Here's a link that may help  

http://www.oscilent.com/spec_pages/PNDescrpt/Load_Cap.htm


Another one from ECS that's a little clearer 

http://ecsxtal.com/pdf/oscir_des.pdf



_______________________________________

Roy
Tauranga
New Zealand
_______________________________________

> -----Original Message-----
> I'm experiencing about a 3% failure rate on my PIC-based product.
Except
> for one time that a wire was cut, the rest of the time it's been the
20MHz
> crystal that's died.  I'm using a 16C58B chip and the crystal is an
> Abracon
> ABL-20.000MHZ-B2.  Is this a normal failure rate for this type of
crystal?
> Also, I'm not sure what the two capacitors should be.  Right now, I'm
> using
> 33pF capacitors with the crystal, but from what I read in the PIC
manual
> that may not be right.  The PIC manual recommends 15pF, but there's a
note
> that it should be 30pF if VDD > 4.5 (I'm using a 78L05 so VDD is 5
volts),
> and Abracon lists "Shunt capacitance" for this crystal at 7pF & "Load
> Capacitance" at 18pF.  Being almost totally ignorant of circuit
design,
> I'm
> not sure what the right capacitors are, so I'm blindly using what
someone
> else told me to use, but could using a capacitor that's too big cause
> these
> failures?
> 
> Thanks
> -Mark
> 

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