I am working on a design for a two channel AC dimmer using a PIC (16F87x
series).  Just wondering if anyone else has done it.  I am running into a
timing issue =96 or rather a like of timing issue.  Here are my thoughts:

Input to the dimmer would be either a 0-5vdc voltage, or a command via the
RS232 input.  The dimmer would have 256 steps between full on and off.
Also, I would like to linearize the output, rather than the common strong
start, linear in the middle and tapers off at the end.  To do this, I though
that I would have a circuit tied to the 60Hz line that would provide when a
zero crossing takes place.  This would start a counter based on T1 =96 this
counter would count up to 256 during =BD cycle of 60Hz wave.   A look up ta=
ble
would be used to compare the current analog value (or command value) to the
linearized value at that slice of time (1 of the 256 slices in =BD of a 60Hz
wave).  When the current analog value exceeds the look up table value then
the AC output would be activated.  At the end of the =BD cycle the process
would start over.

Now for the problem =96 A 60Hz wave has a period of 16.66ms, so half a wave
would be 8.33ms.  If we divide this time into 256 slices that would give us
32.5 us per slice.  A PIC16F87x running at 20MHz can perform an instruction
in 200ns.  So at 200ns/per instruction we could perform 162 instructions in
32.5us.  This is not enough time to go through all 255 steps in the look up
table.  I could reduce the steps to 7 bits (128 steps) and this would
possibly give me enough time for the look up table, but no much else.

So as I said =96 anyone else already been through this that can possibly gu=
ide
me in a better method?  I am trying to hold external hardware down to a
minimum.
Thanks,
David

David V. Fansler
S/V Annabelle
david@dv-fansler.com
www.dv-fansler.com



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