Setting Up avr-gcc, avrdude and USBasp for Bare-Metal AVR Programming

The First Program

Create a folder named blink and save the following as main.c. F_CPU tells the delay functions the clock speed; it must match the actual clock, which is 1 MHz on a new chip with factory fuses.

/* F_CPU must be defined before util/delay.h is included.
   A new ATmega328P runs at 1 MHz (8 MHz RC divided by 8). */
#ifndef F_CPU
#define F_CPU 1000000UL
#endif

#include <avr/io.h>
#include <util/delay.h>

int main(void)
{
    DDRB |= (1 << DDB5);            /* PB5 as output */

    while (1) {
        PORTB ^= (1 << PORTB5);     /* toggle the LED */
        _delay_ms(500);             /* wait half a second */
    }
}

Compile, Convert and Flash

Run these commands in the blink folder. The first compiles and links, the second extracts the flash contents into Intel HEX format, the third shows how much memory the program uses, and the last writes it to the chip.

avr-gcc -mmcu=atmega328p -DF_CPU=1000000UL -Os -Wall -o main.elf main.c
avr-objcopy -O ihex -R .eeprom main.elf main.hex
avr-size main.elf
avrdude -c usbasp -p m328p -U flash:w:main.hex:i

The -Os flag optimises for size and is also required for _delay_ms() to produce accurate delays, because the delay loops are calculated at compile time. In the avrdude command, -c selects the programmer, -p the part (m328p, m16 or m32), and -U flash:w:main.hex:i means write the file to flash in Intel HEX format. avrdude verifies the flash automatically after writing.