Atmega328p and ATTiny84/85, what sort of analogue reference

[Infraviolet]

What sort of analogue reference voltage is used when making measurements on the ADC pins of an atmega328p and/or attiny84 and/or attiny85.

I understand that there is a default 5V reference, as well as internal 1.1V references, and on the 328p there's a pin to input a reference voltage on...

But when using the default reference, it is referenced against a true 5V (to some +/- error) or against the Vcc voltage rail, whatever voltage the rail is presently at.

Say if you have relatively unstable 5V power which actually ranges from 4.6V to 5.4V, would reading a genuinely constant voltage on an ADC pin show it waving up and down as Vcc varied?

I can see use cases where knowing the ADC reading as a proportion of Vcc is helpful, but others where you want to have ADC readings against the true absolute scale so you know that 1023 (or 1024, I know there are some intricacies there, but 1 bit in 1024 isn't too important) means 5.0V rather than 100% of Vcc.

Which is used, and can one swap between such modes, without having to reference against a lower voltage, or sacrifice a pin on the attiny types to dedicate solely to monitoring either Vcc or some absolute zener derived 5.0V reference?

Thanks

[gilshultz]

@Infraviolet

What sort of analogue reference voltage is used when making measurements on the ADC pins of an atmega328p and/or attiny84 and/or attiny85.

That is design dependent and up to the designer. They can use Vcc, many do or generate there reference or use the internal reference.

[rsmls]

@Infraviolet

it is referenced against a true 5V (to some +/- error) or against the Vcc voltage rail, whatever voltage the rail is presently at.

Against VCC. If that happens to be 5V, it's 5V. If it happens to be 5.43126V and the next millisecond 5.6124, then it'll be that.

@Infraviolet

Say if you have relatively unstable 5V power which actually ranges from 4.6V to 5.4V, would reading a genuinely constant voltage on an ADC pin show it waving up and down as Vcc varied?

Yup.

@Infraviolet

Which is used

Anything you can imagine, from VCC to the internal reference to something made with any kind of external reference, in any degree of accuracy/stability. The world is a big place.

@Infraviolet

can one swap between such modes

Yes.

@Infraviolet

without having to reference against a lower voltage

Note sure what you mean here.

@Infraviolet

or sacrifice a pin on the attiny types

If you want to use an external reference, you need a pin for that. If you want to use VCC or internal 1.1V you don't need a pin.

@Infraviolet

some absolute zener derived 5.0V reference?

There's nothing very 'absolute' about a zener reference. Of all choices available, it's way at the bottom of the list of desirability. TL431 is a still popular reference used a lot in non-critical applications; it's a lot better than VCC, and probably better than internal 1.1V as well (depending on implementation). It's also dirt cheap. Many even better alternatives are available if necessary.

All this will be a lot more focused & efficient if you can tell a bit more about the application on your resolution requirements.

[oldcurmudgeon]

The internal reference is stable but the actual voltage is +/- 10%. If you use the internal reference you need to do a calibration for the actual device you have (unless +/- 10% is close enough).