gistfile1.c

#include <avr/io.h>
#include <avr/interrupt.h>

#define F_CPU 1000000UL  // 1 MHz
#include <util/delay.h>

int counter = 0;

int main(void)
{  
	//We have an LED on PORTA0, which starts on
	//Configure PORTA0 as output, and set it high.
	DDRA = (1 << DDRA0);
	PORTA = (1 << PORTA0);
	
	//We have a button on PORTD2 (INT0) which pulls it to ground when pressed
	//Set PORTD2 as input and activate the internal pull-up resistor
	DDRD = (0 << DDRD2);
	PORTD = (1 << PORTD2);
	
	//Configure INT0 to trigger on falling edge and then enable it
	EICRA = (1 << ISC01) & (0 <<ISC00);
	EIMSK = (1 << INT0);
	
	//enable interrupts
	sei();
	
    while(1){
		//do nothing
    }
}

ISR(INT0_vect)
{
	counter++;
	if(counter % 5 == 0) {
		//toggle the LED on PORTA0
		PORTA ^= (1 << PORTA0);
	}
}