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

#define init_val 130

volatile unsigned char a;

int main(void)
{
    a = 20;
    DDRA = 0xFF;
    TCCR0 = (1 << CS02) | (1 << CS00) | (1 << WGM01);
    OCR0 = init_val;
    TIMSK |= (1 << OCIE0);

    sei();
    while(1);
}

ISR (TIMER0_COMP_vect)
{
    a--;
    if(a == 0)
    {
        a = 20;
        PORTA ^= 0xFF;
    }
}