the problem:
i need to biphase code a byte for serial transmission over an rf link.
in other words each '1' becomes '10' and each '0' becomes '01'.
if it makes the code easier then '1' becoming '01' and '0' becoming '10' is ok too.

this _could_ be done with a lookup table, which would be fast, but that's not the point of a tiny
code challenge now is it ?

here's a chunk of code that does the job.

; data starts off stored in lo_int_temp+0
; program_status(fRFToggle) flips each time around the loop to select which nibble we are attacking
btfss    program_status, fRFToggle
swapf    lo_int_temp+0, F            ; select the appropriate nibble

; biphase code bit
movlw    b'01010101'                 ; start biphase coding...
btfsc    lo_int_temp+0, 0
addlw    b'00000001'                 ; if bit 0 is set then W = 01010110
btfsc    lo_int_temp+0, 1
addlw    b'00000001'                 ; if bit 1 is set then W = 010110xx
btfsc    lo_int_temp+0, 2
addlw    b'00010000'                 ; if bit 1 is set then W = 0110xxxx
btfsc    lo_int_temp+0, 3
addlw    b'01000000'                 ; xxxxabcd now encoded as aAbBcCdD, where A = !a
; exits with a coded version of the relevant nibble of lo_int_temp+0 in W


happy hunting,
Simon


p.s. James: how about adding a 'tiny code challenge' section to www.piclist.com to store the gems
these little competitions turn up?

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