void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  ADMUX = (1 << REFS0);
  ADCSRA = (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
  ADCSRA = (1 << ADEN);
}

void loop() {
  // put your main code here, to run repeatedly:
  ADCSRA |= (1 << ADSC);
  while (ADCSRA & (1 << ADSC));
  Serial.println(ADC);
}