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PICList Thread
'Strange results for oscillator frequencies'
1998\12\14@175853 by Dan Rosenfeld

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Hi all,

I wonder if anyone can help me understand the strange results I see
when I point my oscilloscope at the OSC pins of my 16f84. I've got a
wire-wrapped prototype with an RC oscillator connected to OSC1. Current
values of R and C are 10K Ohms and 20pF. I also looked at the same
circuit with the 20pF cap removed--I imagine the wire-wrapping, etc.
provides enough capacitance to get the oscillator going.

Estimating the frequencies with my $20 flea market oscilloscope I get:

               w/20pF cap              w/o cap

       OSC1    290khz                  310kHz

       OSC2    670kHz                  2.2mHhz


So my first question is, why am I not seeing the roughly 2.5mHz predicted
in the data sheet for these values of R and C.

More puzzling, though, is that I don't see anything like the expected 4/1
ratio of OSC1's frequency to OSC2's frequency. In fact, OSC2's frequency is
always at least twice that of OSC1.

Any clues? (BTW, I wouldn't rule out anything obvious--I'm very much a
beginner at this stuff.)


Dan

1998\12\14@181507 by Sean Breheny

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Hi Dan,

The first thing I would try would be to use a bigger capacitor. See if the
results are closer to predicted then. Also, make sure that you are setting
the PIC up in RC mode (using the configuration word in your assembler).
BTW, does the waveform look right on your scope? I think it should look
like a sawtooth wave on OSC1 and a square wave on OSC2.

Good luck,

Sean


At 02:45 PM 12/14/98 -0800, you wrote:
{Quote hidden}

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1998\12\14@182450 by Dave Celsnak

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Scope probes tend to have too much capacitance in them, sometimes
stopping the oscillator all-together.

My 2 cents,
Dave

>Date:         Mon, 14 Dec 1998 14:45:05 -0800
>Reply-To:     pic microcontroller discussion list
<.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
{Quote hidden}

predicted
>in the data sheet for these values of R and C.
>
>More puzzling, though, is that I don't see anything like the expected
4/1
>ratio of OSC1's frequency to OSC2's frequency. In fact, OSC2's
frequency is
>always at least twice that of OSC1.
>
>Any clues? (BTW, I wouldn't rule out anything obvious--I'm very much a
>beginner at this stuff.)
>
>
>Dan


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1998\12\14@183302 by dave vanhorn

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At 03:20 PM 12/14/98 -0800, you wrote:
>Scope probes tend to have too much capacitance in them, sometimes
>stopping the oscillator all-together.


Usually an RC should be ok, but with 20pF cap, the probe's 10pF will be
VERY significant.
Probe it only on the OUT pin if you must, otherwise use RF pickup and a
shortwave receiver to get it.


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1998\12\14@191147 by Sean Breheny

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At 03:20 PM 12/14/98 PST, you wrote:
>Scope probes tend to have too much capacitance in them, sometimes
>stopping the oscillator all-together.

Yeah, I wonder if he is even using an x10 probe? If not, the cap of the
probe could easily be 60pF.

>
>My 2 cents,
>Dave

Sean

+-------------------------------+
| Sean Breheny                  |
| Amateur Radio Callsign: KA3YXM|
| Electrical Engineering Student|
+-------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
EraseMEshb7spam_OUTspamTakeThisOuTcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\12\14@192350 by Dan Rosenfeld

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Sean Breheny wrote:
> The first thing I would try would be to use a bigger capacitor. See if the
> results are closer to predicted then.

I'll try this, thanks.

> Also, make sure that you are setting
> the PIC up in RC mode (using the configuration word in your assembler).

PICStart plus claims it's programming the configuration word
for an RC oscillator.

> BTW, does the waveform look right on your scope? I think it should look
> like a sawtooth wave on OSC1 and a square wave on OSC2.

Yup. That's what I see.

Thanks again,

Dan

1998\12\14@203223 by Dan Rosenfeld

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Sean Breheny wrote:
>
> At 03:20 PM 12/14/98 PST, you wrote:
> >Scope probes tend to have too much capacitance in them, sometimes
> >stopping the oscillator all-together.
>
> Yeah, I wonder if he is even using an x10 probe?

He is not. In fact he would be embarassed if they saw what was attached to his
scope.

> If not, the cap of the probe could easily be 60pF.

This explanation (probe capacitance) make a lot of sense.

With R = 10k P = 20pF the expected OSC1 frequency is 2.66mhz which is almost
exactly(!) 4 times the OSC2 frequency I measured. So I guess the conclusion
is... OSC2 measurement, valid; OSC1 frequency, bogus.

So, thanks very much for clearing this up.

Dan

1998\12\15@013428 by Mike Keitz

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On Mon, 14 Dec 1998 14:45:05 -0800 Dan Rosenfeld wrote:
{Quote hidden}

Both results can be explained with the hypothesis that when the
oscilloscope is connected to OSC1, the oscillator slows way down.  The
scope must have a lot of capacitance.  Connecting it to OSC2 allows the
oscillator network on OSC1 to run at full speed.  With the 20 pF
capacitor, the PIC is indeed running about 2.6 MHz (670 KHz * 4).

Also realize that old analog oscilloscopes aren't very well optimized for
measuring time or frequency.  Expect errors of 10% or more.

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