Ok, this is *way* strange.  I'm trying to use this CRC code from the PIC
lists' web site:

  http://www.piclist.com/techref/microchip/crc16ca.htm

I'm trying to verify the CRC16.ASM file and I'm getting strange
results.  I've made some minor modifications to the code, like set
p=16f877, include "p16f877.inc", and add some debugging code to print
the CRC out the serial port when entering CRC16.  In other words,
nothing that should affect the CRC.

When I run the code through the simulator (gpsim under Linux), I get
correct results.  The CRC matches the ones in the comments as I step
through the code.  However, when run on a real PIC (16F877 @ 4MHz),
the results are wrong.  Note: the results in comments appear to be
correct.  I've verified it with some C code on the desktop.  Here are
the results from a real PIC from the serial port:

  80 6000
  75 27A0
  8A 1FA7
  0B 7D1F
  75 2FFD
  C7 D3AE
  AA 03D3
  75 7A83
  C7 337A
  55 DC72
  43 141C
  1C 0014
  14 0000

Note the bizarre behavior.  The end CRC is actually correct.  However
some (but not all) of the intermediate CRCs are incorrect.  What could
possibly cause this code to act differently on the simulator from a
real PIC?  It's not interacting with any port registers or any outside
stimulus.  It's not time dependent.  There are no interrupts or
anything happening, so nothing could be corrupting the CRC in between
invocations.  I dunno.  This is really strange.  I won't even consider
a bug in gpsim because it's actually producing expected results.  I've
run other code on the PIC and it all works, which suggests the chip is
a fully functional PIC.

Maybe it's time to rewrite this code anyways.  I never heard back from
him and I'm not sure the copyright status, so it may be better to just
start over.  But I'd still like to get to the bottom of this strange
behavior.

-Dave

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