#define F_CPU 16000000L
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

volatile int8_t counter=-1;
volatile uint8_t seg[4]={0b01001100, 0b01000001, 0b01000011, 0b01001000};

ISR(TIMER0_OVF_vect)
    {
    PORTB=255;
    counter++;
    if (counter==4) counter=0;
    PORTA=seg[counter];
    PORTB=~_BV(counter);
    }

void main(void)
    {
    DDRA=255;
    DDRB=0x0F;
    sei();
    TIMSK|=_BV(TOIE0);
    TCCR0|=_BV(CS00)|_BV(CS01);

    while (1)
        {
        }
    }