Crystal capacitance

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clpalmer
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Posted by clpalmer: Wed. Jan 31, 2007 - 03:56 AM
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I've read several tutorials and the like on this, but I'm still a bit confused. If I want to drive an AVR with a 3.6864 MHz crystal and I find one on digikey that states 18pF load capacitance, I should give it ~26pF-32pF (depending on what's taken for Cstray on the board) correct?

This is what I gathered from the various discussions I've found. The reason I was questioning it is that on tuxgraphics.org, the avrusb500 specs show 18pF caps on the crystal. Since I can only seem to find crystals with 18pF or 20pF Cload, I assume that either I'm mistaken and Cload is the value of the caps, or Guido is mistaken and those values should have been higher...

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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AVR Microcontrollers, megaAVR and tinyAVR
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mneary
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Posted by mneary: Wed. Jan 31, 2007 - 04:03 AM
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Truthfully, for one-off projects, the tolerance is fairly wide.

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Mike B
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Posted by Mike B: Wed. Jan 31, 2007 - 07:02 AM
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The AVR application note AVR042 has information about this in section 5 Using crystal and ceramic resonators.

http://www.atmel.com/dyn/resourc...

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leon_heller
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Posted by leon_heller: Wed. Jan 31, 2007 - 01:43 PM
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The capacitors are effectively in series, so you'd need something like 36 pF for a crystal with an 18 pF load capacitance. Allowing for stray capacitance, 33 pF is probably about right. In practice, it isn't very critical unless you want the frequency to be as close as possible to what is marked on the crystal.

Leon

Leon Heller G1HSM

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clpalmer
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Posted by clpalmer: Wed. Jan 31, 2007 - 04:14 PM
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Thanks =)

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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