#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;
wysw++;
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)
{
}
}