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Is this 16 MHz crystal layout OK?

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exscape:

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-- UPDATED:
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Bottom layer cutout highlighted:
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(before the update)

16 MHz crystal, 22 pF load capacitors, Atmega328 microcontroller... and a 100 nF decoupling cap. (I'm not a fan of the pin placement there.)

There's quite a bit of conflicting info about what's best. Some say you "must" have a ground plane, others say you shouldn't (mostly due to stray capacitance?). Some say you should have a ground ring on the top layer, while some seem to recommend a local ground plane... Needless to say, this is pretty confusing as I'm still pretty new to this.

The main requirement is that it works, really. Low EMI and noise coupling etc. is of course preferred, but isn't as critical as having it work. ;)

EDIT: I just realized that the crystal cap ground current will pass through the same trace as the main power will, at least in part. Should I remove that connection (between the right pad of C11 and the left pad of C8)? -- I did so and updated the post above.

Short Circuit:

--- Quote from: exscape on September 09, 2012, 01:41:30 pm ---...
There's quite a bit of conflicting info about what's best. Some say you "must" have a ground plane, others say you shouldn't (mostly due to stray capacitance?). Some say you should have a ground ring on the top layer, while some seem to recommend a local ground plane... Needless to say, this is pretty confusing as I'm still pretty new to this.

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It doesn't matter that much since all these claims are backed by working designs.
In fact, I think it would be quite difficult to screw the oscillator by layout mistake alone.

jimmc:
C11, C12 and the crystal (which is inductive at the frequency of oscillation) form a tuned circuit. For minimum radiation or pickup, ensure that the area enclosed by currents circulating in this loop is minimised.
To do this place C11 & 12 alongside the crystal with their earthed ends close together in the middle.
Personally I would use an earth plane unless it's a multi-layer board with a very thin (0.005") dielectric under the outer layer. For a double sided board (1/16 or 1/32") the additional stray capacity is small compared with the 22pF caps.

As Short Circuit has said, any reasonable layout will work, but it's not difficult, I'd make the change.

Jim

exscape:

--- Quote from: jimmc on September 09, 2012, 08:10:06 pm ---C11, C12 and the crystal (which is inductive at the frequency of oscillation) form a tuned circuit. For minimum radiation or pickup, ensure that the area enclosed by currents circulating in this loop is minimised.
To do this place C11 & 12 alongside the crystal with their earthed ends close together in the middle.
Personally I would use an earth plane unless it's a multi-layer board with a very thin (0.005") dielectric under the outer layer. For a double sided board (1/16 or 1/32") the additional stray capacity is small compared with the 22pF caps.

As Short Circuit has said, any reasonable layout will work, but it's not difficult, I'd make the change.

Jim

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Something
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? (Quick throw-together, I'll reposition slightly/clean it up if it's better than the previous one.)
Note the connection between the crystal ground and the decoupling-cap - OK? :)

jimmc:
I'd be happy with that.
With regards to the decoupling cap, either merge the 'GND' track into the ground plane or only connect it to the ground pin via the track.
I've a slight preference for merging but I'm sure either would work.

Jim

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