Weird Oscillator Problems

This is a really weird problem. I have a circuit that has a serial output to monitor temperatures and functions and noticed the display is going totally wonky. Random character from hell actually, locks up, resets etc. After some time I have narrowed the problem to the oscillator section. The oscillator I'm using is a 4.000MHz crystal and a pair of 22pF capacitors. This is the really strange part, if I remove the capacitors from the circuit the board works prefect. If I leave on cap in it still works perfect. The traces between the PIC and the crystal are extremely short, the crystal is right beside the micro. I don't understand how this can be happening. Anyone have any ideas what might be causing this? If I install a resonator is works fine as well.

The PIC's that I'm working with are the 18F452 and 16F877. I just removed the caps from a different board and the same result. However this board works fine with or without the caps.

CocaColaKid,

I have seen this problem once some time ago.

The cause was a bad PCB.
(see attachment)

Take a Magnifying Glass and inspect your PCB.

Other possible causes:

If you have your PIC or Crystal on a Socket, it could be a bad socket.

Also people have been reporting similar problems with Breadboards.
(Search the Forum for "Breadboard")

And last but not least:
The Crystal itself, have you tried a different brand?

I just talked with Jeff and ME Labs and asked him if he has seen this problem and he said yes. I guess some people don't even run caps. I checked the clkout and noticed a lot of noise when the caps were in circuit. Once removed the wave was nice and smooth. I tried this will about 8 different boards and all of them had the same results. I guess it really depends on the crystals charateristics if you need the caps or not. I tried a resonator and noticed a big difference in the clkout signal. The crystal is so much more accurate.

'Kid,

I've seen similar symptoms with LVP enabled in the configuration fuses and the PGM pin left floating...

Regards, Mike

You might be overdriving the crystal when set up properly. In the Microchip books, they talk about putting a resistor between Osc2 pin and the crystal. Unless it is a specially cut crystal, or your Uncle owns the crystal company, find one that works reliably.

Hope this helps,
Ron