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

double dutyCycle = 0;

int main(void)
{
    DDRD = (1 << PORTD6);

    TCCR0A = (1 << COM0A1) | (1 << WGM00) | (1 << WGM01);
    TIMSK0 = (1 << TOIE0);

    OCR0A = (dutyCycle/100)*255.0;

    sei();

    TCCR0B = (1 << CS00) | (1 << CS02);

    while(1)
    {

        _delay_ms(1000);

        dutyCycle += 10;

        if(dutyCycle > 100)
        {
            dutyCycle = 0;
        }
    }
}

ISR(TIMER0_OVF_vect)
{
    OCR0A = (dutyCycle/100.0)*255;
}