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PIC Microcontoller Basic Math Inc/Dec Methods


From Incrementing/Decrementing "w"

Here are a couple of snippets that will allow you to increment and decrement the "w" register without affecting any file registers if you don't have "addlw"/"sublw" instructions (ie in the case of the low-end processor).

"Reg" can be any register that does not change during the execution of the three instructions. For the low-end parts, any file register can be used because there is no danger of them being updated by an interrupt handler.

To Increment:

 xorlw 0x0FF              ;  Get 1s Complement of Number
  addwf Reg, w             ;  w = Reg + (w^0x0FF)
  subwf Reg, w             ;  w = Reg + ((Reg + (w^0x0FF))^0x0FF) + 1
                           ;  w = w + 1

To decrement, the instructions are re-arranged:

 subwf Reg, w             ;  w = Reg + (2^0x0FF) + 1
  xorlw 0x0FF              ;  Get 1s Complement of Result
  addwf Reg, w             ;  w = w - 1



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