Put a low value resistor in series at the driving pin. Actually, you can use
a Pot and adjust the signal until you see a nice semi-sin wave that still
has a full swing at each input pin. See:

http://www.piclist.com/../noises
aka
http://www.piclist.com/techref/default.asp?url=noises

Where you will hear Pa say:

    Insert a small resistance in series with each very high speed signal
produced
    by the chips. I always leave a pad for an SMD resister at each pin of a
high
    frequency signal source when I make a PCB. I just bridge the pad with a
fine
    trace (which acts as a small resistor) and if noise becomes a problem, I
    slice the trace and install a larger resistance. That is reason number
999
    for http://www.piclist.com/../logic/xtrapins.htm adding resistances to
io pins.
    Think of the resistance as a way of preventing the water valve from
being opened
    all the way. But won't that mess up the signal? delay its propagation?
reduce
    its level? Well, see, even in digital electronics, signals are analog.
Its
    a matter of reducing and delaying a signal A LITTLE in order to decrease
    noise A LOT. The signal path acts as a (very small) capacitor and the
extra
    (very small) resistor helps to convert the square wave of the original
signal
    (with infinite harmonics) into a semi-sin wave (with no harmonics). Just
    ask the guys who designed the RS-232 spec.
    Switching speed is an evil thing

---
James Newton (PICList Admin #3)
mailto:jamesnewton@piclist.com 1-619-652-0593
PIC/PICList FAQ: http://www.piclist.com or .org

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Phil Anderson
Sent: Tuesday, August 15, 2000 12:36
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: terminating a ringing 74HC clock



Fellow Picsters...

I have a clock with CMOS output running around a pc board, roughly 8
inches, to the XTAL input of a PIC. The clock is 16 Mhz, and while scoping
it, I see that it rings badly, i.e. large overshoot on the rising edge and
large undershoot on the falling edge. Spectrum analyzer picks it right up!
Nice 16 Mhz picket fence.

How may I clean up this ringing?

Phil.

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