wvmarle:
That filter cap can't charge beyond +5.5V or -0.5V thanks to the clamping diodes of the Arduino.
Ok, that makes sense when you put it that way. So it doesn't seem like the capacitor could be a source of current that would overload the clamping diodes.
I may nonetheless use some external clamping diodes or give up and put in an optocoupler. This is the third or fourth chip I've smoked with this project, even though the circuit works, so something isn't right.