Issue with internal voltage reference

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I am using the ATmega644 as a low quality voltmeter to test out both ADC and my LCD display. Everything works great when using an external 5V reference, but as soon as I switch ADMUX from 0b00100000 to 0b11100000 (enabling the 2.56V reference) the ADC starts to read maximum voltage even at 0V at my input. I did disconnected the reference pin from 5V and did put a capacitor from ground to it, but it stays at the maximum 256. Any ideas why it will work for an external voltage but not an internal reference voltage?

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What voltage do you see on the AREF pin?

If not around 2.5V, could it be that you made the ref change in the code before disconnecting your voltage at AREF pin? It would then have been in a hard fight with the internal reference circuit.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Ironically I don't have a voltmeter to test the pin on my voltmeter project. If I can't find the fix I will have to wait until next Monday to bring it into class (where the meters are). Just checking, the only thing I have to switch is the ADMUX register, correct?

EDIT: I now notice that I may have accidentally suggested that I changed to reference while it was still wired to Vcc, ignore that please!