list	p=12c671
	include	<p12c671.inc>

	org	0h	; reset vector
reset	goto	start

	org	4h	; peripheral interrupt vector
intv	goto	start

start			; this is where execution begins

	movlw	ADCON1	; set I/O pins for digital operation
	movwf	FRS
	movlw	7
	movwf	INDF

	movlw	TRISIO	; set Tri-state pins
	movwf	FSR
	bsf	INDF,5	; GP5 (pin 2) is set for clock input
	bsf	INDF,4	; GP4 (pin 3) is set for data input
	bsf	INDF,3	; GP3 (pin 4) is cleared for output

	bcf	GPIO,3	; Clear output bit

	clrf	20h	; F20 (transition count) = 0
	clrf	21h	; F21 (last data input bit) = 0

zerlp:	btfss	GPIO,5	; Skip next instruction if clock is one
	goto	zerlp

	clrf	22h	; Set F22 to next data input bit
	btfsc	GPIO,4
	incf	22h,1	

	movf	21h,W	; W <- XOR(F21, F22)
	xorwf	22h,W
	btfss	STATUS,Z	; skip next instruction if W == 0
	goto	out0

			;; there has been a transistion
	movlw	22h	;; F21 <- F22
	movwf	21h

	incf	20h,W	;; W <- F20+1
	andlw	20h,3	;; W <- W & 3
	movwf	20h	;; F20 <- W
	btfss	STATUS,Z	; skip next instruction if F20 == 0
	goto	out1

out0:	bcf	GPIO,3	; data output <- 0
	goto	onelp

out1:	bsf	GPIO,3	; data output <- 1

onelp:	btfsc	GPIO,5	; Skip next instruction if clock is zero
	goto	onelp

	goto	zerlp
	end