Mark Chauvin wrote:
> 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?

No, not if it is used correctly and not abused.

> 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.

It's not up to the PIC manual.  The load capacitance is a function of the
crystal design and is specified by the crystal manufacturer.  There seems to
be a lot of confusion and dead fish waving regarding load crystal load
capacitance on this list.  To some extent, the Microchip manuals don't help.

It's important to understand why both the capacitors on either side of the
crystal are there.  Each is there for a different reason.  Microchip might
have a spec on the maximum capacitance on the oscillator out pin at a given
frequency.  They have very little to say about the capacitance required on
the oscillator in pin.

> 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,

Since you're using the dead fish method, you apparently need a smaller fish.
After all the fancy analysis, 22pF to ground on either side of the crystal
actually works quite well most of the time.  But remember most of the time
isn't all of the time.  For short leads with nothing else connected and
20MHz crystal rated at 18pF load, 22pF caps should work well.

Despite all that, you still need to understand your high failure rate.  Is
the unit being subjected to shock and vibration?  If so, maybe a crystal
isn't appropriate.


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