ATtiny861A: ADC reference voltage should be 1 Volt under AVcc?
Let me start by &quot;Small chip die has specifications X and large die from same company has better specifications&quot; is not an argument for any manufacturer. Of course the larger, more expensive die will have better stuff in many dimensions.
But, are you using bipolar or single ended channels? The page you are looking on is the limitation for bipolar channels, possibly due to some internal stage.
Two pages higher, it says for single ended channels &quot;Aref External = 2.0V to AVcc&quot;.
Whether an internal reference track has a certain option is also not a valid argument, as you do not know all the paths internally on the chip itself. So you cannot just say &quot;If I have it switch over internally it can do this, but externally it can't, that's bullshit, I will just do it externally&quot;, because internally it may also switch over some stages, that work better, but need to be hard-related to VCC to be able to do that. It may not be likely, but it is possible.
So if you do try it with bipolar channels and nowhere else you can find a limitation to their use that also implies a lower Vref and this is something you want to produce more than one of, then you should still at least test it to further limits than it will ever go in the real world.
For example if you make it for -20deg C to +50 deg C, it needs to be verified to work correctly for extended periods of -25deg C to +60 deg C. As such also with main incoming supply voltage. If the board is designed for 11V to 13V, it needs testing for 10V to 15V, or some such. And not just for a minute, but at the very, very least for the better part of a day at each extreme and possibly several points in between.
And for proper testing, you should test each parameter with each possible setting for each other parameter. I.e. 10V to 15V in 1V or 0.5V increments, half a day each at every temperature between -25 and +60 at 5 degree increments, in the example above.
And that needs to be done with several randomly picked items from each production run.
EDIT:
Doing tests like that with slightly less rigour would be wise, even if you stay inside the datasheet's parameters for all parts. But then you can probably stick to &quot;Test all voltages for a couple of minutes at a temperature below zero, one above room temp and at room temp&quot;. Unless it is a very critical part, of course.