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'[PIC]: Crystal reccomendation'
2000\12\08@004209 by Dave Bell

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Olin Lathrop (spam_OUTolin_piclistTakeThisOuTspamEMBEDINC.COM) wrote:

>> I have to say you are probably better served buying the complete
>> oscillator from Digi-key.
>> I just got 10 of them (20 Mhz.) at $2.50 each. No muss, no fuss, they
>> just work.
>> Small package, (4 pins in an 8 pin dip), and low power.

>That can be a convenience for a low volume product, but will cost at
>least double what the crystal and two caps cost.  One good reason for
>having an external oscillator is if you are driving multiple devices from
>the same clock.

I ended up picking up some oscillators for this job.
1) It's only a couple-off project, so no big cost constraint.
2) Bound to be more reliable, and easier to get working right.
3) The greater precision is an advantage here.
4) I realized I could use the OSC_OUT (At CLK/4) as a deglitch
  clock for a BiPhase-L output that got added at the last minute!

Thanks for all the suggestions!

Dave

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2000\12\08@074352 by mike

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On Thu, 7 Dec 2000 21:39:13 -0800, you wrote:

>Olin Lathrop (.....olin_piclistKILLspamspam@spam@EMBEDINC.COM) wrote:
>
>>> I have to say you are probably better served buying the complete
>>> oscillator from Digi-key.
>>> I just got 10 of them (20 Mhz.) at $2.50 each. No muss, no fuss, they
>>> just work.
>>> Small package, (4 pins in an 8 pin dip), and low power.
>
>>That can be a convenience for a low volume product, but will cost at
>>least double what the crystal and two caps cost.  One good reason for
>>having an external oscillator is if you are driving multiple devices from
>>the same clock.
>
>I ended up picking up some oscillators for this job.
>1) It's only a couple-off project, so no big cost constraint.
>2) Bound to be more reliable, and easier to get working right.
>3) The greater precision is an advantage here.
>4) I realized I could use the OSC_OUT (At CLK/4) as a deglitch
>   clock for a BiPhase-L output that got added at the last minute!
OSCOUT will be the same as the input frequency - it's only fosc/4 in
RC mode. It might be possible to get the PWM or compare output to run at Fosc -
can't remember.
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2000\12\08@133159 by Dan Michaels

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Mike Harrison wrote:
.........
>>4) I realized I could use the OSC_OUT (At CLK/4) as a deglitch
>>   clock for a BiPhase-L output that got added at the last minute!
>OSCOUT will be the same as the input frequency - it's only fosc/4 in
>RC mode.
>It might be possible to get the PWM or compare output to run at Fosc -
>can't remember.
>

By using the 2 LSBs of the 10-bit PWM, you can get pulses out that
are only a quarter cycle long. With a 20 Mhz xtal, I use this technique
to generate 50 nsec pulses at a 5Mhz rate. Since the PR2 register
is totally programmable, you are not locked into only a few
PWM frequencies. You would set PR2 = 00h, IIRC, to get this rate.

Can also get 100/175/... nsec:

-___-___ or --__--__ or ---_---_

50ns        100ns       175ns     <-- pulse width

The center one will be 50% duty cycle at 5 Mhz even. You cannot get
the period < 200 nsec ~ 1/5Mhz for 20Mhz xtal.

- danM

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