piclist 2002\10\03\043131a >
Thread: use VCC for crystal bypass caps?
face BY : Russell McMahon email (remove spam text)

> This came up in a beginner thread, but I'd like to get a definitive
> Can I use VCC for the "ground" side of the crystal's phase shift caps?
> Theory says VCC is equivilent to ground from an AC perspective, and it
> would certainly make most PCB layouts a bit cleaner, but I'd like to
> hear from someone who has done this regularly and thinks that it actually
> works!

It works.
The books generally say not to do it. The reasons given often don't apply.
They SAY that you will introduce noise into the oscillator (which you may).
I don't see that as a bad thing in many cases. You are liable to get a
degree of clock jitter or "spreading" and this is liable to reduce the peak
harmonic intensity. The effect is liable to be small. Use of varicaps as the
crystal caps may be indicated :-).

For applications requiring high clock stability or purity this may not be a
good scheme but for most real world applications it seems fine.

Jinx notes that startup may be affected - this is an area of the blackest of
magics and if you have startup problems this may make them worse - or

I use the arrangement for circuits on breadboard or Vero board (US = vector
board ?).

       Russell McMahon.

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