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PICList Thread
'Mislabelled crystal?'
1997\06\03@141608 by Brian Scearce

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I don't think this is specifically PIC-related, so please feel free
to reply to me privately if you think the list wouldn't be interested
in your answer.

I was having a problem the other day where my 20MHz 16C63/JW seemed
to be running at more like 6-7MHz.  I wrote up a description of
the problem and almost submitted it to the list, but then I decided
to spend $0.75 of my own money buying another 20MHz crystal first.
Sure enough, the new crystal works fine.

I suspect and hope that the first crystal was just mislabelled.
Or is this something that could happen again?  Can damage to a
crystal change its frequency, rather than just break it?

Further details: I have no oscilloscope, but as far as my blinkenlights
programs showed, the first crystal worked steadily and reliably,
just at the wrong rate.  I have the OSC1/OSC2 capacitors installed
as recommended in the data book (22pF), and the OSC TYPE bits are
set to "HS" (assuming that the Picstart Plus software is doing its
job).  I don't think it's a marginal 16C63; I'm using two of them,
and in any case the new 20MHz crystal works with both.

Brian

1997\06\03@155221 by John Payson

picon face
> I was having a problem the other day where my 20MHz 16C63/JW seemed
> to be running at more like 6-7MHz.  I wrote up a description of
> the problem and almost submitted it to the list, but then I decided
> to spend $0.75 of my own money buying another 20MHz crystal first.
> Sure enough, the new crystal works fine.
>
> I suspect and hope that the first crystal was just mislabelled.
> Or is this something that could happen again?  Can damage to a
> crystal change its frequency, rather than just break it?

A crystal, like many other resonant systems, can either oscillate at
a "base" frequency, or at (approximate) multiples thereof.  For example,
an undamped "A" string on a violin may resonate at 440Hz, 880Hz, 1320Hz,
etc.  The third harmonic may be produced by playing the string while
lightly touching the string 1/3 of the way up the fingerboard (pushing
down firmly would generate 660Hz, not 1320Hz); even after releasing the
lightly-touching finger, the string will often continue to vibrate at
the higher pitch.

In the case of a 20Mhz crystal, most manufacturers design the crystal
so it's fundamental mode of operation is at 6.667MHz (20/3) but then
modify the shape so that it prefers to oscillate at the third harmonic
rather than the fundamental (analagous to the light finger touch on the
violin string).  Apparently, however, that particular crystal favors the
fundamental instead for some reason.  This may result from a manufacturing
defect, or from damage the crystal has received somehow (either mechanical
shock or excess drive level).

FYI, the only time I've had trouble with a crystal oscillating at the wrong
mode was when a 32Khz crystal wanted to run at about 160Khz.  Same phenom-
enon but the other direction.

1997\06\04@004227 by Mike Smith

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face
{Quote hidden}

They can be fragile - eg dropping one can either stuff it completely or
shift its frequency.  Alternatively the seal on the case may have bean
damaged, with similar effects.

MikeS
<mikesmith_ozspamKILLspamrelaymail.net>

1997\06\05@063639 by Frank A. Vorstenbosch

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Brian Scearce <.....blsKILLspamspam.....BEST.COM> wrote:
>
> I don't think this is specifically PIC-related, so please feel free
> to reply to me privately if you think the list wouldn't be interested
> in your answer.
>
> I was having a problem the other day where my 20MHz 16C63/JW seemed
> to be running at more like 6-7MHz.  I wrote up a description of
> the problem and almost submitted it to the list, but then I decided
> to spend $0.75 of my own money buying another 20MHz crystal first.
> Sure enough, the new crystal works fine.

Sounds to me that the first crystal was actually a third-overtone one.
These are commonly used for higher frequencies because otherwise the
crystal would be too thin.  By using an LC filter, the oscillator can
filter out the fundamental frequency and actually oscillate on three
times the fundamental.  The same trick can be used to use higher overtones
(5th,7th, etc) for even higher frequencies.
The Pierce oscillator that's used in the PICs could probably be modified
with such a filter, but it is cheaper to get 'real' 20MHz crystals.

Frank

------------------------------------------------------------------------
Frank A. Vorstenbosch                           Phone:  +44-181-636 3391
Electronics & Software Engineer                    or:  +44-181-636 3000
Eidos Technologies Ltd., Wimbledon, London      Mobile:  +44-976-430 569

1997\06\06@061521 by lmclaren

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Brian Scearce wrote:
{Quote hidden}

I believe that what you are seeing is the wrong capacitive load on the
xtal.
Try replacing the 30pf? capacitors with say 20pf capacitors.
I have found xtals 20Mhz and above very fussy about the capacitive load.

regards

--
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