;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; RANDOM
; Generates a pseudorandom number. Works best if called from a loop so that
; the value of its workspace variable (consisting of lowB and hiB) is
; constantly stirred.	

	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 16-bit random number.
lowB Res d'1' ; LSB of 16-bit random number.
temp Res d'1' ; Temporary counter for delay.
temp2 Res d'1' ; Temporary counter for delay.

; Device data and reset vector
 org 0

start mov !rb,#0  ; Output to show sequ onence LEDs.
             MOVLW d'13'                ; Arbitrary starting values
             MOVWF lowB
             MOVLW d'99'                ; for the shift register.
             MOVWF hiB
start_loop   CALL Random                
             MOVF lowB,w                ; Display 8-bit random numbers
             MOVWF 6h
             CALL delay                 ; on LEDs connected to RB.
             GOTO start_loop            ; Endless loop
; Random number generator.
Random       MOVF hiB,w                 ; First, ensure that hiB and lowB aren't
             IORWF lowB,w               ; all zeros. If they are, NOT hiB to FFh.
             BTFSC status,z             ; Otherwise, leave hiB and lowB as is.
             COMF hiB                   
             MOVLW 0x80                 ; We want to XOR hiB.7, hiB.6, hiB.4
             BTFSC hiB,d'6'             ; and lowB.3 together in W. Rather than
             XORWF hiB                  ; try to line up these bits, we just
             BTFSC hiB,d'4'             ; check to see whether a bit is a 1. If it
             XORWF hiB                  ; is, XOR 80h into hiB. If it isn't,
             BTFSC lowB,d'3'            ; do nothing. When we're done, the
             XORWF hiB                  ; XOR of the 4 bits will be in hiB.7.
             RLF hiB,w                  ; Move hiB.7 into carry. 
             RLF lowB                   ; Rotate c into lowB.0, lowB.7 into c. 
             RLF hiB                    ; Rotate c into hiB.0. 
             RETLW 0h                   ; Delay loop for program demo. Remove when using just Random. 
delay        DECFSZ temp                
             GOTO delay
             DECFSZ temp2               
             GOTO delay
             RETLW 0h                   

             
             
             end