void config_adc() {
  ADMUX = 0;

  /* AVCC with external capacitor at AREF pin */
  ADMUX |= (1 << REFS0);

  ADCSRA = 0;
  // Set the ADC prescaler to 128 for a 16MHz clock, giving an ADC clock of 125kHz
  ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
  
}

void init_adc() {
  ADCSRA |= (1 << ADEN);   // Enable ADC 
  ADCSRA |= (1 << ADIE);   // Enable ADC interrupts
}