328p-readvcc.ino

#include <Arduino.h>

#include <stdint.h>
#include <avr/io.h>

uint16_t vcc_millivolts(uint8_t delay_us = 50)
{
    // select AVCC as reference and switch the internal band-gap (1.1V)
	ADMUX = 0b01011110;

    // stabilize input
    _delay_us(delay_us);

	// start ADC conversion and wait to finish
    ADCSRA |= (1 << ADSC);
    while (!(ADCSRA & (1 << ADIF)));

	// AVCC = Vbg * 1023 / ADC
	return (1100UL * 1023) / ADC;
}

void setup()
{
    Serial.begin(9600);
}

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