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Thread: use VCC for crystal bypass caps?
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face BY : Wagner Lipnharski email (remove spam text)



William Chops Westfield wrote:
> This came up in a beginner thread, but I'd like to get a definitive
> answer. 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!
>
> Thanks
> Bill W
>
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Hi Bill,
In theory it really doesn't matter where you anchor your AC signals. Ground
or VCC can act the same.

I just see two big issues.

1) Normally we use to lay out larger real estate copper for ground. Not
only this, but normally your Ground will be connected to other Grounds
around, what will create a larger ballast and will spread even more your
Ground potential to suck down noise and abnormal energy.  What this mean?
in a direct path, Ground will always be the easier way for those signals to
travel.  By other side, VCC as usual designed, uses not so heavy copper
rails, a VCC track of 1/8" wide with so many turns and twists can represent
a higher impedance for a high frequency point of view.

2) A blow up capacitor with its imminent short circuit, from output
oscillator clock pin to ground, is not a big issue, but try to do the same
to VCC and you will have a fried oscillator faster than you can say
"oops!".

Most microchips try to balance their internal low impedance much more to
ground than to VCC.  Even discrete components designs, you can see much
more component leads to ground than to VCC. This, shows a natural tendency
to a lower impedance in general to ground than VCC.

Of course the large and small capacitors at any power supply offer a low
impedance to frequencies, so, for a decoupling capacitor, it really doesn't
matter much where you anchor it, if, or course, the path is free and direct
impedance is low for such frequency.

Perhaps one problem to consider is that the return path via VCC will be
always longer than via Ground, this can create unexpected issues and
intermittent problems.

/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
Wagner Lipnharski - UST Research Inc
Orlando FLorida - USA - http://www.ustr.net
/_/_/_/ Atmel AVR Consultant /_/_/_/

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