list      p=p16f84
	#include p16f84.inc


;===============================================================
;
;    CRC-8 for Dallas iButton products
;
;
;    From Maxim/Dallas AP Note 27
;
;    "Understanding and Using Cyclic Redundancy Checks with 
;    Dallas Semiconductor iButton Products"
;
;    The Ap note describes the CRC-8 algorithm used in the 
;    iButton products. Their implementation involves a 256 byte
;    CRC table. This algorithm is implemented here. In addition
;    two other algorithms are shown. One uses nibble arrays and
;    the other uses boolean arithmetic.
;
;    
;    18JAN03 - T. Scott Dattalo


  cblock	0x20

	crc

  endc

  org 0

	goto	start

;===============================================================
crc_8:
	xorwf	crc,f
	clrw

	btfsc	crc,0
	xorlw	0x5e

	btfsc	crc,1
	xorlw	0xbc

	btfsc	crc,2
	xorlw	0x61

	btfsc	crc,3
	xorlw	0xc2

	btfsc	crc,4
	xorlw	0x9d

	btfsc	crc,5
	xorlw	0x23

	btfsc	crc,6
	xorlw	0x46

	btfsc	crc,7
	xorlw	0x8c

	movwf	crc

	return

;===============================================================
start:

	clrf	crc
	movlw	1
	call	crc_8	; crc = 0x5e

	clrf	crc
	movlw	2
	call	crc_8	; crc = 0xbc

	clrf	crc
	movlw	4
	call	crc_8	; crc = 0x61

	clrf	crc
	movlw	8
	call	crc_8	; crc = 0xc2

	clrf	crc
	movlw	0x10
	call	crc_8	; crc = 0x9d

	clrf	crc
	movlw	0x20
	call	crc_8	; crc = 0x23

	clrf	crc
	movlw	0x40
	call	crc_8	; crc = 0x46

	clrf	crc
	movlw	0x80
	call	crc_8	; crc = 0x8c

  ; dallas example:

	clrf	crc
	movlw	0x02
	call	crc_8	; crc = 0xbc

	movlw	0x1c
	call	crc_8	; crc = 0xaf

	movlw	0xb8
	call	crc_8	; crc = 0x1e

	movlw	0x01
	call	crc_8	; crc = 0xdc

	movlw	0x00
	call	crc_8	; crc = 0xf4

	movlw	0x00
	call	crc_8	; crc = 0x15

	movlw	0x00
	call	crc_8	; crc = 0xa2

	movlw	0xa2
	call	crc_8	; crc = 0x00

	goto	start

  end