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-----
> From: Mike Keitz [SMTP:mkeitz@JUNO.COM]
> Sent: Thursday, April 16, 1998 8:28 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Shared Clock?
>
> On Thu, 16 Apr 1998 09:36:15 +0200 Caisson <caisson@TELEBYTE.NL>
> writes:
> >> Van: Solon Caceres Moreno <cdmb@MULTINET.COM.CO>
> >> Aan: PICLIST@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.
>
> 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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