I've run into this problem with a few of my chips.  I suspect the major
reason is the extra capacitance from the breadboards I use.  One person
I talked to awhile ago measured 20pF extra capacitance from wire to wire
on their breadboard...  (I doubt you'll ever find your breadboard that
high, but they still have enough to mess with sensitive circuits)

But I've never needed the extra speed badly enough to really look into
it hard.  Use a wire wrap socket for the pic, bend the osc pins out and
up so they don't go into the breadboard, and solder the osc (or another
socket) directly to those pins with a flying lead to ground (or to the
capacitors).

-Adam

Fowler, Paul B. wrote:

>...and what can cause my PIC16F84A-20P to work with a 4MHz resonator but not
>a 20MHz resonator?
>
>I have tried two different 20 MHz resonators and the chip doesn't work with
>either...  4MHz is just fine.  The application is a simple LED turn on and
>keep on, so speed should make no difference.
>
>Any ideas?  - Programmed with MPLab and the properties for the project
>specify 20MHz.  XT is specified as the type...
>
>I don't need the extra speed at the moment, but I am curious what I might be
>missing.  My tools to track this problem are not advanced - no oscillator,
>etc...  A logic probe is about all I have.
>
>Thank you,
>Fowler
>
>-----Original Message-----
>From: Rick C. [mailto:rixy@VISUALLINK.COM]
>Sent: Sunday, March 10, 2002 10:34 PM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [PIC]: PIC crystal...
>
>
>Maximum speed of the chip.
>Rick
>
>Shirollah Ghadirian wrote:
>
>>Hi all,
>>
>>When it says on the chip 04 or 08 for the speed of the chip, is it the
>>crystal value or 1/4 of the crystal value.
>>
>>Thanks in advance.
>>
>>--
>>http://www.piclist.com hint: The PICList is archived three different
>>ways.  See http://www.piclist.com/#archives for details.
>>
>
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>
>
>
>

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