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PICList Thread
'Shared Clock?'
1998\04\16@063616 by Caisson

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> Van: Solon Caceres Moreno <spam_OUTcdmbTakeThisOuTspamMULTINET.COM.CO>
> Aan: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Onderwerp: q: Shared Clock?
> Datum: donderdag 16 april 1998 2:10
>
> I Am designing a project with three PICS (16C71, 2X16C57).  I can use an
> only clock to control the three PICS?
>
> Thanks in advance,

Yes, If you supply an external clock on all three OSC1 -pins.
You could try to give one PIC an oscillator, and connect (again) all three
OSC1 -pins. But due to power-consumption by the OSC1 -pin this could
fail.

Let me know if (when u use the latter possebility) it works.

Greetz,
 Rudy Wieser

P.S.
 The documents about the different PIC's show how the external clock
should be provided.  with a 74AS04 (3/4 of it actually).

1998\04\16@113227 by Mike Keitz

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On Thu, 16 Apr 1998 09:36:15 +0200 Caisson <caissonspamKILLspamTELEBYTE.NL> writes:
>> Van: Solon Caceres Moreno <.....cdmbKILLspamspam.....MULTINET.COM.CO>
>> Aan: EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU
>> Onderwerp: q: Shared Clock?
>> Datum: donderdag 16 april 1998 2:10
>>
>> I Am designing a project with three PICS (16C71, 2X16C57).  I can
>use an
>> only clock to control the three PICS?
>>
>> Thanks in advance,
>
>Yes, If you supply an external clock on all three OSC1 -pins.

The official method is to use an external clock source connected to the
OSC1 pins of all the PICs.  The PICs should be set to LP, XT, or HS mode
depending on the clock rate.  The external clock must meet the input high
and low voltage specifications for the OSC1 pin.  Many of the canned
"TTL" oscillators don't.  If using an oscillator can (which I think is
much better than trying to build a gate oscillator), be sure it is rated
for CMOS level output.

>You could try to give one PIC an oscillator, and connect (again) all
>three
>OSC1 -pins. But due to power-consumption by the OSC1 -pin this could
>fail.

The PIC's "oscillator" is actually an amplifier with input to OSC1 and
output from OSC2.  Microchip does not recommend connecting anything to
the OSC2 pin other than the crystal circuit.  However, it is possible to
take a clock signal from the OSC2 pin of an oscillating PIC.  For
example, the OSC2 pin of a PIC with crystal can be connected to the OSC1
pin of another PIC with no crystal, providing it an external clock.  The
load must be light and the wiring short to not interfere with the
oscillator.  Since the load has capacitance, the capacitor usually
connected to OSC2 can be reduced or removed.

For more than two PICs, connect them in a chain with OSC2 of one driving
OSC1 of the next one.  All PICs will run at the same frequency but not
exactly in phase.  It may be a good idea to couple through capacitors of
0.01 uF or so to let each PIC establish its own DC bias.

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1998\04\16@120410 by Mauro, Chuck

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Mike,

Good advice!  That's what I love about this list - little pearls of info
that can be tucked away in a folder, to brought out when a similar
problem comes up in one's own work...  I wish I had time to contribute
more than I receive...

Ciao,

Chuck Mauro
RAPID Engineering
KLA-Tencor

> {Original Message removed}

1998\04\16@122527 by Matt Bonner

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Mike Keitz wrote:
> The official method is to use an external clock source connected to the
> OSC1 pins of all the PICs.  The PICs should be set to LP, XT, or HS mode
> depending on the clock rate.  The external clock must meet the input high
> and low voltage specifications for the OSC1 pin.  Many of the canned
> "TTL" oscillators don't.  If using an oscillator can (which I think is
> much better than trying to build a gate oscillator), be sure it is rated
> for CMOS level output.

If power is an issue, you can set the configuration fuses for LP no
matter what the frequency.  All the fuses are doing is establishing the
amount of feedback to drive the crystal.  An external oscillator doesn't
require this feedback.  Most oscillators probably have enough drive for
a handful of PICs so, for reliability, don't use OSC2 as an output.

--Matt

1998\04\16@165905 by Alex Torres
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> From: Caisson <caissonspamspam_OUTTELEBYTE.NL>

> > I Am designing a project with three PICS (16C71, 2X16C57).  I can use
an
> > only clock to control the three PICS?
> >
> > Thanks in advance,
>
> Yes, If you supply an external clock on all three OSC1 -pins.
> You could try to give one PIC an oscillator, and connect (again) all
three
> OSC1 -pins. But due to power-consumption by the OSC1 -pin this could
fail.

OSC1 ? May be OSC2 !

>
> Let me know if (when u use the latter possebility) it works.

What about using pic's generators in series ?

 OSC1.1    OSC2.1-----OSC1.2   OSC2.2
  |___________cristal_____________|
  |                                                                |
cap                                                          cap
  |                                                                |
gnd                                                          gnd

=============
Alex Torres, Kharkov, Ukraine (exUSSR)
@spam@altorKILLspamspamgeocities.com
2:461/28 FidoNet
http://www.geocities.com/SiliconValley/Lab/6311

1998\04\17@015141 by Pasi T Mustalahti

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On Thu, 16 Apr 1998, Mike Keitz wrote:

> For more than two PICs, connect them in a chain with OSC2 of one driving
> OSC1 of the next one.  All PICs will run at the same frequency but not
> exactly in phase.  It may be a good idea to couple through capacitors of
> 0.01 uF or so to let each PIC establish its own DC bias.

PTM: How about this: connect 2N+1 PIC's in series: osc21-osc12,
osc22-osc13..osc2(2N1)-osc1(2N+1). And then the Xtal connected between
osc1 of the first PIC to osc2 of the last osc2(2N+1)-osc11 :)
That would be a real loop just for the fun of it.

We have resistors in series and parallel. How about uP's in series,
Would the 'resistance' be 1/R=sum(1/r) or R=sum(r) ? ;)

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1998\04\17@063221 by Caisson

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> Van: Alex Torres <spamBeGonealtorspamBeGonespamGEOCITIES.COM>
> Aan: TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU
> Onderwerp: Re: Shared Clock?
> Datum: donderdag 16 april 1998 14:47
>
> > From: Caisson <RemoveMEcaissonspamTakeThisOuTTELEBYTE.NL>
[Cut]
> > Yes, If you supply an external clock on all three OSC1 -pins.
> > You could try to give one PIC an oscillator, and connect (again) all
> three
> > OSC1 -pins. But due to power-consumption by the OSC1 -pin this could
> fail.
>
> OSC1 ? May be OSC2 !

No, I did mean OSC1.  Those are the input-pins.  OSC2 are Output pins.

{Quote hidden}

That probably wouldn't work, because OSC2.1 is OSC1.1 _inverted_.
Your OSC2.2 has the same phase as OSC1.1. The crystal wouldn't
see any voltage difference ...
Maybe it would work with three PIC's ?

Oh, an After-thought : What would happen if one of the chained PIC's
would go into Sleep ? :-)

> =============
> Alex Torres, Kharkov, Ukraine (exUSSR)
> altorEraseMEspam.....geocities.com
> 2:461/28 FidoNet
> http://www.geocities.com/SiliconValley/Lab/6311

Greetz,
 Rudy Wieser

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