> MicroChip says the lowest voltage I can operate the
> 16C558 on is 3.0V
>
> ISD says I have to supply 2.7 to 3.3V to the ISD chip
>
> Also, I need to clock with a XTAL (I need the timebase),
> will it work reliably at 3.0V???

No problem with 3.0 to 3.3 then. The only problems I've ever had with pics
at low voltage has been oscillator startup, and even then only at 2.5
volts or lower.

If you are sleeping, be prepared for longer startup than at 5 volts, lots
longer. Like really long if you are using a 32khz xtal.

And make sure you erase extra long when doing your development of 3 volt
projects.

Cheers,
Bob