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'[PIC]: Oscillator options, or Oops, I bought a ser'
2003\05\11@190705 by Mike Hord

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So I was digging through the parts box in search of a 20 Mhz crystal , and I
turned up a few.  Remembering when I bought them and not trusting myself, I
looked up the part number at Digikey and sure enough, they're listed as
series cut crystals.

Digikey Part Number CTX-062.

My question is this:  can I rescue this crystal from oblivion and use it on
my current demo board project?  I'd really like to get fairly good accuracy
out of the crystal for this project, since I'll be using it as a develop
platform so I can finally flush my breadboard.

I might be okay with as high as a 200 ppm error...HELP!!!

Mike H.

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2003\05\11@213830 by Larry Bradley

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I found about 160 ppm error with a 20 mhz serial xtal using 18 pf
capacitors. As long as you know this, you can take it into account when
doing timing via TMR0, 1 etc.

Larry

At 06:06 PM 5/11/2003 -0500, you wrote:
{Quote hidden}

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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2003\05\12@084312 by Dave VanHorn

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>>
>>My question is this:  can I rescue this crystal from oblivion and use it on
>>my current demo board project?  I'd really like to get fairly good accuracy
>>out of the crystal for this project, since I'll be using it as a develop
>>platform so I can finally flush my breadboard.
>>
>>I might be okay with as high as a 200 ppm error...HELP!!!

Therein lies the rub.
It will almost certainly run, but it won't run at the nameplate frequency
in a parallel circuit.
Don't even try to put it there.  All crystals have a series and a parallel
resonant point, fairly close together, but they cannot be at the same point.
It's behavior is only certified at one point though.  I'm not clear on what
compromises they might make to the behavior on one side, to optimize the
other.

I would suggest putting it in circuit, and checking the frequency with a
counter or SW receiver.

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2003\05\12@090205 by Dave Tweed

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Dave VanHorn <.....dvanhornKILLspamspam.....CEDAR.NET> wrote:
> It's behavior is only certified at one point though.  I'm not clear on what
> compromises they might make to the behavior on one side, to optimize the
> other.

None whatsoever. It's simply a question of which mode the crystal was
calibrated for during the manufacturing process. If it was calibrated to a
certain frequency in series-resonant mode, there's simply no way you can
pull it to the same frequency in parallel mode.

-- Dave Tweed

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2003\05\12@093224 by Dave VanHorn

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>
>None whatsoever.

I talked to a fellow at california crystals years ago, he told me that they
did things to optimize the operating point, that compromised the other
point, but he didn't want to clarify it.  I've taken that as an operating
principle to use xtals in their proper mode, and not to try taking them
places they might not want to go. :)
I don't need even a 1 in 10000 incidence of "osc won't start reliably"..

>It's simply a question of which mode the crystal was
>calibrated for during the manufacturing process. If it was calibrated to a
>certain frequency in series-resonant mode, there's simply no way you can
>pull it to the same frequency in parallel mode.

Absolutely.

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2003\05\12@164244 by Olin Lathrop

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> My question is this:  can I rescue this crystal from oblivion and use
> it on my current demo board project?  I'd really like to get fairly
> good accuracy out of the crystal for this project, since I'll be
> using it as a develop platform so I can finally flush my breadboard.
>
> I might be okay with as high as a 200 ppm error...HELP!!!

How much trouble is it worth to save a <$1.00 part?  I would buy the right
crystal and keep the othwer ones around.  Some day they may be just what
you want, or you need a stand alone crystal oscillator you can make with a
few CMOS inverters.


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