>
>
>> -----Original Message-----
>> From: piclist-bounces@mit.edu
>> [mailto:piclist-bounces@mit.edu] On Behalf Of Adam stambler
>> Sent: Friday, December 08, 2006 11:47 AM
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [PIC] Oscilator Problem
>>
>> Sorry, my description of my problem wasn't exactly accurate.
>> I tried the circuit with my full program instead of just a
>> turn on LED test.
>>
>
> I built a PIC project that used a crystal and found the crystal to have
> flaky behavior.  Rather than futz with it I removed it and replaced it
> with a canned oscillator.  But your question got me to wondering why it
> didn't work.  A crystal is a cheaper solution that an oscillator.  I did
> some digging and found this:
>
> http://www.springerlink.com/index/R181295K26346647.pdf
>
> Also this:
>
> http://www.citizen.co.jp/english/crystal/attention/index.html
>
> Which says to use a variable resistor in series with the crystal, and
> adjust it until the crystal starts oscillating.  This tells you the
> internal negative resistance of the crystal, and you can use that value
> to choose values for C1, C2, and Rd
>

I think the negative resistance refered to in the Citizen page, above, is
just a method of modeling the amplifier gain. The oscillator will just
oscillate when there's a loop gain of 1. They're putting Vr in series with
the crystal and adjusting it until the oscillator stops. They want this
resistance to be 5 to 10 times the rated effective series resistance of
the crystal. This allows the effective series resistance of the crystal to
go up 5 or 10 times before the oscillator dies.

I don't think Rd helps in getting the oscillator to run, but, instead, is
used to limit drive to the crystal. I don't think this is needed other
than for 32kHz "tuning fork" type crystals, which are very sensitive to
overdrive.

Harold

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