Oscillator intrigue

[Adrian]

I have a PCB with PIC16F877. The circuit works well with a 4.0mHz resonator OR a 4.0mHz crystal, but refuses to oscillate with crystal and the normal 22pf caps (I used Hi-K ceramics). Has anyone come across this before. I am assuming that either (a) there is enough stray capacitance around without the caps or (b) I admit the earthy ends of the 22pfs are down to the ground plane but spaced either side of the crystal. Maybe they are too far apart but not yet tried shifting them. Any clues?
Thank you

Adrian

[w7ami]

Adrian,

The placement of the capacitors isn't to critical at 4 MHz. Good design would tell you to keep the leads short. Are you certain of the values of the capacitors? The oscillator won't start if you accidentally use a value to much higher. 22pf is given as a good starting point. You might try 10 pf or so since the crystal does oscillate with no capacitors in the circuit. Look for solder bridges around the capacitors while you are at it.

Terry

[Adrian]

Thanks Terry
I guess you must be right with the values and/or a whisker of wire somewhere. I'll strip it out and try again.

Thanks

Adrian