ATmega 0 series. setting internal Vref required with external Vref

[Simon]

Reading through the ADC section of the ATmega1608 carefully I came across a requirement when using an external reference voltage to set the internal voltage reference to a voltage as close to it and higher than it as possible.

Why? is there some sort of leakage from the inter reference that by default is 0.55V to the external reference pin or the reference circuitry?

[harerod]

Simon, thank you for bringing this to my attention. I have been using the tinyAVR 1-series for a while, but never with external ADC reference. I looked through Microchip's appnotes for ATtiny, but couldn't find any information beyond what is found in the datasheet (e.g. ATtiny1616 -> ADC operation -> "external reference").

"When using the external reference voltage V_REFA , configure ADCnREFSEL[0:2] in the corresponding VREF.CTRLn
register to the value that is closest, but above the applied reference voltage. For external references higher than
4.3V, use ADCnREFSEL[0:2] = 0x3."

Looking at this I can only suspect some structural diode.

The implication of this requirement is that other peripherals (e.g. DAC) are limited in their range of reference, when the ADC uses the external reference.

I don't have an isolated tinyAVR 1-series ready for measurement. If anybody had one, I would be curious as how  currents (supply, reference input) and analog accuracy change, when this requirement is violated.

[Simon]

The mega 0 series does not have a DAC. In light of this I really don't know why they do not have the internal reference register in the ADC registers. In fact you could be told to select the next voltage up from you external reference thus setting the internal one or the same bit set VDD or one of the internal values, it would make the coding easier. 1 bit to say internal or external followed by 3 bits that set the voltage including VDD as an internal option.

What they have done is so "mechanical"

[harerod]

Ah, thanks for tacitly telling me to clean my glasses. 
Anyways, the ATmega1608 and ATtiny1616 seem to share some properties. Good to know.

[Simon]

all the new mega's and tiny's are just Xmega's with a new name that will run on 5V. I don't know what microchip are playing at other than confusing everyone. They may as well just call everything what it is: Xmega!