volatile bool convDone;
volatile unsigned int value;
volatile unsigned long lastTime, convTime;

void setup() {
  Serial.begin (115200);
  Serial.println ("ADC Interrupt Example");

  //  ADC Setup
  ADCSRA = 0b10001111;      // Enable ADC & Interrupt, pre-scaler = 128
  ADMUX  = 0b01000000;      // Use AVCC and A0 as input to ADC
  sei();                    // Set global interrupt flag

  convDone = true;
}

void loop() {
  
  if (convDone) {
    // start converting voltage on A0
    ADCSRA |= bit(ADSC) | bit(ADIE);  
    lastTime = micros();
    convDone = false;
  }
  
  static unsigned long printTime;
  if(millis() - printTime > 500) {
    printTime = millis();
    Serial.print(convTime);
    Serial.print("uS - Val: ");
    Serial.println(value);
  }
}

// ADC Interrupt Service Routine
ISR(ADC_vect) {
  convTime = micros() - lastTime;
  value = ADC;
  convDone = true;
}