Arduino internal VRef (Atmega 328/328P based board only)

arduino_vref.cpp

//Code from: https://code.google.com/archive/p/tinkerit/wikis/SecretVoltmeter.wiki
#include <Arduino.h>

//Reads internal Arduino VRef
long readVcc() { long result;
    // Read 1.1V reference against AVcc
    ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
    delay(2);
    // Wait for Vref to settle
    ADCSRA |= _BV(ADSC);
    // Convert
    while (bit_is_set(ADCSRA,ADSC));
    result = ADCL;
    result |= ADCH<<8;
    result = 1126400L / result;
    // Back-calculate AVcc in mV
    return result;
 }

void setup() {
    Serial.begin(57600);
}

void loop() {
    Serial.println(readVcc(), DEC );
    delay(1000);
}