2 PICs and 1 Oscillator?

weg22 · Joined: 08 Jul 2005 Posts: 91

Hi all,

I'm designing a PCB with 2 PICs on it. I was wondering if I can save space by using just one oscillator (i.e. feeding the output of the oscillator into both PICs CLK pin)?

Thanks,
-weg

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Do you truly have an oscillator ?

A crystal looks like this:
http://rocky.digikey.com/WebLib/ECS/Web%20Photos/HC-49US.jpg

An oscillator looks like this:
http://rocky.digikey.com/WebLib/CTS/Web%20photos/MXO45.jpg
(Though it can also be a small surface mount package).

weg22 · Joined: 08 Jul 2005 Posts: 91

Yes, I have the MX045HS 10MHz oscillator.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

You can do this, but ideally you would have the oscillator drive two
buffers which are placed near the oscillator's output pin. Each buffer
would then drive the clock signal going to each separate PIC. The
buffers could be something like 74AHC04 gates. A series resistor
might be used on the output of each buffer, to reduce ringing, if the
trace has to run for more than a short distance.

You may not want to add the separate buffer chip(s). So another
solution would be to use 22 ohm series resistors instead of the buffer
chips. The two resistors would be placed very close to the output pin
of the oscillator. The traces leaving the resistors (one to each PIC)
would be of equal length, and preferably they would be short (2 inches
or 5 cm each).

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

I have done this with nothing at all.
I just had the osc. run to both pics on a single board.
In my case both pics DIDN'T have to
be synced up in any way.
I used this part.
http://catalog.digikey.com/scripts/partsearch.dll?Detail?name=CTX116-ND

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I was just trying to be correct. I have Mr. Johnson's book on High Speed
Digital Design and I did consult it for this. The professors are always
deathly afraid of daisy-chaining clocks, etc. In the real world I might
have to do that, and I'd just use a single series resistor at the oscillator
output. I'd look at the signals to make sure that I had clean waveforms.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I doubt you have to worry about "high speed" stuff at 10MHz unless you have very long traces. I have run a 20MHz TTL oscillator through a couple of paralled HC logic gates as a buffer, then driving an 18" backplane driving a PIC and an HC logic gate on daughter card every 1.5". I put pads for a terminating RC at the end of the backplane, but didn't need to install it. The PIC expects a near sine wave from a crystal, so the waveform can be real bad without being a problem.
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