#define F_CPU   8000000L //Running at 8Mhz

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

double dutyCycle = 0;

int main(void)
{
    DDRB = (1 << PORTB0); // Set PB0 to an outpu

    TCCR0A = (1 << COM0A1) | (1 << WGM00) | (1 << WGM01); // Non inverting Fast PWM. Fast PWM (Mode 3)
    TIMSK = (1 << TOIE0);

    OCR0A = (dutyCycle/100)*255; // Get the dutycycle setup

    sei(); // Setup external interupts

    TCCR0B = (1 << CS00); // No prescaler

    while(1)
    {
        _delay_ms(100); // Wait 100ms

        dutyCycle += 10; // Increase pwm with 10 percent

        if(dutyCycle > 100) // If dutycycle is 100 percent, reset it.
        {
            dutyCycle = 0;
        }
    }
}

ISR(TIMER0_OVF_vect)
{
    OCR0A = (dutyCycle/100)*255; // Update dutycycle after every timer interupt.
}