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'[Pic]: Measuring resonator frequency'
2002\02\01@075455 by Dans_PIC_Stuff

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How can I connect a scope to a resonator to measure it's actual frequency?
I took a 16F84 with a 4MHz resonator to my Electronics AC circuits class and
tried testing it on one of their scopes but it didn't look too prety and it
made the PIC's program slow down dramatically.  Any tips?

  -Dan

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2002\02\01@081751 by Peter Onion

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On 01-Feb-02 Dans_PIC_Stuff wrote:
> How can I connect a scope to a resonator to measure it's actual frequency?
> I took a 16F84 with a 4MHz resonator to my Electronics AC circuits class and
> tried testing it on one of their scopes but it didn't look too prety and it
> made the PIC's program slow down dramatically.  Any tips?

Does your 'scope probe have a "x10" setting ?

If so try using that as it will have less of a effect on the oscillator circuit.
(Don't forget that the scope will display one tenth of the signal apmlitude).

Also it sounds that the oscillation is marginal if connecting a scope probe can
upset the clock speed of the PIC.  Are you using the recomended capacitors in
the circuit ?

Peter.

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2002\02\01@134747 by Andrew Warren

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Dans_PIC_Stuff <spam_OUTPICLISTTakeThisOuTspammitvma.mit.edu> wrote:

> How can I connect a scope to a resonator to measure it's actual
> frequency? I took a 16F84 with a 4MHz resonator to my Electronics AC
> circuits class and tried testing it on one of their scopes but it
> didn't look too prety and it made the PIC's program slow down
> dramatically.  Any tips?

Dan:

Put the scope probe on the Xtal OUT pin, not Xtal IN.

-Andy

=== Andrew Warren -- .....aiwKILLspamspam@spam@cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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2002\02\01@152015 by Dans_PIC_Stuff

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I'm using this as the resonator.  It has built in caps.
http://www.ecsxtal.com/pdf2/ZTT.PDF

ZTT-4.00MG
 -Dan


{Original Message removed}

2002\02\01@152222 by Dans_PIC_Stuff

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I'll try that next class.  I also tried using 2 hhannels, one on each
resonator pin of the PIC.
  -Dan
{Original Message removed}

2002\02\01@152954 by Martin Peach

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If you have a probe on the oscillator input the probe capacitance will load
the input and skew the frequency. Similarly it's usually a waste of time
looking at the input pin of an op-amp with a scope, because the op-amp is
trying to make it a virtual ground (if it's wired as an amplifier).
/\/\/\/*=Martin

{Original Message removed}

2002\02\01@171034 by Dave Dilatush

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Dans_PIC_Stuff wrote...

>How can I connect a scope to a resonator to measure it's actual frequency?
>I took a 16F84 with a 4MHz resonator to my Electronics AC circuits class and
>tried testing it on one of their scopes but it didn't look too prety and it
>made the PIC's program slow down dramatically.  Any tips?

Best thing to do is put the PIC into a simple program loop that toggles
an I/O pin every N clock cycles; then measure the frequency at this pin
and multiply by 4 * N to get the actual resonator frequency.

That way you avoid any kind of loading on the resonator or oscillator
that might influence the frequency.

Dave

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2002\02\02@053243 by Roman Black

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Dave Dilatush wrote:
>
> Dans_PIC_Stuff wrote...
>
> >How can I connect a scope to a resonator to measure it's actual frequency?
> >I took a 16F84 with a 4MHz resonator to my Electronics AC circuits class and
> >tried testing it on one of their scopes but it didn't look too prety and it
> >made the PIC's program slow down dramatically.  Any tips?
>
> Best thing to do is put the PIC into a simple program loop that toggles
> an I/O pin every N clock cycles; then measure the frequency at this pin
> and multiply by 4 * N to get the actual resonator frequency.
>
> That way you avoid any kind of loading on the resonator or oscillator
> that might influence the frequency.


That's how I do it for accuracy, but in the
field a good trick is to use your scope lead
on "10x" which is usually about 1 megohm
impedance and touch on the osc OUTPUT pin,
it's pretty obvious from the scope display
which is IN and OUT. I have also grabbed a
10 megohm resistor and added that to an
"alligator" tip, you lose some amplitude but
you can get an ok measurement of freq.
-Roman

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2002\02\02@053733 by Peter L. Peres

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Make sure you use a 1:10 probe (10Meg), that the caps are connected to the
crystal, and that the PIC has been programmed in XT or HS oscillator
mode, then scope the osc out pin (the one near the +5V pin).

Peter

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