> =A0 =A0 =A0 =A0But the thing is, in any pictures I've found online of PIC=
-based
> PCBs, no-one seems to actually be doing this isolating "ring". =A0So is
> it really something I should be concerning myself with? =A0Or ideally,
> does anyone know a way of geting multiple "GND"-named polygons on a
> layer in Eagle?

I don't run Eagle, but I can answer the question on the why of it.

The crystal load caps should be returned only, and directly, to the
ground pin on the CPU nearest the oscillator pins. That pin should be
the only point where the crystal system touches the rest of the
ground. Otherwise, you are allowing oscillator currents (at the
fundamental and all harmonics) to flow into your plane and they will
radiate.  This is also a door that opens both ways. The oscillator
done improperly is more prone to being upset by externally applied
fields and other currents flowing in the rest of the ground system.

This is the right way to do it, whether or not other people do it that
way.  Ask those guys if their designs pass part 15 first time, every
time, and in fact are barely above the noise floor of the analyzer
instead of barely below the fail line.


Think of it this way.  It isn't strictly true that current always
takes the shortest path.  It actually takes every possible path, it
just prefers the short one.   If you want quiet PCBs, return current
to where you got it from, by low impedance direct traces.

Also, the caps are in series and will need to be a bit less than twice
the specified Cl value, another one that is commonly done wrong but
usually "works".

:)

As to how to do that in eagle, can you place a "keep out" that the
flood will respect?

--=20
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