Quote Originally Posted by The Master View Post
I see. The datasheet says XT is for 200KHz to 4MHz. Anything above that is HS. I checked the datasheet for PIC16F87 too and that says the same thing yet ive been using one with a 20MHz resonator without setting it to HS. Ill change the 16F877A to HS when i get home
I think you've just been lucky so far. Some of the various oscillator modes (XT, HX, etc) change the 'gain' inside the PIC, drive the crystal/resonator/whatever harder at higher freq's.
The F87 is a bit newer than the F877A. Maybe it's a bit more tolerant of that sort of thing.