void configure_input(void){
  DDRD &=~(1<<PD2);
  PORTD |=(1<<PORTD2);
}

void init_INT0(){
  EIMSK |= (1 << INT0);
  EICRA |= (1 << ISC01);
  EICRA &=~(1 << ISC00);
}

void init_adc() {
  ADCSRB = 0x02;
  ADMUX = 0x40;                 // Reference: AVCC, Input Channel: ADC0
  ADCSRA = 0xA7;                // Enable ADC, Auto Trigger, Prescaler 128
  ADCSRA |= (1 << ADATE) | (1 << ADEN) | (1 << ADIE); // Enable ADC interrupt Enable the ADC Start the first conversion
  //ADCSRA |= (1 << ADEN);        // Enable the ADC
  //ADCSRA |= (1 << ADIE);        // Start the first conversion
}
ISR(ADC_vect) {
  uint8_t lowByte = ADCL;
  uint8_t highByte = ADCH;
  uint16_t adc_value = (highByte << 8) | lowByte;
  Serial.println(adc_value);
}
ISR(INT0_vect) {
  ADCSRA |= (1 << ADSC); // Start ADC conversion
}
//ADCSRA = 0xA7; 
void setup() {
  Serial.begin(9600);
  configure_input();
  init_INT0();
  init_adc();
  sei(); // Enable global interrupts
}

void loop() {
  // Main loop can stay empty, ADC results are printed in the ISR.
}