Thanks for the info Pepe34, & BenF,
I wrote the analog reference entry based on my, and others, reading of the datasheet and some experiments I did on the pin, but I was never satisfied with the ambiguity in the datasheet, and have always thought about writing to the Atmel engineers for clarification - or to see if they had a schematic of the AREF system.
I missed entirely the 32K input impedance reference - but this still doesn't make it a voltage divider unless the 32k is connected on one end to ground. which the math you post seems to indicate. Does the datasheet mention there is a 32k load resistor to ground?
If there really is some input impedance input at the AREF pin, which I have always suspected, then the 5K recommendation doesn't seem to make any sense and we should tell people to just hook up a low impedance voltage source to AREF.
I've got a lot of chips around and I'll do some experiments with current draw and the like, but I expect to find no problems with connecting low impedance sources to the pin. Except of course, that the voltage will override the expected reference voltage.
Using the resistor does
make it possible to switch sources on the fly - but at the expensive of fussing with the voltage divider effect. I guess if the input is grounded through a 32K resistor when it's in the EXTERNAL setting a resistor to a uC pin could be used to generate the voltage, then switched off for the 1.1 or 5V setting.
Let me know if you think my interpretation is incorrect. If anyone wants to try some experiments, I'll be glad to send out new chips to users who think they may have damaged their chips in the service of science.
This is from the 328 datasheet.
AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is generated
from the internal bandgap reference (VBG) through an internal amplifier. In either case, the
external AREF pin is directly connected to the ADC, and the reference voltage can be made
more immune to noise by connecting a capacitor between the AREF pin and ground. VREF can
also be measured at the AREF pin with a high impedance voltmeter. Note that VREF is a high
impedance source, and only a capacitive load should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no
external voltage is applied to the AREF pin, the user may switch between AVCC and 1.1V as reference
selection. The first ADC conversion result after switching reference voltage source may
be inaccurate, and the user is advised to discard this result.
"If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage."
This does seem to indicate that something bad may happen - other than just malfunction, but just malfunction seems like one way it could be interrpreted.