At 07:07 AM 7/14/2005 -0500, you wrote:

>Can I assume in the crystal mfg's literature that "Load capacitance" 
>specifies the recommended values for c1 & c2?  That would mean I'm using 
>capacitors that are 83% bigger than recommended.

No, you're using about the correct value, from that pov. The capacitors
are effectively in series in a Pierce oscillator and there is some
stray/input capacitance, so 33pF is about right for the 18pF load
capacitance. That means that the oscillator will be nominally on the marked
frequency, not a bit off. This is not your main problem.

HOWEVER, drive power and starting are different requirements from getting
the frequency spot-on... most crystal MANUFACTURERS (the guys who you
will complain to if you start getting 1% failures in 1E6 crystals)
recommend that you use a (expensive) 'scope current probe and calculate the
actual drive power of the crystal with different series resistors (plot it
on a graph). And you look at what maximum value of resistor will
cause the oscillator to fail to start, over temperature and allowing
for aging, unit to unit variation, power supply variations and so on.
Then you pick a value (if possible) that allows reliable and fast
starting and keeps the drive power within the recommended range.
Too much drive power can lead to accelerated aging, deterioration of
characteristics, and outright failure. Problems are more common with
tuning-fork style crystals, high frequency crystals, and the tiny
SMT crystals that can only handle 50-100uW rather than 1mW or more that
the HC49 crystals can handle. The crystal manufacturers don't typically
give you the information except as typical values of the series
resistance. The IC manufacturer doesn't give the range of the gm of the
oscillator section. So, there is not really a guarantee from anyone
that it will work.

If you can use a larger crystal (full height HC49) or a resonator (we've
used many thousands of 20MHz 3-pin resonators with zero problems),  or
drop the supply voltage to 3.3V, you'll probably get much higher reliability
without major changes. The ones you have out there now may be ticking time
bombs waiting to fail. 3% is a VERY high reported failure rate.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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