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'[PIC]: My 16F877/20 enters in coma at 20 Mhz !!'
2001\05\25@140052 by joan

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Hi, PicDoctors,

I am trying to work with a 16F877/20 at 20 MHz. I program it
with the HS option bit set, and have tried different capacitor
values : 27 pF and 47 pF. Nothing at all : I look with my scope
in the OSCOUT pin and the encephalogram is flat.

If I substitute the 20 MHz for a 4 Mhz one, my 16F877 comes to live
again (even without changing the HS bit for the XT one). If I
use the 4 Mhz quartz with the XT bit set, it is also OK.

Please, PicDoctors, can some of you suggest how can I reanimate
my 16F877 at 20 MHz ?

Thanks !

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    Joan Ilari                 e-mail : spam_OUTjoanTakeThisOuTspamilari.org
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2001\05\25@143151 by Quentin

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Using ICD currently on a 16F876/20 . Using 15pF caps on a production
board. Works perfectly.
Just realised my OSCI setting is still set at XT . Forgot to change it
to HS when I went from 4 Mhz prototype to 20 Mhz production.
Then again, I also have not tested it without ICD.
Maybe you should also check that your PIC is initialized properly.

Quentin

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2001\05\26@052004 by Peter L. Peres

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Try a 20MHz crystal specified for parallel operation. Most 20MHz crystals
that are unspec'd are either series or 6.66 MHz 3rd overtone and will not
move at 20MHz without a tank circuit in a normal 'gate' oscillator.

Peter

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2001\05\26@060534 by joan

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What have I to ask for when in my electronics shop : a parallel operation
20 Mhz crystal quartz ? What about the cutting type of crystal ? Is there
any page where all this is explained ?

Gracies !

Joan

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2001\05\27@144504 by Peter L. Peres

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> What have I to ask for when in my electronics shop : a parallel
> operation 20 Mhz crystal quartz ? What about the cutting type of
> crystal ? Is there any page where all this is explained ?
>
> Gracies !
>
> Joan

Joan, ask for a parallel operation 20MHz crystal for microprocessor use
(probably 100 ppm tolerance). Ask for the recommended caps for it. Better
sources will tell you this (like Mouser etc). Buy two identical crystals.
If you ever have a doubt about having damaged one, swap them. If it's
still the same problem it's very unlikely that it's the crystal. This also
applies for other key parts in the circuit.

good luck,

Peter

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2001\05\27@145542 by David VanHorn

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At 09:07 PM 5/27/01 +0300, Peter L. Peres wrote:
> > What have I to ask for when in my electronics shop : a parallel
> > operation 20 Mhz crystal quartz ? What about the cutting type of
> > crystal ? Is there any page where all this is explained ?
> >
> > Gracies !
> >
> > Joan
>
>Joan, ask for a parallel operation 20MHz crystal for microprocessor use
>(probably 100 ppm tolerance). Ask for the recommended caps for it.

Beware. The loading capacitance value (typical 20pF) needs to be doubled,
and then cut back a bit for typical apps.
http://www.dvanhorn.org/Micros/All/Crystals.php   has the details.


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2001\05\28@025024 by Peter L. Peres

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> Beware. The loading capacitance value (typical 20pF) needs to be
> doubled, and then cut back a bit for typical apps.
> http://www.dvanhorn.org/Micros/All/Crystals.php has the details.

Better sources tell you directly what caps to use (not the real load cap).
So be sure NOT to double those. Double if they tell you 'load cap
is X pF'. Do not double if they say 'use X pF capacitors'. Beware, Dave.
;-)

Peter

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2001\05\28@025442 by David VanHorn
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At 09:05 AM 5/28/01 +0300, Peter L. Peres wrote:
> > Beware. The loading capacitance value (typical 20pF) needs to be
> > doubled, and then cut back a bit for typical apps.
> > http://www.dvanhorn.org/Micros/All/Crystals.php has the details.
>
>Better sources tell you directly what caps to use (not the real load cap).
>So be sure NOT to double those. Double if they tell you 'load cap
>is X pF'. Do not double if they say 'use X pF capacitors'. Beware, Dave.
>;-)

How would they know what your parasitics are in your circuit?
Granted, they can probably give you a value within 20% or so.

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