In the hunt for just one more ATmega328 line that can be used as an external 0-level interrupt input I'm wanting to use the RESET/ pin. The user guide defines the RESET/ pin as open-drain when the DWEN fuse is programmed. The guide's Alternate Functions of Port C section also says that the RESET/ pin can be used as a Pin Change interrupt input PCINT13.
So it looks like I could attach an external interrupt open-drain/open-collector pull-down to RESET/ to generate interrupts and still preserve debugWire functionality. Anyone ever tried this? I'm wondering what would happen if the external int source stuck open-drain low when debugWire tried to take over the pin. My guess is that debugWire would choke.