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'[PIC]: Share crystal between 2 PICs'
2003\05\07@022149 by Thomas N

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Hi everyone, I have 2 PIC16F877s on my design.  I am wondering if I can use one crystal for both micros.  Is there any problem?  Do I just directly connect the crystal to both pins of both micros?  Please help! Thanks!Thomas

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2003\05\07@023226 by Stuart Meier

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Thomas we did this theme recently see

groups.google.com/groups?hl=en&lr=&ie=UTF-8&frame=right&th=2de7735f9be483
d0&seekm=000901c2dcf4%2423289bb0%240300a8c0%40main#link5

Stuart
{Original Message removed}

2003\05\07@081916 by Olin Lathrop

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> Hi everyone, I have 2 PIC16F877s on my design.  I am wondering if I can
> use one crystal for both micros.  Is there any problem?  Do I just
> directly connect the crystal to both pins of both micros?

The simplest answer is to let one PIC drive the crystal normally, then
have its OSC OUT drive the other PIC's OSC IN.  Leave the second PIC's OSC
OUT floating.

I don't like this unless the two PICs are physically very close on the
same board.  A better answer is to put a buffer between the two.  The
buffer input should be as close to the first PIC OSC OUT pin as you can
manage.

I have driven multiple PICs this way from a single oscillator several
times.


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2003\05\07@091346 by Eduardo Garcia

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Hi thomas!
Remember isolate the OSC out from other pic with a 22pF capacitor.

See literature about IC mt8870. It explain a little about this techinic.

{Original Message removed}

2003\05\07@095155 by Dave VanHorn

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>
>I don't like this unless the two PICs are physically very close on the
>same board.  A better answer is to put a buffer between the two.  The
>buffer input should be as close to the first PIC OSC OUT pin as you can
>manage.

And remember to take into account the extra loading on the osc pin, or your
osc won't start reliably.
This trick done badly caused me to have to fly to Taiwan one Christmas day
because they had stopped the production line due to problems.
I warned the engineer on the project about this, but he insisted that since
all 20 of his protos worked, that there would be no problem..
In production, 50% wouldn't start.  Different date codes, different output
drive capability...

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2003\05\07@104049 by

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Or use one of the pre-built osc's in DIP (or
simmilar) package with MOS/TTL output, and drive
the OSC-In pin of both PIC's in parallel.
That should work without problems, AFAIK.

Jan-Erik.

> Hi everyone, I have 2 PIC16F877s on my design.  I am wondering if I can
> use one crystal for both micros.  Is there any problem?  Do I just
> directly connect the crystal to both pins of both micros?

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