----- Original Message ----- 
  From: Roger Weichert 
  To: Microcontroller discussion list - Public. 
  Sent: Wednesday, August 12, 2009 8:06 PM
  Subject: [PIC] Advise on replacing an obsolete micro with a PIC



  Hi Guys,

  I do repairs to a range of control boards which are around 20 years old but 
  still expensive to replace .. aprox $1200 . or to fit a replacement system . 
  around $1500.

  They are extremely reliable and generally very easy to repair if they fail.

  The only problem is when one comes in with a failed microcontroller . the 
  part is not available. The label has a TI logo . CF 30140 .. N A 9116 .. 
  Phillipines.

  I am considering mounting a small pcb on stilts, (connected to the main 
  board where the old micro was) just large enough to fit a smd PIC ... plus 
  any additional components needed.

  Obviously there are a lot of things to consider, so I just thought I'd ask 
  for any advice on things I need to look out for.

  SUPPLY . Crudely, the boards only use a 5.6v zener to regulate the supply to 
  the original micro and two other housekeeping IC's . so I assume that needs 
  to be addressed.

  The board runs off 12V ac, via a rectifier into a 22000uF cap. Then it has a 
  82 ohm resistor, a zener to drop it to 9 volts . then another resistor (120 
  ohm) and the 5.6 volt zener mentioned above  ..  crude !! .. but they've 
  been running for 20 + years.

  CRYSTAL . the original uses a 4.9152 MHz crystal.

  If I kept it in the same spot and perhaps fitted a 4MHz one initially (to 
  simplify the timing in the software until I get it going properly) . is it 
  likely to cause any problems being 25 - 30 mm from the new micro . or should 
  I just plan to fit one on the new board?

  The software requirements are pretty basic, so I don't envisiage any major 
  problems there . just 4 input links that set 'cycles per minute' and the 
  'output pulse ratio' . another input to monitor any 'overcurrent' condition 
  (Hi or Lo) . and four output pins to drive some fets (via an interface chip) 
  at 50 - 60 cycles per minute . 6 - 12 hours a day, everyday!

  Any suggestions . thanks.

  rogerdw 
Sounds like a GREAT little job.
You didn't mention how many pins the Micro had.
You may be able to skip the Xtal, using the internal clock in the PIC.
I love these kinds of retrofits.

Tom 

 *
  |  __O    Thomas C. Sefranek  tcs@cmcorp.com
  |_-\<,_   Amateur Radio Operator: WA1RHP
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