#ifndef F_CPU
#define F_CPU 20000000UL // 20 MHz clock speed
#endif

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

ISR(TIMER1_CAPT_vect) {
  PORTC ^= 0xFF;
}

int main(void) {
  // Initialize Periphery

  // quick and dirty debug LED
  DDRC = 0xFF;

  // pin D6 (ICP) is digital input with pullup
  DDRD &= ~(1 << PD6);
  PORTD |= (1 << PD6);

  // Initialize Timer1

  // prescaler: 64
  TCCR1B = (1 << CS11) | (1 << CS10) \
      // input capture on rising edge
      | (1 << ICES1);

  // mode: normal
  // TCCR1A unchanged

  // enable input capture interrupt
  TIMSK1 = (1 << ICIE1);

  // global interrupt enable
  sei();

  while(1) {
    // loop
  }
}