Post by thread:[PIC]: Fast manchester decode (code challenge)

I've been playing with some RF modules, and I'm using manchester encoding to to provide a DC balanced code. Encoding 4 bit nibbles into 8 bit bytes with a lookup table is very fast and requires little memory, but decoding is slightly trickier if you also want to check the code is valid. I came up with the following that uses 1 byte of GPR and executes in 13 cycles if the manchester code is valid, or jumps to an error handler in 8 cycles if not. I don't need the speed, but I'm pretty sure this can be done in less cycles, anyone see a better way? Mike ; on entry manchester encoded byte held in w, ; if code is valid, decoded byte held in w on exit ; if code is invalid, jumps to error handler ; if valid incomming byte looks like a !a b !b c !c d !d xorlw 0x55 ; Invert the second phase of each bit. ; First and second phases should now match ; if valid i.e. aabbccdd movwf enc ; Take a copy of the result rrf enc,f ; enc = xaabbccd xorwf enc,w ; (w)aabbccdd ; (enc)xaabbccd XOR ; (w)x0x0x0x0 andlw 0x55 ; Clear bits 1,3,5,7. The value in W is now zero if ; the manchester code was valid i.e. both phases ; of each bit are complimentry skpz ; If valid skip to decoding goto man_err ; invalid code, jump to error handler ; enc still holds xaabbccd, our target is 0000abcd ; bits 0 & 1 are already in the correct position movf enc,w ; w = xaabbccd andlw 0x03 ; w = 000000cd btfsc enc,4 iorlw 0x04 ; w = 00000bcd btfsc enc,5 iorlw 0x08 ; w = 0000abcd ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

Michael Rigby-Jones <spam_OUTpiclistTakeThisOuTMIT.EDU> wrote: > I came up with the following that uses 1 byte of GPR and executes in > 13 cycles if the manchester code is valid, or jumps to an error handler > in 8 cycles if not. > > I don't need the speed, but I'm pretty sure this can be done in less > cycles, anyone see a better way? Mike: ; Enter with received byte in REG. Jumps to INVALID if the ; Manchester code is invalid. RADIX = DEC RRF REG,W ;Copy REG's odd-numbered bits to W's ;even-numbered bit positions. XORWF ENC,W ;Each even-numbered bit of W holds "1" ;if REG's odd bits are complements of ;REG's even bits. XORLW 11111111B ;Now, each even-numbered bit of W holds ;"0" if REG's odd bits are complements ;of REG's even bits. ANDLW 01010101B ;Mask off all but W's even bits. BNZ INVALID ;If any of them are non-zero, the word in ;REG was not valid. Jump. -Andrew === Andrew Warren - .....fastfwdKILLspam@spam@ix.netcom.com