The analog pins can certainly be used as digital pins.
You can use the serial pins too except that they have pull-ups connected internally. It would be OK to use them as outputs, not quite as useful to use them as inputs.
The analog pins can certainly be used as digital pins.
You can use the serial pins too except that they have pull-ups connected internally. It would be OK to use them as outputs, not quite as useful to use them as inputs.