Now I'd normally use weak pullups & then switch a ground to a PIC digital IO, but on this occasion, the place where the switch is going is *very* tight for space & I could really do without having to feed 0V just for the switch. Now where the switch is going to be located, there's already 4.5V present (this with a fresh battery) ...this voltage fades down to about 3.2V as the battery life fades, so I was wondering if I could just utilise that 4.5V to feed a PIC IO pin & trigger an IOC interrupt...so in essence, I'm applying about 1.2V (max) higher than the PIC's own supply - will this cause a problem?

Another question ...when is a high a high & when is a low a low ...for example with 3.3V supply, has anyone chec ed to see what voltage is deemed a 'high' & triggers (ie can it be less than the supply) & vice versa?