Trouble reading V_BG

Question

I would like to read MUX 14 with the ADC. On page 263 of the Atmega328 data sheet it shows 1110b on the MUX corresponding with reading V_BG.

I need to read this so I can calibrate my sensor readings taken with the 5v reference to those taken with the 1.1v reference. Because the 5v power supply may vary and the 1.1 is more stable, I need to be able to translate the readings taken with a 5v reference based on how the internal 1.1v is read.

Based on the code for analogRead, I wrote this:

int readVBG(int _analogReference) {
 uint8_t low, high;
 ADMUX = (_analogReference << 6) | 14;
 sbi(ADCSRA, ADSC);
 while (bit_is_set(ADCSRA, ADSC));
 low = ADCL;
 high = ADCH;
 return (high << 8) | low;
}

_analogReference is either 1 or 3 depending on the reference voltage desired.

I expect to read 1023 when _analogReference is 3 and about 225 when _analogReference is 1. Instead I get smaller values which I'm guessing are from a floating (unconnected) analog pin.

• Ignacio Vazquez-Abrams: How are you initializing ADCSRA?
• MattD: As I understand it, these are registers on the atmega328. If they were just variables, then the loop would never exit.
• MattD: Ignacio, are you suggesting that I need to do more than twiddle a single bit in ADCSRA?
• Majenko: You might find this of interest: hacking.majenko.co.uk/making-accurate-adc-readings-on-arduino

Answer 1

The problem is that a 2ms delay is required after setting the ADMUX bits before starting the conversion. Thank you Majenko for the link.

int readVBG(int _analogReference) {
 uint8_t low, high;
 ADMUX = (_analogReference << 6) | 14;
 delay(2); // Wait for voltage to settle
 sbi(ADCSRA, ADSC);
 while (bit_is_set(ADCSRA, ADSC));
 low = ADCL;
 high = ADCH;
 return (high << 8) | low;
}

The above code is specifically for the Arduino UNO. Majenko's link above has #ifdef statements which support other devices.

• Nick Gammon: Can you accept this answer please so that StackExchange doesn't keep "bumping" it in the hope someone will give an accepted answer. :)