I've been seeking more digital outs and ins. Here's what I have ascertained:
IN 1 / POT 1 cannot be used as a digital out: doing a digitalWrite() will reboot GRAINS (!!!)
Same for IN 2 / POT 2
IN3 can be used as a digital out (and analog in)
AUDIO IN can be used a digital out (!!) (and analog in)
AUDIO OUT can be used as a digital in or out
DIGITAL OUT can be used as a digital in or out
So it appears we can have a maximum of 4 digital outs, for a 4-track drum sequencer for example.
The question is: AUDIO IN is analog in being pushed through a filter. Surprisingly it seems to work as a digital out. The question is: is it okay to use AUDIO IN as a digital out, or would pushing current back through the filter stress it in some way?