Measure Vcc using 1.1V bandgap

Question

For an battery driven application I want to measure the Vcc (using an Atmega 32u4). That is my code:

int readBandGap() {
 ADMUX = (B01 << REFS0) // VCC
 | (0 << ADLAR) // right-adjusted result
 | (B011110 << MUX0) // 1.1V
 ;
 // switch off any ADC conversion
 ADCSRA &= ~(1 << ADEN);
 // enable ADC
 ADCSRA |= 1 << ADEN;
 // start ADC
 ADCSRA |= 1 << ADSC;
 // wait until finished
 while (bit_is_set(ADCSRA, ADSC));
 // first read low, then high!
 uint8_t low = ADCL;
 uint8_t high = ADCH;
 // switch off any ADC conversion
 ADCSRA &= ~(1 << ADEN);
 return (high << 8) | low;
}
void setup() {
 Serial.begin(9600);
}
void loop() {
 int value = readBandGap();
 float vcc = 1.1 * 1023 / value;
 Serial.print(value);
 Serial.print(" ");
 Serial.println(vcc);
 delay(1000);
}

My Olimexino 32u4 board has the ability to switch the voltage between 5V and 3.3V. There are two strange things:

1. the first measurement is always significantly different than the later measurements

2. with 5V I'm getting 450, followed by ~367; with 3.3V I'm getting 422, followed by ~395. I would have expected a higher Vcc resulting in a lower measured value (bandgap should be measured compared to Vcc).

Any ideas what is wrong with my approach?

• Majenko: I have known good Vcc measuring code here: hacking.majenko.co.uk/making-accurate-adc-readings-on-arduino - You might like to compare it with your code - especially the bit that delays to allow Vref to settle.
• Thomas S.: I've tried that code now and readVcc always reports ~1100, no matter whether I'm using 3.3 or 5V. Maybe for the 32u4 some different flags have to be used.
• Majenko: Given that code was written for the 328P with fewer ADC inputs, yes, you probably need to adjust the MUX settings to get the right channel.

Answer 1

The bandgap voltage reference needs some time to stabilize, after you enable it. See http://jeelabs.org/2012/05/12/improved-vcc-measurement/

Doing 3 dummy measurements, gives it enough time to do that.

Answer 2

After tweaking a lot, I've now have found following code to work quite good.

float readVcc() {
 ADMUX = (B01 << REFS0) // VCC
 | (0 << ADLAR) // right-adjusted result
 | (B011110 << MUX0) // 1.1V
 ;
 // enable ADC
 ADCSRA |= (1 << ADEN);
 // seems necessary
 delay(2);
 // start ADC
 ADCSRA |= 1 << ADSC;
 // wait until finished
 while (bit_is_set(ADCSRA, ADSC));
 // first read low, then high!
 long value = ADCL;
 value |= ADCH << 8;
 return 1.1 / value * 1023;
}