Just saw this note in the Attiny84 spec (page 145, section 16.13.1):
Internal voltage reference options may not be used if an external voltage is being applied to the AREF pin.
I may be have been bit by this on my most pcb design. The voltage I want to convert is on the ADC0 pin, and I was planning to use VCC as the reference (for maximum dynamic range).
Is use of vcc as a reference considered to be "an internal voltage reference option"?