Example of measuring atmega VCC using the bandgap method (no external hardware or voltage divider needed).

arduino_bandgap_vcc.ino

/*
Example of measuring atmega VCC using the bandgap method (no external hardware or voltage divider needed).
Based on: https://jeelabs.org/2012/05/04/measuring-vcc-via-the-bandgap/
*/

// Arduino Nano / Atmega328 tested

static int vccRead () {
  ADMUX = bit(REFS0) | 14; // use Vref=VCC and measure internal bandgap channel 1.1V
  delayMicroseconds(250);  // wait for bandgap to stabilize
  bitSet(ADCSRA, ADEN);    // enable ADC
  bitSet(ADCSRA, ADSC);    // start conversion
  while (ADCSRA & (1<<ADSC)); // wait till conversion stops
  return ADC; // return value in 0-1023 range
}

void setup() {
  Serial.begin(9600);
  Serial.println("bandgap:");
}

// adc = 1100 / vcc * 1024
void loop() {  
  byte x = vccRead();
  Serial.print("vcc=");
  Serial.println(1100 * 1023L / x); // power supply (VCC) voltage
  delay(500);
}