Crystal decoupling caps

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Hey guys

Quick question; I know its common (and good?) practice to use decoupling capacitors for the crystal. Being 2 0.1uF to ground correct?

How come whenever I try to do that my AVR halts completely until I remove both caps?

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Quote:
Being 2 0.1uF to ground correct?
ABSOLUTELY WRONG! It depend on the crystal used but usually between 15pF and 27pF.
Quote:
How come whenever I try to do that my AVR halts completely until I remove both caps?
For the same reason that your car will not move with a foot on the brake (see above) and one foot on the accelerator. :?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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There are no "crystal decoupling caps".

There ARE caps that are needed to make a standard oscillator circuit. As John pointed out, those are typically 15pf to 27pf wilth 22pf being quite common.

Caps as large as you suggest will cause the oscillator to completely stop oscillating. I suggest that you use caps with a more suitable value.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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They are not crystal decoupling caps, they are crystal loading caps. The crystal needs a certain capacitive load to operate at the frequency that is printed on it.

And no we don't know what load capacitance your crystal needs, but it typically is 16-20pF, but generally speaking, if you put two 22pF or 27pF capacitors there it should work.

For power pins, you do need 100nF decoupling caps.

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Atmel has Application Note AVR042: AVR Hardware Design Considerations, which discussed these Xtal caps, By-Pass Caps, Programming Header Pinouts, and other useful info.

JC