AREF is the analog reference pin for the A/D Converter.
5.2.9. ADC[7:6] (TQFP and VFQFN Package Only)
In the TQFP and VFQFN package, ADC[7:6] serve as analog inputs to the A/D converter. These pins are
powered from the analog supply and serve as 10-bit ADC channels.
inner pull-up is no exist ...
what to do with unused these pins (AREF & ADC)?
I suppose that everything can be grounded, but how is it correct within the framework of the "AVR-architecture"?
similar to what to do with the pins of the ISP connector? (they are used only for programming, do not participate in the circuit)