I've inherited a board developed by an outside contractor that uses a
PIC16F874.  I've lots of time on other processors but this is the first
I've really dealt with a PIC up close and personally.

 

We make slot machines and this board is the reel controller.  Almost all
the time it happily runs along take commands from what amounts to a PC
running DOS that is the main computer in the machine.  One of the
functions that the reel controller has is sending the reels (stepper
motors with 200 steps but 22 stops or symbols on the reel strips) to a
known safe losing position if there is a game failure.  A goof early on
before I was at the company was that the reel strips were designed with
no thought to where a bad hardware failure would tend to send them.  So
on one game family ( and yeah our most popular) if all reels go to motor
step 0 the top prize is displayed on the reels.

 

To get around that the PC sends a command to the PIC to change the home
or fault positions to some known safe point with a different stop on
each reel.  The values are stored in the internal EEPROM.  Again almost
all the time - well over 99.99% of the time this is all cool.

 

BUT some times the game is happily chugging along and the reel
controller apparently loses its mind.  It decides there was a failure
and the reels get sent to home positions (the going home action is a
slower spin so we know it is a sent home problem not a bad decision
about where to send them) but for some reason the reels all go to step 0
with is 4 steps off of the stop for top prize.  Not good for a slot
machine to fail and display what looks like a big win.  Not good to the
tune of some half a million in payouts a year.

 

Okay amusing story done, does anyone know under what conditions a PIC
might have trouble reading the internal EEPROM?  Design decisions in
making the board were some I would not have made and the programming
style is not one I would have used but I see no failure mode in the code
that would lead to this.

 

The original contractor told me that a colleague told him that there was
a known problem with the PIC16F87x and EEPROM that was fixed in the
PIC16F87xA but I've not seen that anywhere else so far.  Anyone else
know something like that?  White wiring an RC network to the board to
let us use the A part (a change to MCLR/Vpp) is fine but changing
thousands of board on the chance it might fix something is a tad
expensive.  If it saves a half mill a year it is a great investment
though.

 

Any PICophile knowledge that could help me here?

 

Bill Killian

 



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