> I'm wondering why you need to synchronize so closely. Are they going to
all
> do the same thing? Is this a tester, maybe?

They will all be generating 8-bit PCM sine waves of different frequencies
from a LUT. The momma PIC will then add these, and multiply by a
(user-selectable) constant to give a 14-bit PCM output that will drive a D/A
converter. The baby PICs will be using a constant sampling rate --- which
means that if each LUT is properly padded, all the baby PICs can finish
looking up the table, and have an 8-bit PCM value ready --- there will then
be n cycles before they need to lookup again to maintain the sampling rate.
While constant initial phase for the waves would be helpful, it is not
essential -- what must *not* happen is the momma sampling a baby PIC more
than once, or missing out on a baby PIC for one cycle (of the sampling
rate).

Clocking the baby PICs from a master external oscillator is no problem.
Interrupts would be ideal, but besides the I/O pin requirements, I don't
think having a PIC jump to an ISR in the middle of looking up the LUT is
what I'd like. If they were sync'd, there would be time enough during the n
idle cycles for the momma to receive one byte from all of the baby PICs
serially and sequentially.

Since this is a proof-of-concept project, I wouldn't mind using bigger PICs
and just use parallel I/O for bytewide transfer on a common bus. The baby
PIC would just leave the data there during the idle cycles, and the momma
would be free to pick it up whenever it wanted. Then again, if the PICs
weren't synchronized, there would need to be some sort of ISR to ensure that
only one device mastered the bus at any given time and tri-stated its output
port otherwise. If the PICs were synchronized, I'd still need the interrupt
for bus mastering, but I would know that it would be taking place during the
n idle cycles, instead of interrupting the PIC in the middle of the LUT. Of
course, the brute force solution would be to just plug tristate buffers in
front of each PIC and use the momma to just strobe the buffers whenever
needed, but I'd like to avoid that.

Cheers,
Ishaan

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