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April 2002

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Subject:
From:
Rob Legg <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 22 Apr 2002 15:30:02 -0400
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You'll find extensive coverage on this topic if you research google group
threads on sci.electronics.design.
http://groups.google.com/groups?q=sci.electronics.design&hl=en&btnG=Google+S
earch&meta=

If the problem is just start-up time, this can be increased by splitting the
final capacitor value into two parts connected as a divider between ground
and the chip supply rail. Their transient charge accumulation, at start,
should be set to bias the device linearly, without waitng for the parts to
charge through the feedback biasing resistor.

Gain is improved also, if the downstream C value is larger than the input C
value - of course the net series effective C should be maintained at the
specified value (11pf in your circuit).

If the problem is xtal mortality, degeneration, drift, then make sure you
are not overdriving the thing. a resistor in series with the output gate,
before the first capacitor, absorbs excessive drive power.

general app notes:
http://www.wenzel.com/documents/library.html
http://www.foxonline.com/main_other.htm
http://www.ctscorp.com/reeves/fdefault.htm

----- Original Message -----
From: "Jason" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, April 22, 2002 12:24 PM
Subject: [TN] 20 Mhz crystal start-up woes


> Has anyone had difficulty with crystal start-up.
> Currently we are having problems with a circuit that uses a PIC16C711-
> 201/P, 2 @ 22pF load caps and a 20 Mhz AT crystal. The configuration is
for
> fundamental operation.
>
> We are using a high temp screen (85 deg C storage before functional check)
> to try and weed out problems but have still found modules not functional
at
> the production plant.
> Sleepy crystals have been discussed but we have experimented with three
> different manufactures of crystals we have ruled this out.
> One of our action items is to redesign and use 10 Mhz, but right now we
> aren't sure of the root cause of failure so there really isn't a guarantee
> that this plan will work.
>
> What are some specific design parameters that could be measured to arrive
> at the root cause?
>
> Thanks,
> Jason
>
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