Crystal - Diferences

[terciof]

Hi All!

I have a PIC16F877A and I want it to run at 20Mhz.

In CCS Wizard the oscilator I need to choose High Speed Crystal(>4Mhz).
But when I choose HS the PIC doesn´t work.
If I use 4Mhz cristal with XT configuration it runs ok.

My question: What is the diferences between them?

The crystals its self is the same, just the frequency changes.

Thanks.

Tercio

[Ttelmah]

HS, increases the gain of the oscillator to deal with the extra losses at higher frequencies. You say the crystal stays the same. Presumably you mean that you are using the same 'type'?. Are you sure that the type you are using actually meets the specs required for the PIC oscillator?. This becomes _much_ more important as the frequency rises, with the oscillator actually working with crystals that are well 'off spec', at lower frequencies, but failing when higher frequencies are used. What voltage are you running the PIC?. What load capacitance have you got on the crystal?. Have you looked at the MicroChip application notes on crystal selection?.

Best Wishes