main.c

#include <stdint.h>

#include <avr/io.h>

#include <avr/interrupt.h>

#define OC0_F 263 //Hertz

#define F_TOSC 32768

#define T0_PRE 1

#define OC0_VAL(_foc) (F_TOSC/(_foc * 2 * T0_PRE) -1)

int main(void)

{

  DDRB = (1<<PB5);

  ASSR = (1<<AS0);

  OCR0 = OC0_VAL(OC0_F); //preload for CTC

  TCCR0 = (1<<WGM01)|(1<<COM00)|(1<<CS00); // Prescaler:1, CTC, toggle on compare

/*  wait for timer-tick IF necessary (i.e. you want to change TCCR0/OCR0/TCNT0):

  while(ASSR & ((1<<TCN0UB)|(1<<OCR0UB)|(1<<TCR0UB)) )

    ;

*/

  TIMSK = (1<<OCIE0);

  sei();

  while(1)

  {

  //  mainloop

  }

  return 0;

}

ISR(TIMER0_COMP_vect)

{

  PORTB ^= (1<<PB5);

}