Noch eine Frage zum Interrupt

#ifndef F_CPU

#define F_CPU 16000000UL

#endif

#include <avr/io.h>

#include <util/delay.h>

#include <avr/interrupt.h>

#include <stdio.h>

#include "lib_lcd.h"

#ifndef TIMER1_OVF_vect

#endif

#define TASTER1 (PORTC&1)==1

#define TASTER2 (PORTC&1)==1

char Sekunden     = 0;

char Minuten      = 0;

char Stunden      = 0;

// INTERRUPT

ISR (TIMER1_OVF_vect)             //Interrupt-Routine

{

  cli();                  //Interrupt abschalten

  Sekunden++;          

    if (Sekunden >= 59) 

    {

      Sekunden = 0;

      Minuten++;

      if (Minuten >= 59) 

      {

        Minuten = 0;

        Stunden++;

        if (Stunden >= 23) 

        {

          Stunden = 0;

        }

      }

    }

  }

  sei();                  //Interrupt zulassen

}

int main ()

{

  char text_buffer[17];

  TCCR1B = 0x02;              //Timer System Frequency Prescaler 8

  TIMSK0 |= (1<<TOIE1);          //Interrupt alle 100ms (Timer 1)

  sei();                  //Interrupt erlauben

  lcd_init();

  while (1)

  {

    if (TASTER1)

    {

      Minuten++;

    }

    if (TASTER2)

    {

      Stunden++;

    }

  sprintf(text_buffer,"%02u:%02u:%02u",Stunden,Minuten,Sekunden);

  lcd_string_xy(0,1,text_buffer);

  }

}