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'Another Crystal Question'
1999\02\12@080755 by verhage

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I ran out of time last night, but was able to put 2 18pF caps on the
crystal connected to OSC1 and OSC2 of the PIC programmed as a MIM
Module.  Still no joy.  How sensitive are crystals to their caps?  I
have 2 22pF caps to try next.  Is it possible that a difference of 4
pF will make a difference?

Thanks.
Lloyd

1999\02\12@102513 by dave vanhorn

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At 07:06 AM 2/12/99 +0000, Lloyd Verhage wrote:
>I ran out of time last night, but was able to put 2 18pF caps on the
>crystal connected to OSC1 and OSC2 of the PIC programmed as a MIM
>Module.  Still no joy.  How sensitive are crystals to their caps?  I
>have 2 22pF caps to try next.  Is it possible that a difference of 4
>pF will make a difference?
>
>Thanks.
>Lloyd


Not likely. If the xtal isn't singing now, I'd suspect it or the processor
is dead.
You did return the caps to the processor's ground, right? Otherwise it
could cause the osc to stop.

1999\02\12@102848 by verhage

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> >I ran out of time last night, but was able to put 2 18pF caps on the
> >crystal connected to OSC1 and OSC2 of the PIC programmed as a MIM
> >Module.  Still no joy.  How sensitive are crystals to their caps?  I
> >have 2 22pF caps to try next.  Is it possible that a difference of 4
> >pF will make a difference?

> Not likely. If the xtal isn't singing now, I'd suspect it or the processor
> is dead.
> You did return the caps to the processor's ground, right? Otherwise it
> could cause the osc to stop.

I soldered to caps on the underside of the PCB.  They each connected
a lead/pad of the crystal to the ground plane.  Otherwise, I didn't
modify anything else on the PCB.

Thanks,
Lloyd

1999\02\12@103332 by dave vanhorn

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>I soldered to caps on the underside of the PCB.  They each connected
>a lead/pad of the crystal to the ground plane.  Otherwise, I didn't
>modify anything else on the PCB.
>
>Thanks,
>Lloyd

Check for a short to ground on each side, I assume this is the MIM pcb?
If so, I'd say return the processor for an exchange.

1999\02\12@170247 by paulb

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Lloyd Verhage wrote:

> I ran out of time last night, but was able to put 2 18pF caps on the
> crystal connected to OSC1 and OSC2 of the PIC programmed as a MIM
> Module.  Still no joy.

 Noted Dave's comments.  Can you just reassure me that you *are*
assessing crystal operation using the receiver and not by test equipment
connected to the crystal (which will load it, possibly to the point of
non-operation)?

 Lack of crystal operation is certainly a critical factor in the
function of the device, but by no means the only one.

 Your comments indicate you are actually prototyping your own PCB
design, so everything must be taken stepwise.  You are using an
absolutely standard pre-programmed PIC from Clement?

 Have you checked the voltage on the PIC MCLR (Pin 1)?  In fact, you
have presumably checked all rail voltages?  You have made sure *both*
Vss rails are securely grounded (pins 8 *and* 19)?  There should be a
track on the PCB between these two.  The crystal is pins 9 and 10.  As
Dave mentioned, the crystal should be adjacent to pin 9 and its caps
should return directly to the connection of pins 8 and 19.
--
 Cheers,
       Paul B.

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