Externer Interrupt oder TimerInterrupt und Pin abfragen?

// ATtiny841

#define F_CPU 8000000UL  // Systemtakt in Hz

#include <avr/io.h>

#include <stdio.h>

#include <avr/interrupt.h>

#include <util/atomic.h>    

volatile uint32_t millis_count;    // ISR Timer 0 Variable

volatile uint32_t second_count;    // ISR Timer 0 Variable

uint32_t last_millis;

uint32_t last_second;

// *** Funktion Deklarationen *** //

uint32_t millis();

uint32_t seconds();

ISR(TIMER0_COMPA_vect);

void set_Timer0();          // Timer 0, millis, seconds

int main(void)

{  

  set_Timer0();        

    while (1)  

    {   

    last_millis = millis();

    last_second = seconds();            

  }

}

/* *** Funktionen *** */

void set_Timer0 ()    // millis, seconds

{

  cli();                  

  TCNT0 = 0;                // Register Reset

  TCCR0A = 0;

  TCCR0B = 0;

  TIMSK0 = 0;

  TCCR0A = (1 << WGM01);          // CTC Modus

  OCR0A = 124;                     // TOP, (F_CPU/PRESCALER)/1000-1

  TIMSK0 |= (1 << OCIE0A);        // Compare Match einschalten

  TCCR0B |= (1 << CS01) | (1 << CS00);  // Prescaler 64, Timer starten

  sei();

}

uint32_t millis()

{

  uint32_t value = 0;

  ATOMIC_BLOCK (ATOMIC_RESTORESTATE)

  {

    value = millis_count;

  }

  return value;

}

uint32_t seconds()

{

  uint32_t value = 0;

  ATOMIC_BLOCK (ATOMIC_RESTORESTATE)

  {

    value = second_count;

  }

  return value;

}

ISR(TIMER0_COMPA_vect)

{

  static uint32_t local_millis = 0;

  millis_count++;        // Zähler für Millisekunden

  local_millis++;

  if (local_millis > 999) {

    second_count++;      // Zähler für Sekunden

    local_millis = 0;

  }

}