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

uint8_t i = 0;
uint8_t links = 0;

ISR(TIMER0_OVF_vect)
{

    if(!i) {
        links = 1;
    } else if ( i == 8 ) {
        links = 0;
    }

    if (links) {
        PORTB = ~(1 << i++);
    }
    else {
        PORTB = ~(1 << --i);
    }
}

int main(void)
{
    DDRB=0xFF;
    PORTB=0xFF;
    TCCR0=0x05;
    TIMSK=0x01;
    sei();
    while(1);
}