Ah!  Now we're getting to the body of the matter.

Dave VanHorn wrote:

> I've seen this in EMI testing, and I can tell you, watching the pen
> repeatedly slam against the top of the chart recorder is NOT a
> pleasant experience.   In this case, the xtal caps were "grounded",
> but they didn't return to the uP ground pin, instead an 8 mil track
> connected them to a 100 mil track, about an inch from the uP ground.
>  The track happened to resonate at some harmonic of the osc (I forget,
> it's been a few years), so the system looks a lot like a gamma-matched
> quarter wave antenna.

  OK, I agree to that, the PCB was obviously designed wrong, you fixed
it.  No contest whatsoever.  So have you actually tried prototyping a
PIC or AVR PCB and compared the options (for argument's sake, on the
same design) of returning the load caps to Vdd, or Vss (which for the
PIC at least requires mounting SMD caps underneath the chip)?

> I would return it to where the chip designer told me to, on the vague
> assumption that they knew more about the internals of the chip than I
> do.

  I don't actually read the spec sheet as "telling" you to return these
caps to ground.  I think it assumes you will return them to ground, and
diagrams accordingly, but no more *tells* you to do so than it tells you
to keep them short!  I think if you realize that it is a good idea to
keep the crystal and load capacitor traces as short as possible, then
you may also deduce that in a symmetrical circuit, one supply rail is as
good as another, so the closest is just as likely best.

> I'm not sure I buy this model. I see current pulled from VCC to charge
> the caps, dumping into ground, and current dumped through the chip
> into it's GND pin to discharge them.

  But I re-phrase this to read: "I see current pulled from ground to
charge the caps, dumping into VCC, and current dumped through the chip
into it's VCC pin to discharge them."

> Provided you have a nice plane layer for each supply, there's probably
> no difference.

  No difference either way.  The RF current is, and you desire it to be,
local, so how heavy you have made the ground planes becomes irrelevant
in this single respect.  Half the current into each supply.  Whichever
supply you return the caps to takes its half directly, and the other
half always flows through the bypass.

> I live in two-layer-world, where my ground has a lot lower impedance
> than my VCC, and chips are bypassed to ground locally,

  My punt is that if you put a SMD bypass either under the chip or on
the other side of the PCB, directly between the supply pins, you'll do
well if you return the load caps direct to the Vdd pin.

  If you put SMDs under the footprint of the PIC you can return them to
ground, especially if you have chosen to take the ground to other points
underneath the PIC.  This is more difficult though if you have decided
to reticulate Vdd or pass other signals underneath the PIC instead.
--
  Cheers,
        Paul B.