On 14 Jul 2005 at 5:57, Mark Chauvin wrote:

> Also, I'm not sure what the two capacitors should be.  Right now, I'm using 
> 33pF capacitors with the crystal,

> and Abracon lists "Shunt capacitance" for this crystal at 7pF & "Load 
> Capacitance" at 18pF. 

That is probably just about perfect choice of capacitor. The capacitors are effectively in 
series, so the load capacitance is 33/2, or 16.5pF. Add some stray capacitance, and you 
are probably between 18 and 20.

If you are having failures it is most likely mechanical shock or overdriven crystals, but I 
have very little experience there. But it seems to me that overdriving would be a long-
term problem so if the device is used 24 hours a day it would be more of a problem than 
in something used occasionally.

Adding a resistor in series with the crystal will reduce the drive, but unless you can 
actually determine that overdriving is the problem, it may make startup less reliable. 
Also if you have the choice of oscillator on your PIC you may choose the lower power 
"XT" rather than "HS".

If you decide to switch to a resonator rather than a crystal, be forewarned that many 
brands of resonator will not start up with the PIC at 20MHz.

Cheerful regards,

Bob

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