Atmega88 Timer2 mit externem Takt

#include <avr/io.h>

#include <avr/interrupt.h>

#include <avr/sleep.h>

#define F_CPU 1000000

int main (void)                   //Benutzt Timer2, hat 8Bit

{ 

   DDRB |= (1 << PB0);                // LED an Pin12

  DDRB |= (0<<PB6);                // PB6 Eingang

   TCCR2B |= (1 << WGM12);               // Timer 2 in CTC mode

   ASSR |= (1<<AS2);                 // Ext. Taktquelle an TOSC1

   TIMSK2 |= (1 << OCIE1A);             // Enable CTC interrupt

   sei();                       // Enable global interrupts

  TCCR2B |= ((0<<CS20)|(0<<CS21)|(1<<CS22));    // Prescaler  31,2Hz

  OCR2A   = 64;                   // Vergleichswert

for (;;) 

   {

           set_sleep_mode(SLEEP_MODE_PWR_SAVE);

        sleep_mode();                         // in den Schlafmodus wechseln

    }

    }

ISR(TIMER2_COMPA_vect) 

{ 

   PORTB ^= (1 << 0);                 // Toggle Pin12

}