OpenChrono/firmware/OpenChrono/adc.cpp

/*
 * adc.cpp
 *
 * OpenChrono BB speed measurement device.
 *
 * https://provideyourown.com/2012/secret-arduino-voltmeter-measure-battery-voltage/
 */

#include <Arduino.h>

#include "adc.h"

long readVcc(void) {
    // Read 1.1V reference against AVcc
    // set the reference to Vcc and the measurement to the internal 1.1V reference
#if defined(__AVR_ATmega32U4__) || defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
    ADMUX = _BV(REFS0) | _BV(MUX4) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
#elif defined (__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__)
    ADMUX = _BV(MUX5) | _BV(MUX0);
#elif defined (__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
    ADMUX = _BV(MUX3) | _BV(MUX2);
#else
    ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
#endif

    delay(2); // Wait for Vref to settle
    ADCSRA |= _BV(ADSC); // Start conversion
    while (bit_is_set(ADCSRA,ADSC)); // measuring

    uint8_t low  = ADCL; // must read ADCL first - it then locks ADCH
    uint8_t high = ADCH; // unlocks both

    long result = (high<<8) | low;

    result = 1125300L / result; // Calculate Vcc (in mV); 1125300 = 1.1*1023*1000
    return result; // Vcc in millivolts
}