|
用PIC單片機和Dallas的iButton通訊時，編了段產生和校驗CRC的子程序。 #include ;------------------------------- 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 WEND |
