I'm designing a circuit that will use a 2.5v reference for the AREF. Should AVCC be powered from 5V or 2.5V?
AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that PC6...4 use digital supply voltage, VCC.
So it should be connected to 5V.