PIC Microcontoller Bit Math Method

Converting a 2-bit number to a 4-bit mask

From: Mike Keitz. There is a trick method that works for 4 bits. It replaces a 2-bit number in a variable with a 4-bit mask:

        incf    BitP, W  ;W = 0001 0010 0011 0100 
        btfsc   BitP, 1  ;If 0 or 1, result is almost correct now. 
        iorwf   BitP, F  ;BitP = [0000] [0001] 0011 0111 
        incf    BitP, F  ;BitP = 0001 0010 0100 1000 

Jinx says:

Bob Ammerman gave me this one. I used it to convert a device number to a pin bit number. I call it "bobbit" ;-)

         movf    portb,w
         andlw   b'00000011'
         movwf   temp1

         movlw   .1
         btfsc   temp1,1
         movlw   .4
         btfsc   temp1,0
         addwf   wreg,w
         btfsc   temp1,2
         swapf   wreg,w
         movwf   temp2

portb = xxxxxx00, temp2 = 00000001
portb = xxxxxx01, temp2 = 00000010
portb = xxxxxx10, temp2 = 00000100
portb = xxxxxx11, temp2 = 00001000

Russell McMahon says:

Here's a lateral thought :-).

If you can decide which of the 4 output lines is output 1,2,3,4 (as will usually be the case) you can solve this as follows Assumes bits are in LSBs and no other bits set. AND high bits if required.

Input in W in each case

   Add 1 to W
   If result = 3 then result = 8

Or

   If W = 2 then W = 7
   Add 1 to W