#include <util/delay.h>
#define LED_PIN   PORTD3
#define INT0_PIN PORTD2

#define NO_ARDUINO_LAYER         // comment this line to run with the Arduino layer

# ifdef NO_ARDUINO_LAYER
int main() {
  setup();
  while (1) {
    loop();
  }
  return 0;
}
#endif

void setup() {

  EICRA = 0;               // clear register
  bitSet(EICRA, ISC01);    // The falling edge of INT0 generates an interrupt request.
  EIMSK = 0;               // clear register
  bitSet(EIMSK, INT0);     // enable External Interrupt Request 0
  bitSet(DDRD, LED_PIN);   // set pin as output
  bitSet(PORTD, INT0_PIN); // enable pin pull-up
  bitSet(PORTB, 5);        // flash L to indicate setup() is running.
  _delay_ms(1000);
  bitClear(PORTB,5);
  interrupts();            // enable interrupts
}

void loop() {
  if bit_is_set(EIFR, INTF0) {  // check falling edge at INT0_PIN
    bitSet(EIFR, INTF0) ;       // clear INTF0 flag
    bitToggle(PORTD, LED_PIN);  // toggle LED level
  }
}