Arduino - PWM frequency and timers

A little known feature of many AVR chips is the ability to measure the internal analog voltage reference. This trick can be used in all sorts of ways such as:

Monitoring battery voltage to your Arduino
Checking to see if A/C power is running
Improve accuracy of analogRead() in many situations

The way to perform these feats is use the internal reference to actually measure Vcc. In the code following, we will actually measure the internal voltage reference, and then use this value to calculate our actually Vcc. Here is the code:

long readVcc() {
  // 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) ;
  #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
}

long readVcc() {

// 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) ;

#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

}

There are some limitations in accuracy due to tolerances on the internal voltage reference. You can however, calibrate the scale factor for greater accuracy. This code runs on all Arduino variants as well as the ATtinyx4 series chips.

Bron: https://www.instructables.com/Secret-Arduino-Voltmeter/

Other example

Another option which does not require a voltage divider is to use Vcc as a reference to measure both the analog input and the internal 1.1 V “bandgap“ reference. Measuring the 1.1 V against Vcc is an indirect way to measure Vcc. This is not supported by the Arduino core library, but you can do it by programming directly the control registers of the ADC:

// Return the supply voltage in volts.
float read_vcc()
{
    const float V_BAND_GAP = 1.1;     // typical
    ADMUX  = _BV(REFS0)    // ref = Vcc
           | 14;           // channel 14 is the bandgap reference
    ADCSRA |= _BV(ADSC);   // start conversion
    loop_until_bit_is_clear(ADCSRA, ADSC);  // wait until complete
    return V_BAND_GAP * 1024 / ADC;
}

// Return the supply voltage in volts.

float read_vcc()

{

const float V_BAND_GAP = 1.1; // typical

ADMUX = _BV(REFS0) // ref = Vcc

| 14; // channel 14 is the bandgap reference

ADCSRA |= _BV(ADSC); // start conversion

loop_until_bit_is_clear(ADCSRA, ADSC); // wait until complete

return V_BAND_GAP * 1024 / ADC;

}

Beware that the very first reading after boot may be bogus.

Source: https://newbedev.com/does-adc-conversion-to-a-voltage-rely-on-the-actual-value-of-the-5-v-pin

Zoeken

Arduino – Calculate VCC

Other example