Quarz zu langsam?

#define F_CPU 16000000UL

#define sbi(ADDRESS,BIT) ((ADDRESS) |= (1<<(BIT)))

#define cbi(ADDRESS,BIT) ((ADDRESS) &= ~(1<<(BIT))

#include <avr/io.h>

#include <util/delay.h>

#include <avr/interrupt.h>

ISR (TIMER1_COMPA_vect)

{

  static int cnt=0;

  if(cnt>0){

      PORTD=0xff;

      cnt=0;

  }else{

    PORTD=0x00;

    cnt++;

  }

}

void init(void)

{

  DDRD=0xff;

  PORTD=0x00;

  sbi(TCCR1B,WGM12); //ctc ocr1a

  sbi(TCCR1B,CS12);

  sbi(TCCR1B,CS10); // 1/1024

  OCR1A=15625; // 1 sek

  sbi(TIMSK, OCIE1A);

  sei();

}

int main(void)

{

  init();

  while(1)

  {

    _delay_ms(1);

  }

}