Microchip A2D.SRC

       list    p=16F84,t=ON,c=132,n=80,st=off


	__CONFIG   _CP_OFF & _WDT_OFF & _PWRTE_ON & _HS_OSC

        include "P16F84.INC"

  cblock   0x0C

    count
    t1,t2
    error_code

    total_lo,total_hi
    count_lo,count_hi
  endc

;                /-----<A2D_OUT_BIT
;                R
;                R  Rf
;                R
;          Rin   |
;Vin ----RRRRR---.------>A2D_IN_BIT
;                |
;                -
;                -
;                |
;               Gnd
;  Resistor "Rin" [10K] from unknown voltage to comparator input.
;  Resistor "Rf" [10k] from Feedback pin to comparator pin.
;  Capacitor Cint from comparator pin to ground.0.56uF mylar film

A2D_PORT	EQU	PORTA
A2D_IN_BIT	EQU	0
A2D_OUT_BIT	EQU	1


ERROR_STUCK_HI	EQU	1
ERROR_STUCK_LO	EQU	2




        ORG     0               ;Reset Vector

        GOTO    Main

        ORG     4               ;Interrupt Vector


Main
	bsf	STATUS,RP0
	clrf	PORTB		;Make portb i/o pins outputs
	movlw	1
	movwf	PORTA
	bcf	STATUS,RP0

	clrf	error_code

start:


loop
 	clrf	count_lo
 	clrf	count_hi

	call	a2d_convert2
	
	movf	count_lo,W
	movwf	PORTB

	goto	loop


;--------------------------------------------------------
; a2d_convert
;
; 2I/O pins

a2d_convert:

	call	precharge

a2d_loop
	RLF	A2D_PORT,W
	xorlw	0xff
	movwf	A2D_PORT

	clrc
	movlw	1

	btfss	A2D_PORT,A2D_OUT_BIT
	 addwf	count_lo,f
	skpnc
	 addwf	count_hi,f

	addwf	total_lo,f
	skpnc
	 addwf	total_hi,f

;	clrf	t1
;	decfsz	t1,f
;	 goto	$-1

	btfss	total_hi,4
	 goto	a2d_loop


	return	

;--------------------------------------------------------
; a2d_convert
;
; I/O pin

a2d_convert2:

    ;Put the A2D_PORT's tris into the FSR so that we can
    ;access it with out having to do those damn bank selects

	movlw	TRISA
	movwf	FSR
	movlw	1
	clrf	count_lo
	clrf	count_hi
	clrf	total_lo
	clrf	total_hi
	clrf	t1

    ; Step 1 is the pre-charging cycle where the capacitor 
    ;is made to pass through the low-to-high threshold and
    ;then again through the high-to-low threshold.
    ; This ensures that the voltage on the capacitor is the
    ;same for each time we pass through the algorithm.

    ;Make the capacitor go through the low to high threshold

	bsf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

a2d2_l1

	call	delay16
	call	delay16
	call	delay16
	call	delay16

	bsf	INDF,A2D_OUT_BIT

	call	delay16

	btfsc	A2D_PORT,A2D_OUT_BIT
	 goto	a2d2_l2

	bsf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

	decfsz	t1,f
	 goto	a2d2_l1

;	movlw	ERROR_STUCK_LO
;	movwf	error_code
;	goto	display_error



   ;now pass through the high to low threshold

a2d2_l2
	clrf	t1
	bcf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

a2d2_l3

	call	delay16
	call	delay16
	call	delay16
	call	delay16

	bsf	INDF,A2D_OUT_BIT

	call	delay16

	
	btfss	A2D_PORT,A2D_OUT_BIT
	 goto	a2d2_l4

	bcf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

	decfsz	t1,f
	 goto	a2d2_l3

;	movlw	ERROR_STUCK_HI
;	movwf	error_code
;	goto	display_error

a2d2_l4
    ;If we get then we've successfully precharged the capacitor
    ;Also, the capacitor voltage is just below the high-to-low
    ;threshold (Vil) so let's begin by ramping it positive

	bsf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

a2d2_l5
	addwf	total_lo,f
	skpnc
	 addwf	total_hi,f
	btfsc	total_hi,0
	 return

	call	delay16
	call	delay16
	call	delay16
	call	delay16

	bsf	INDF,A2D_OUT_BIT

	call	delay4

	btfss   A2D_PORT,A2D_OUT_BIT
	 goto	a2d2_low

	addwf	count_lo,F

	;The cap voltage is too high
	bcf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

	skpnc
	 addwf	count_hi,F

	goto	a2d2_l5

a2d2_low
	;The cap voltage is too high
	bsf	A2D_PORT,A2D_OUT_BIT
	bcf	INDF,A2D_OUT_BIT

	goto	$+1
	goto	a2d2_l5

	return	

;--------------------------------------------------------
; precharge 

precharge:

	movlw	1
	clrf	total_lo
	clrf	total_hi

	bcf	A2D_PORT,A2D_OUT_BIT

    ; Make the capacitor voltage go below the trip threshold

pc_low_loop

	btfss	A2D_PORT,A2D_IN_BIT
	 goto	pc_high

	addwf	total_lo,f
	skpnc
	 addwf	total_hi,f

	skpc
	 goto	pc_low_loop

	movlw	ERROR_STUCK_HI
	movwf	error_code
	goto	display_error

    ; Now make the capacitor voltage go above the trip threshold

pc_high
	BSF	A2D_PORT,A2D_OUT_BIT
	clrf	total_lo
	clrf	total_hi

pc_high_loop

	btfsc	A2D_PORT,A2D_IN_BIT
	 goto	precharged

	addwf	total_lo,f
	skpnc
	 addwf	total_hi,f

	skpc
	 goto	pc_high_loop

	movlw	ERROR_STUCK_LO
	movwf	error_code
	goto	display_error

precharged:

	clrf	total_lo
	clrf	total_hi

	bcf	A2D_PORT,A2D_OUT_BIT

	return

;--------------------------------------------------------
;
; error - write an error code to portb. if the error code is
;         zero then toggle portb between 0 and 0xff

display_error:
	bsf	STATUS,RP0
	clrf	PORTB		;Make portb i/o pins outputs
	bcf	STATUS,RP0

error_loop:
	movf	error_code,W
	movwf	PORTB
	skpz
	 goto   error_loop

    ;If the error code is zero, then flash the LEDs

	call	big_delay
	comf	PORTB,f
	call	big_delay
	goto	error_loop

big_delay:
	clrf    t1
	clrf	t2

	decfsz	t1,f
	 goto	$-1
	decfsz	t2,f
	 goto	$-3
	return

delay16 NOP
delay15 NOP
delay14 NOP
delay13 NOP
delay12 NOP
delay11 NOP
delay10 NOP
delay9  NOP
delay8  NOP
delay7  NOP
delay6  NOP
delay5  NOP
delay4  RETURN

        END