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'Help -- Load Caps for osc'
1999\12\30@225721 by Ann & David Scott

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William K. Borsum wrote

>Also, be sure the load caps
>are correct--most watch
>crystals I get from Digikey
>or Mouser are around 12.5
>pF--remember this includes
>trace capacitance, etc.  NOT
>critical, but it does seem to
>help to be close.

I've made it work before but never quite understood it.  What does this
mean?  The 2 caps on the crystal should match the capacitance of the
crystal?  Can you use the catalog value for the crystals pF as given in the
catalog?

Happy New Year
DScott

1999\12\31@041026 by William K. Borsum

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At 09:51 PM 12/30/99 -0600, you wrote:
>William K. Borsum wrote
>
>>Also, be sure the load caps
>>are correct--most watch
>>crystals I get from Digikey
>>or Mouser are around 12.5
>>pF--remember this includes
>>trace capacitance, etc.  NOT
>>critical, but it does seem to
>>help to be close.
>
>I've made it work before but never quite understood it.  What does this
>mean?  The 2 caps on the crystal should match the capacitance of the
>crystal?  Can you use the catalog value for the crystals pF as given in the
>catalog?

I do.  Also check the MicroChip data sheets for their recommendations.
There are a number of Ap-Notes on the subject as well. In general values
from 10 to 30 work reasonably well, along with that 100 K carbon Comp
series resistor.

Remember that watch crystals start fairly slowly--a few seconds is not
unusual.

Kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>San Diego, California, USA


'Help -- Load Caps for osc'
2000\01\02@214030 by paulb
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Ann & David Scott wrote:

> I've made it work before but never quite understood it.  What does
> this mean?  The 2 caps on the crystal should match the capacitance of
> the crystal?  Can you use the catalog value for the crystals pF as
> given in the catalog?

 OK, the crystal has a rated load capacitance, which is what it wants
to see for exact frequency.  What it sees is this:

Osc1 o------+--------+       Where:  C1 = loading cap on Osc1
           |        |               C2 = pin capacity of Osc1 input
          === C1   === C2           C3 = loading cap on Osc2
           |        |               C2 = pin capacity of Osc2 output
Crystal    +--------+-- Ground
           |        |
          === C3   === C4
           |        |
Osc2 o------+--------+

 This is actually pretty simple because we make C1 = C3 and assume that
C2 = C4 although for some peculiar reason, these are not specified in
the datasheets.  10pF sounds a round guess however (implied from other
CMOS spec sheets).

 Let's say your crystal was specified at 15pF.  This would be achieved
with each of the pairs C1+C2 = C3+C4 = 30pf, i.e., two 30pF in parallel.
If C2=C4 is approximately 10pF, then C1 and C3 would each be 20pF or in
practice, a 22pF (nominal) capacitor.

 If you really want accuracy, then C1 becomes a 8.2pF in series with a
3-30pF trimmer.  Also, as the internal capacities C2 and C4 may vary
with temperature and voltage, it is somewhat preferable if the loading
capacitors are bigger rather than smaller.  (This obviously influences
crystal selection, but only for high accuracy.)
--
 Cheers,
       Paul B.

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