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

int freq = 0;

ISR(TIMER0_COMPA_vect){           // Interrupt Service Routine
  static unsigned char teiler=0;
  teiler++;
  if(teiler>=freq){               // Teilfaktor in ISR
    teiler=0;
    if (freq>0) PORTB ^= 1;       // Ausgang PB0 invertieren (PINB = 1)
  }
}

void setup(){
  PORTB = 0xff;
  DDRB = 0xff;
  TCCR0B = 2;  
  TIMSK |= 1<<OCIE0A;   // Timer/Counter0 Output Compare Match A Interrupt Enable
  sei();                // globale Interruptfreigabe
}
 
void loop()
  {
  for (int n=1; n < 20; n++)
    {
    delay(2000);
    freq=n;  
    }
  cli();
  delay(50000);
  freq=0;
  sei();
  }