#include <avr/io.h>

now put the cursor on that line and Alt-G (go to definition) which will open the file in your editor.

Do you see how it works? It uses a symbol such as __AVR_ATmega16__ to choose to include iom16.h or whatever from amongst about 300 other device specific .h files. That __AVR_ATmega16__ symbol in turn is defined when you pass -mmcu=atmega16 to the compiler. And in your project you will ave selected to build for ATmega16 and that is what lead to that -mmcu value being passed.

My example here uses mega16. Perhaps you build for something else? If so you can trace the path to the point at which its own device specific header is included.

In such headers it then defines the names of all the SFRs and all the bit names in those SFRs

The point I was trying to make about io.h to you is that it is simply a lot of this:

#if defined (__AVR_AT94K__)
#  include <avr/ioat94k.h>
#elif defined (__AVR_AT43USB320__)
#  include <avr/io43u32x.h>
#elif defined (__AVR_AT43USB355__)
#  include <avr/io43u35x.h>
#elif defined (__AVR_AT76C711__)
#  include <avr/io76c711.h>
#elif defined (__AVR_AT86RF401__)
#  include <avr/io86r401.h>
#elif defined (__AVR_AT90PWM1__)
#  include <avr/io90pwm1.h>
#elif defined (__AVR_AT90PWM2__)
#  include <avr/io90pwmx.h>
#elif defined (__AVR_AT90PWM2B__)
#  include <avr/io90pwm2b.h>
#elif defined (__AVR_AT90PWM3__)
#  include <avr/io90pwmx.h>
#elif defined (__AVR_AT90PWM3B__)
#  include <avr/io90pwm3b.h>
#elif defined (__AVR_AT90PWM216__)
#  include <avr/io90pwm216.h>
<and so on for about 300 different models of AVR...>

In other words you write:

#include <avr/io.h>

int main(void) {
    DDRB = 0xFF;
    while(1) {
        PORTB ^= 0xFF;
    }
}

and if you then compile this with:

avr-gcc -mmcu=atmega8 -Os main.c -o main.elf

then that long list in io.h will include the header file for atmega8 because the symbol __AVR_ATmega8__ will be defined by the use of -mmcu=atmega8. Whereas if you use:

avr-gcc -mmcu=atmega328p -Os main.c -o main.elf

then the header for mega328 will be used because __AVR_ATmega328P__.

In each case the addresses of PORTB and DDRB will then be supplied by the header file. So the same code can be built for two (or many more) AVR using the symbols DDRB and PORTB and because of -mmcu the correct values for the address of those locations will be defined by the appropriate header file.