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手机阅读
PIC单片机实现Dallas的iButton通讯时，编了段产生和校验CRC的子程序。
#include <P16CE625.INC>
;-------------------------------
cblock 0x20
datBuff:8 ;assign 8 byte data buffer
bit_cnt ;bit counting
CRC_COUNT ;number of bytes for CRC
CRC_RESULT ;CRC resulr
crc_temp ;temporary data buffer during CRC
endc
;-------------------------------
#define skp0 btfsc
#define skp1 btfss
;===============================
org 0x000
movlw .7
movwf CRC_COUNT
movlw datBuff
movwf FSR ;FSR point to data buffer
call CRC_CHECK
movwf datBuff+7
goto $
;===============================
;CHECK THE CRC FOR ? BYTES DATA
;For the polynomial of X^8 + X^5 + X^4 + 1
;Derived from Dallas's iButton standard
;Before calling, FSR = data buffer
; CRC_COUNT = number of bytes for CRC
CRC_CHECK ;~~~~~~~~~~~~~~~
clrf CRC_RESULT ;initialize the CRC buffer
_crc_00
movlw .8 ;number of bits for one byte
movwf bit_cnt ;set bit counter
movf INDF,w ;get one data byte
movwf crc_temp ;copy to temporary location
_crc_01
rrf crc_temp,f ;get LSB in C
skpc ;is this LSB=1?
goto _crc_02 ;go if LSB=0
movlw 0x01 ;do if LSB=1
xorwf CRC_RESULT,f
_crc_02
rrf CRC_RESULT,w ;get LSB of CRC
skpc ;test LSB
goto _crc_03 ;go if LSB=0
movlw 0x18 ;do if LSB=1
xorwf CRC_RESULT,f ;polynomial implementation
_crc_03
rrf CRC_RESULT,w ;whole byte right rotate
rrf CRC_RESULT,f
decfsz bit_cnt,f ;bit counting
goto _crc_01 ;go on until aa 8 bits done
incf FSR,f ;pointer update to next byte
decfsz CRC_COUNT,f ;byte counting
goto _crc_00 ;go on until all byte done
movf CRC_RESULT,w ;get CRC, Z set if CRC=0
return ;return with CRC in W
END