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'Decoupling capacitor'
1995\11\04@185200 by MK Walton

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I am having problems with a circuit based around a pic 16c55 jw at 20Mhz
The circuit is curently on a prototyping board, and sometimes works
fine, but mostly goes completely up the spout!

I can touch the power supply rails and all hell breaks loose, and just
pushing wires that I know have a good connection has the same effect.

There is a single 100nF decoupling capacitor in the circuit which so I am
told should remove this problem, but doesn't, some circuits I have seen
using PICs have a similar capacitor next to each chip.

Is this the cause of the problem, and if so how many do I need?

Cheers

Martin Walton (spam_OUTmk-waltoTakeThisOuTspamuwe.ac.uk)

1995\11\04@204229 by Paul Christenson [N3EOP]

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>I am having problems with a circuit based around a pic 16c55 jw at 20Mhz
> The circuit is curently on a prototyping board, and sometimes works
> fine, but mostly goes completely up the spout!

> I can touch the power supply rails and all hell breaks loose, and just
> pushing wires that I know have a good connection has the same effect.

The problem is probably a combination of the 20 MHz and the proto board.
Proto boards just aren't designed for high frequency designs.  Among
the problems with the boards is that the contacts that run under the
plastic are effectively capacitors, which go lower in impedence as the
frequency increases.

Normally, 20 MHz is not going to be bothered by the small impedences
presented by the board.  But, digital signals are square waves, which
have a high harmonic content.  These harmonics can crossover relatively
easily, and can confuse the digital circuitry.

Just a guess, but I have seen it happen before.  You may be able to get
it to work if you keep all of the jumpers as short as possible.

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