From the ATmega328 datasheet:
ADC7:6 (TQFP and QFN/MLF Package Only)
In the TQFP and QFN/MLF package, ADC7:6 serve as analog inputs to the A/D converter.
These pins are powered from the analog supply and serve as 10-bit ADC channels.
Note that ADC pins ADC7 and ADC6 do not have digital input buffers, and therefore do not
require Digital Input Disable bits.
Unfortunately, ADC6 and ADC7 are analog input only, and do not interface with the digital input or output
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