In my case, I have spare input ports on my Arduino so no need to emulate a pot - I'm using the pot connected to the Arduino's ADC anyway to give an internal 0-255 to represent desired temperature - so using the processor means a SMOP(*) only needed.

For the Hitachi processor, though, not so sure. Had a quick look at data sheet last night (my, that thing's old!) and it looks as if it has the potential for a couple of interrupt lines but not sure if they would trigger on level change in either direction or just one. I would try to do as much as possible in the CPU and avoid external circuitry, though.

(*) Simple Matter Of Programming