My circuit has a battery charger and an ATMega88. The ATMega needs to sense the state of the battery charger - but if the battery is too low, the ATMega may not have power.
I know it's bad to drive a CMOS input more than 0.3V above VCC - you can power chips through the protection diodes, cause latch-up, etc.
I see three possible solutions:
1) Use a dual-supply bus transciever. http://www.ti.com/lit/ds/symlink/sn74lvc1t45.pdf This adds more cost than I'd like.
2) Put a 100K resistor inline with the signal, so that "not much" current flows into the powered-off circuit. I think this shouldn't cause latch-up or burn anything out, but could still power-up some circuitry and cause undesired operation.
3) Use divider resistors to step the signal down to <0.3V, and feed it into an ADC pin. This requires extra work and time to sense - and I'm still just a bit nervous about feeding any voltage/current into unpowered CMOS.
So I'm inclined to go with option 3, but hoping there's a better option I haven't thought of.