William Ruehl wrote:
>
> PICsters,
>    What is the best way to have two 16F84's on the same board share
> the same crystal? Can the chips share the crystal in parallel? or is
> there
> a more elegant approach?
>
> Bill Ruehl

  One question:  Why not just use 2 crystals or 2 ceramic resonators?

  2 easy possibilities;

  a)  Put the F84's fairly close to each other, connect crystal across #
1 F84 with caps as per normal, connect the Osc2/ClkOut pin of that one
to the Osc1/ClkIn pin of #2 (treating it like an external oscillator.)
Want the Osc. trace from Pic #1 to #2 short, here.  And stray
capacitance from that lead may slightly affect the frequency of
oscillation of the Xtal.  So if you buffer this output through a gate,
that improves things (perhaps a spare inverter?)  If careful, you can
skip the gate.  YMMV.

  b)  BETTER: Set up a standard inverter-style oscillator & use that to
drive both PICs, through a second inverter, into Osc1/ClkIn.  See the
data sheets for how to make such an oscillator <G>

  Also could just use a clock oscillator module & drive both F84's off
that.

  A caveat:  You aren't guaranteed that the 2 PICs are running on the
same clock phase, they should stay at a constant skew, though.  So don't
assume this!  (i.e. When Pic #1 is in the Q1 state, Pic # 2 may be in
any of the Q1, Q2, Q3 or Q4 states, for all we know.  So if you want to
pass data back & forth quickly, you cannot do it on a 2-instruction loop
etc. <G>)

  Anyone on list know of any way to Q-state synchronize a pair of F84's,
please let ME know!  Though I suppose you could do it with starting with
a slow clock, watching for a rising edge on one output, and holding the
first PIC to raise that edge in "hold" (i.e. no clock state change)
until the other PIC's output goes high, then speeding up both PIC clock
rates (use a programmable clock divider system, for all this.)  Fancy
hard way to do an un-necessary thing, methinks <G>

  Mark