;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; PULSOUT port, pin, time
; Generates an output pulse in 10-cycle (10 us at 4 MHz) units,
; based on a 16-bit (1 to 65,535) value. The pulse is the reverse
; of the pin's state when pulsout is called. For instance,	if the
; specified pin is initially 1, Pulsout will invert it to make a
; negative-going pulse.

	P = pic16c55
	#include <16c55.inc>   ; processor assembler definitions
	_CONFIG _xt_osc & _wdt_off & _protect_off
 reset start

 org 8
hiB Res d'1' ; MSB of time.
lowB Res d'1' ; LSB of time.
pin Res d'1' ; Pin number to pulse (0-7).

temp Res d'1' ; Temporary variables for time delay
temp2 Res d'1' ; between pulses

; 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'                 ; All outputs.
             TRIS 5h
             CLRF 5h                    ; Start with 0s.
             MOVLW d'10'                ; 10 x 10-cycle pulse
             MOVWF lowB
             CLRF hiB                   
             MOVLW d'2'                 ; Pin 2.
             MOVWF pin
             MOVLW d'0'                 ; of port ra.
             CALL Pulsout               ; Pulse pin high for 100 cycles.
             CALL delay                 ; Wait a while between pulses.
             GOTO start                 ; Do it again.

; Upon entry, the desired pin must already be set up as an output.
; The w register contains a number representing the output port (0-2) for
; RA through RC. Variable "pin" contains the pin number (0-7). The variables
; hiB and lowB are the MSB and LSB of the time argument.

Pulsout      MOVWF fsr                  ; Point to the port number.
             MOVLW 5h                   ; Add offset for port RA.
             ADDWF fsr
             MOVF pin,w                 
             CALL Pinz                  ; Get bit mask from the table.
             MOVWF pin                  ; Put the mask into pin
             COMF hiB                   ; Take twos complement
             COMF lowB                  ; of the 16-bit counter
             INCF lowB                  
             BTFSC status,z             ; If zero, lowB overflowed,
             INCF hiB                   ; so carry into hiB.
             MOVF pin,w                 ; Invert the selected pin.
             XORWF indirect

; The main timing loop. Remove the nops for 5-cycle resolution.
Pulsout_loop GOTO $+1                   ; Two-cycle "nop."
             GOTO $+1                   ; Two-cycle "nop."
             NOP                        
             INCF lowB                  ; lowB = lowB+1.
             BTFSC status,z             ; Overflow in lowB?
             INCFSZ hiB                 ; Yes: hiB=hiB+1, watch for overflow.
             GOTO Pulsout_loop          ; If not overflow, do it again.
             MOVF pin,w                 ; Invert pin (back to initial state).
             XORWF indirect
             RETLW 0h                   

; Delay routine for demonstration. Not required by Pulsout.

delay        DECFSZ temp                ; Time delay to provide spacing
             GOTO delay
             DECFSZ temp2               ; between pulses.
             GOTO delay
             RETLW 0h                   
             
             
             end