小弟在atmaga8里面看到的，上面並沒有說不能使用數位i/o，不知道大大在何處看到的？
ADC7..6 (TQFP and MLF
Package Only)
In the TQFP and 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.
p61
‧ AIN1 – Port D, Bit 7
AIN1, Analog Comparator Negative Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function
of the Analog Comparator.
‧ AIN0 – Port D, Bit 6
AIN0, Analog Comparator Positive Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function of
the Analog Comparator.
