PROGRAM PWM

/* Copyright (C) 2010 Enrico Rossi
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

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

/* from 50 to 950 */
uint16_t duty_cycle;

/* IRQ routine at 500hz this is called 500 times x second */
ISR(TIMER1_OVF_vect)
{
    /* if the duty is at 95% the restart from 5% */
    if (duty_cycle > 950)
        duty_cycle = 50;

    /* increment the duty cycle by 1 step at the time wich is
      0.1%, in this way I will complete 100% in 2 seconds (1000 step) */
    duty_cycle++;

    /* set the level where the output pin OC1A will toggle */
    OCR1A = duty_cycle;

    /* set the level where the output pin OC1B will toggle
      this will be the inverse of OC1A level */
    OCR1B = 1000 - duty_cycle;
}

void counter_setup(void)
{
    /* Pin OC1A & B set to output */
    DDRD |= _BV(PD4) | _BV(PD5);

    /* Clear OC1A & B on compare match, set it at BOTTOM.
      Waveform generation mode on 14, see datasheet */
    TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM11);
    TCCR1B = _BV(WGM13) | _BV(WGM12);

    /* TOP value*/
    ICR1 = 1000;

    /* start with a duty of 50% */
    OCR1A = 500;
    OCR1B = 500;

    /* enable interrupt on timer overflow */
    TIMSK = _BV(TOIE1);
}

int main(void)
{
    /* prepare everything */
    counter_setup();

    /* start with a duty cycle of 50% */
    duty_cycle = 500;

    /* enable interrupt */
    sei();

    /* start the counter with prescaler = 8 */
    TCCR1B |= _BV(CS11);

    /* Infinite loop doing nothing, everything is
    handled by the IRQ routine called 500 times x second. */
    for (;;);

    /* just for correct looking code, disable irq and terminate */
    cli();
    return(0);

SOURCE http://enricorossi.org/blog/2010/avr_atmega16_fast_pwm/