piclist 2001\02\17\173658a >
Thread: Oscillator for multiple pic's
face BY : David VanHorn email (remove spam text)

At 05:28 PM 2/17/01 -0500, jhancock wrote:
>What value of resistor should be used in the clock leads?  Is this for
>isolation of the pic clocks?

No, there's no need to isolate them. It's for EMI control.

You're talking about feeding a track a signal with edges that have energy
up through 500 MHz.
Adding some fixed resistance on the source end cuts down on the EMI quite a
bit, by slowing down the risetimes.
Check the PIC spec, but I know you don't need that fast a rise time. Stands
to reason, if it can take a crystal input, which is nearly a sine, then it
can probably take a pretty much slowed down square wave.

The resistors should all be on the driving end of the tracks, not the pic end.
As to value, if you have a scope, then as much as you can take.
If no scope, start out at about 100 ohms, and see what that does for you.

Dave's Engineering Page: http://www.dvanhorn.org
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