The PIC Source Book: Pulsin

measures the width of a pulse in 10-instruction-cycle units. Returns a 16-bit value and an error code.

Tachometers, duty-cycle meters, capacitor checkers, and precise interval timers can all be implemented with Pulsin.

To use Pulsin, you must set the desired pin to input and supply three arguments: the pulse direction in state (0 = 1-to-0, 255 = 0-to-1), the pin number (0 to 7) in pin, and the port number (0 = RA, 1 = RB, etc.) in w. When Pulsin returns, the 16-bit pulse width will be in hiB and lowB, and an error code in w. State and pin will have been modified by Pulsin for its own use, so your program should keep copies of these variables if needed.

The error codes returned in w are as follows:

• 0 = No error.
• 1 = Timed out waiting for setup before initial edge. Pin was already in the specified state (255 = 1, 0 = 0) when Pulsin was called. Pulsin waits up to 65,535 units of 10 instruction cycles (0.65535 seconds at 4 MHz) for setup.
• 2 = Timed out waiting for initial edge of pulse. Pulsin waits up to 65,535 units of 10 instruction cycles for the initial edge.
• 3 = Timed out waiting for final edge of pulse. Pulsin waits up to 65,535 units of 10 instruction cycles for the final edge.

Pulsin, like the other routines that accept pin and port arguments, requires the short table Pinz. Remember that tables must be located in the first 256 words of a 512-word program memory page. The same restriction applies if you decide to use a jump table (see Branch) to process the error code in w; the destinations of the calculated jumps must be in the first half of a program memory page.

One hint on using Pulsin: always assign the port number to w immediately before calling the routine. If you don't, some other instruction that uses w may change its contents, causing an error.

Demonstrating Pulsin.

To see Pulsin in operation, connect the circuit below to an erasable PIC or PIC emulator, such as the Parallax downloader. Assemble and run PULSIN.SRC. When you adjust the function generator, the binary value displayed on the LEDs will change.

;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.bubblesoftonline.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; PULSIN port (in w), pin, state
; Measures the width of an input pulse in 10-cycle (10 us at 4 MHz) units.
; Returns a 16-bit (0 to 65,535) value. If no pulse occurs within
; 65,535 10-cycle intervals, the routine returns an error code in w.
; Routine is edge triggered. If the input pin is already in the target state
; when the routine begins, it will wait for the state to change, then wait
; for the specified edge. The routine waits 65,535 x 10 cycles for
; each edge, so it normally returns within 0.65535 seconds (4 MHz).
; However, if each transition is delayed by close to the full 65,535 x 10
; cycles, the routine can take nearly 2 seconds to return (4 MHz).

	P = pic16c55
	#include <16c55.inc>   ; processor assembler definitions
	_CONFIG _xt_osc & _wdt_off & _protect_off
	reset start
	
	org 8
pin Res d'1'  ; Pin number to sample (0-7).
state Res d'1'  ; Trigger: 255 = 0-to-1 transition.
hiB Res d'1'  ; MSB of measurement.
lowB Res d'1'  ; LSB of measurement.

; Device data and reset vector
	org 0
; Table to convert pin number (0-7) into bit mask (00000001b to 10000000b).

Pinz         ADDWF pcl                  
             RETLW d'1'                 
             RETLW d'2'
             RETLW d'4'
             RETLW d'8'
             RETLW d'16'
             RETLW d'32'
             RETLW d'64'
             RETLW d'128'

start        MOVLW d'0'                 ; Outputs to display result on LEDs.
             TRIS 6h
             MOVLW d'0'                 
             TRIS 7h
             MOVLW b'00001111'          ; All inputs
             TRIS 5h
             MOVLW d'0'                 ; Negative pulse.
             MOVWF state
             MOVLW d'2'                 ; Pin 2.
             MOVWF pin
             MOVLW d'0'                 ; Port ra.
             CALL Pulsin                
             MOVF lowB,w                ; Write Pulsin result to ports RB
             MOVWF 6h
             MOVF hiB,w                 ; and RC for viewing on LEDs.
             MOVWF 7h
             GOTO start                 ; Do it again.

; Upon entry, the desired pin must already be set up as an input.
; The w register contains a number representing the input port (0-2) for
; RA through RC. Variable pin contains the pin number (0-7). Variable state
; contains 255 for a 0-to-1 (positive) pulse, or 0 for a 1-to-0 (negative) pulse.

; Upon exit, the 16-bit count is in variables lowB and hiB.
; Variable pin contains the mask bit used by pulsin (not the pin number
; anymore, but a value with a 1 in the pin position), and state has been
; ANDed with pin. W contains the error code: 0= no error; 1= pin in
; already in target state when Pulsin was called--timeout waiting
; for transition; 2= timeout waiting for initial edge; 3= timeout waiting
; for final edge.

Pulsin       CLRF lowB                  ; Clear the 16-bit counter.
             CLRF hiB                   
             MOVWF fsr                  ; Point to the port number.
             MOVLW 5h                   ; Add offset for port RA.
             ADDWF fsr
             MOVF pin,w                 ; Get bit mask from the table.
             CALL Pinz                  
             MOVWF pin                  ; Put the mask into pin.
             ANDWF state                ; Mask extra bits out of state.
Pulsin_Hold  MOVF indirect,w            ; Sample the port.
             ANDWF pin,w                ; Mask off bits other than pin.
             XORWF state,w              ; Check pin against state.
             BTFSS status,z             ; If pin <> state, set up for edge.
             GOTO Pulsin_doneH
             INCF lowB                  ; Otherwise, lowB = lowB+1.
             BTFSC status,z             ; Overflow in lowB?
             INCFSZ hiB                 ; Yes: hiB=hiB+1, watch for overflow.
             GOTO Pulsin_Hold           ; Sample the pin again.
             RETLW d'1'                 ; Overflow in hiB? Timed out: return.
Pulsin_doneH CLRF lowB                  
             CLRF hiB                   
Pulsin_Edge1 MOVF indirect,w            ; Sample the port.
             ANDWF pin,w                ; Mask off bits other than pin.
             XORWF state,w              ; Check pin against state.
             BTFSC status,z             ; If pin = state, wait until change.
             GOTO Pulsin_doneE
             INCF lowB                  ; Otherwise, lowB = lowB+1.
             BTFSC status,z             ; Overflow in lowB?
             INCFSZ hiB                 ; Yes: hiB=hiB+1, watch for overflow.
             GOTO Pulsin_Edge1          ; Sample the pin again.
             RETLW d'2'                 ; Overflow in hiB? Timed out: return.
Pulsin_doneE CLRF lowB                  
             CLRF hiB                   
Pulsin_Edge2 MOVF indirect,w            ; Sample the port.
             ANDWF pin,w                ; Mask off bits other than pin.
             XORWF state,w              ; Check pin against state.
             BTFSS status,z             ; If pin <> state, we're done.
             GOTO Pulsin_done
             INCF lowB                  ; Otherwise, lowB = lowB+1.
             BTFSC status,z             ; Overflow in lowB?
             INCFSZ hiB                 ; Yes: hiB=hiB+1, watch for overflow.
             GOTO Pulsin_Edge2          ; Sample the pin again.
             RETLW d'3'                 ; Overflow in hiB? Timed out: return.
Pulsin_done  RETLW 0h                   ; Done: pulsewidth in hiB, lowB.
             
             
             end

; PULSIN port (in w), pin, state
; Measures the width of an input pulse in 10-cycle (10 us at 4 MHz) units.
; Returns a 16-bit (0 to 65,535) value. If no pulse occurs within
; 65,535 10-cycle intervals, the routine returns an error code in w.
; Routine is edge triggered. If the input pin is already in the target state
; when the routine begins, it will wait for the state to change, then wait
; for the specified edge. The routine waits 65,535 x 10 cycles for
; each edge, so it normally returns within 0.65535 seconds (4 MHz).
; However, if each transition is delayed by close to the full 65,535 x 10
; cycles, the routine can take nearly 2 seconds to return (4 MHz).

	device	pic16c55,xt_osc,wdt_off,protect_off
	reset	start
	
	org	8
pin	ds	1		; Pin number to sample (0-7).
state	ds	1		; Trigger: 255 = 0-to-1 transition.
hiB	ds	1		; MSB of measurement.
lowB	ds	1		; LSB of measurement.

; Device data and reset vector
	org	0
; Table to convert pin number (0-7) into bit mask (00000001b to 10000000b).

Pinz	jmp	pc+w
	retw	1,2,4,8,16,32,64,128

start	mov	!rb,#0		; Outputs to display result on LEDs.
	mov	!rc,#0
	mov	!ra, #1111b	; All inputs
	mov	state,#0	; Negative pulse.
	mov	pin,#2		; Pin 2.
	mov	w,#0		; Port ra.
	call	Pulsin
	mov	rb,lowB 	; Write Pulsin result to ports RB
	mov	rc,hiB		; and RC for viewing on LEDs.
	jmp	start		; Do it again.

; Upon entry, the desired pin must already be set up as an input.
; The w register contains a number representing the input port (0-2) for
; RA through RC. Variable pin contains the pin number (0-7). Variable state
; contains 255 for a 0-to-1 (positive) pulse, or 0 for a 1-to-0 (negative) pulse.

; Upon exit, the 16-bit count is in variables lowB and hiB.
; Variable pin contains the mask bit used by pulsin (not the pin number
; anymore, but a value with a 1 in the pin position), and state has been
; ANDed with pin. W contains the error code: 0= no error; 1= pin in
; already in target state when Pulsin was called--timeout waiting
; for transition; 2= timeout waiting for initial edge; 3= timeout waiting
; for final edge.

Pulsin	clr	lowB		; Clear the 16-bit counter.
	clr	hiB
	mov	fsr,w		; Point to the port number.
	add	fsr,#RA 	; Add offset for port RA.
	mov	w,pin		; Get bit mask from the table.
	call	Pinz
	mov	pin,w		; Put the mask into pin.
	AND	state,w		; Mask extra bits out of state.
:Hold	mov	w,indirect	; Sample the port.
	AND	w,pin		; Mask off bits other than pin.
	XOR	w,state		; Check pin against state.
	jnz	:doneH		; If pin <> state, set up for edge.
	inc	lowB		; Otherwise, lowB = lowB+1.
	snz			; Overflow in lowB?
	incsz	hiB		; Yes: hiB=hiB+1, watch for overflow.
	jmp	:Hold		; Sample the pin again.
	retw	1		; Overflow in hiB? Timed out: return.
:doneH	clr	lowB
	clr	hiB
:Edge1	mov	w,indirect	; Sample the port.
	AND	w,pin		; Mask off bits other than pin.
	XOR	w,state		; Check pin against state.
	jz	:doneE		; If pin = state, wait until change.
	inc	lowB		; Otherwise, lowB = lowB+1.
	snz			; Overflow in lowB?
	incsz	hiB		; Yes: hiB=hiB+1, watch for overflow.
	jmp	:Edge1		; Sample the pin again.
	retw	2		; Overflow in hiB? Timed out: return.
:doneE	clr	lowB
	clr	hiB
:Edge2	mov	w,indirect	; Sample the port.
	AND	w,pin		; Mask off bits other than pin.
	XOR	w,state		; Check pin against state.
	jnz	:done		; If pin <> state, we're done.
	inc	lowB		; Otherwise, lowB = lowB+1.
	snz			; Overflow in lowB?
	incsz	hiB		; Yes: hiB=hiB+1, watch for overflow.
	jmp	:Edge2		; Sample the pin again.
	retw	3		; Overflow in hiB? Timed out: return.
:done	ret			; Done: pulsewidth in hiB, lowB.

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