PIC Microcontroller Bit Math Method

CRC 16/32 Bit

By Dr. Imre Bartfai

SX version also available

; What you need is to define the polynomial. E. g. for the CCITT one:
poly0 equ 0x21
poly1 equ 0x10
; Do not forget to initialize <crcval> = <crch:crcl> before the 1st call
; e. g. with 0xFF or with zeroes, as the particular CRC algorithm
; requires.
;This subroutine calculates a 16-bit CRC
;--------
; CrcUpd: update <crcval> using <W>
;
;
 CBLOCK crcblk
 crch
 crcl
 saved
 oldcch
 i
 ENDC
CrcUpd movwf saved ; j = W
 movlw d'8' ; W = 8
 movwf i ; i = W
_loop movfw crch
 movwf oldcch ; temporary save
 clrc ; clear carry for rlf
 rlf crcl ; crc << 1
 rlf crch
 movfw saved ; the char read
 xorwf oldcch,F ; test with old high
 btfss oldcch,7 ; if bit set, apply mask
 b _notset ; otherwise skip
 movlw poly0
 xorwf crcl,F
 movlw poly1
 xorwf crch,F
_notset clrc ; for rlf
 rlf saved,F ; next bit of saved
 decfsz i,F
 b _loop
 return
;
; End CrcUpd

---

John Payson replies: Here's a version which should be faster: 38 instructions with no loops. When invoked, it will update crcH and crcL based upon the new input value. Note that crcL is entirely a function of crcH, while crcH is a function of crcH, xor'ed with the 'old' value of crcL. Basically, each bit in crcH will affect certain other bits in the output; each bit in crcL will affect only the corresponding bit in crcH [which, on the next iteration, will affect bits in both crcL and crcH].

; Simple CRC routine
; from John Payson 1998年10月23日
 xorwf crcH
; Compute the LSB first [based upon MSB]
 movlw 0
 btfsc crcH,0
 xorlw 33
 btfsc crcH,1
 xorlw 66
 btfsc crcH,2
 xorlw 132
 btfsc crcH,3
 xorlw 8
 btfsc crcH,4
 xorlw 49
 btfsc crcH,5
 xorlw 98
 btfsc crcH,6
 xorlw 196
 btfsc crcH,7
 xorlw 136
; Swap crcL with W
 xorwf crcL,f
 xorwf crcL,w
 xorwf crcL,f
; Next compute the MSB [note W holds old LSB]
 btfsc crcH,0
 xorlw 16
 btfsc crcH,1
 xorlw 32
 btfsc crcH,2
 xorlw 64
 btfsc crcH,3
 xorlw 129
 btfsc crcH,4
 xorlw 18
 btfsc crcH,5
 xorlw 36
 btfsc crcH,6
 xorlw 72
 btfsc crcH,7
 xorlw 145
 movwf crcH

---

Interested:

---

---

---

---

.