Minimum crystal capacity

[Oznog]

I have a crystal in a tiny can case I might want to use for board space reasons.

It says 18 pF load on the bag from the vendor. It occurs to me that I don't know what this means.

Is that the maximum capacitive load? Or a minimum load to remain stable or a maximum load?

The PIC18F252 spec sheet, 22.2 item D100 says OSC2 has a 15pF load. But I also add a 18pF to each leg for stability. At least one of those caps is in parallel with the OSC2 pin. How does the load cap get calculated? Is this too high? Or do I maybe not need the stability caps when a small crystal is loaded this way (wishful thinking)? Getting rid of the caps would certainly help my board space issue too.

 

[bonxer]

Go to **broken link removed** and get some 18 or 22 pF capacitors from him. He's got good prices and low shipping. If you can find them in a store locally, that would be great. Radio shack doesn't carry them for some reason.

Anyway, pretty much you just connect each of the oscillator pins to one of the capacitors, and connect the other ends of the capacitors to ground. You don't really need to worry about the value of the capacitors.

Unless you are making a program that is really really really time sensitive, and you notice an unacceptable drift, just go ahead and stick in either some 18's or 22's. Whatever ones you go with, just make them match. Don't worry about calculating anything either. Works for me. 8)

 

[Nigel Goodwin]

I agree, stick the crystal in circuit and bang a couple of caps to ground, the exact value doesn't matter - anything from 10pF to 33pF will be fine, just keep them both the same.