di3227.txt
;******************************************************************************
;
; LISTING 1 
;
; "Make noise with a PIC," EDN, August 7, 2003, pg 77
;
Copyright (c) 2003 Peter Guettler, APS Software Engineering GmbH, Koeln 
;  Non-commercial use granted.                
; -------------------------------------------------------------------------- 
;  Filename : RND32E.ASM
; -------------------------------------------------------------------------- 
;  Scope    : 31 Bit Pseudorandom generator                                                 
; -------------------------------------------------------------------------- 
;  Project  : RND32
;  Version  : 1.01                                                           
;  Date     : 28-FEB-2003                                                   
;  Autor    : P.Guettler                                                     
; -------------------------------------------------------------------------- 
;  History :                                                                
;   Vers: Datum:      Name:                                                  
;   V1.01 28-FEB-2003 GUT(APS)  Comments improved. 
;   V1.00 10-JUN-2001 GUT(APS)  Program optimized. 
;                               Cycle time is now constant 15Tc. Additional
;                               variable TEMP ("trade space for speed")
;   V0.00 04-MAR-2001 GUT(APS)  First release                               
; -------------------------------------------------------------------------- 
;  Info     :                                                                
;
;   The program generates random numbers with a 31 bit shift register.                                                                            
;   The 31 bit shift register requires only two feedback taps in contrast to
;   a 32 bit shift register which requires 4 taps.
;   Cycle time is 2^31 - 1 = 2147483647 states. At 4MHz oscillator frequency
;   the program generates noise with spectral components ranging from 15.5uHz 
;   up to 33.3kHz. 
;   The random pattern repeats each 17.8 hours and is sufficient "random" for
;   most audio frequency purposes.
;   
;   For a comprehensive table of Linear Feedback Shift Register Taps see
;   XILINX app.note.  XAPP 052 of July 7, 1996.                                                                         
; ************************************************************************** 

; Target: Microchip PIC12C508(A) / PIC12C509(A)

; Lower memory area: program
; Upper memory area: unused

	LIST P=12C509A

; Configuration word:
;
; MCLRE = 0	(disabled)
; CP	 = 0/1	(code protection on/off)
; WDTE	 = 0	(disabled)
; FOSC1 = 1	(internal oscillator)
; FOSC0 = 0
;  ==>  00AH  (without code protection)
;       002H  (with code protection)

; -------------------------------------------------------------------------- 
; Constants
; -------------------------------------------------------------------------- 
INADR	EQU	00H	; Indirect Address register
TMR0	EQU	01H	; Timer 0
PCL	EQU	02H	; lower 8 bits of Program Counter
STATUS	EQU	03H	; Status register
FSR	EQU	04H	; File select register
OSCCAL	EQU	05H	; Calibration register
GPIO	EQU	06H	; General Purpose I/O register

F	EQU	1	; Direction of Operation = F-Register
W	EQU	0	; Direction of Operation = W-Register

#DEFINE Z	STATUS,2	; zero flag - bit 2 of STATUS reg
#DEFINE C	STATUS,0	; carry flag - bit 0 of STATUS reg
#DEFINE PA0	STATUS,5	; page select bit (12C509 only)


;	 Sig.	Port	 I/O	Pin	Description
#DEFINE ENBL	GPIO,4; I	3	High = Enable

; Generated random signal is output at GPIO,0 
; and each shifted by one bit at GPIO,1 and GPIO,2

; -------------------------------------------------------------------------- 
; Variables
; -------------------------------------------------------------------------- 
RND0	EQU	07H	; RND shift register  byte 0
RND1	EQU	08H	;  "                  byte 1
RND2	EQU	09H	;  "                  byte 2
RND3	EQU	0AH	;  "                  byte 3
TEMP	EQU	0BH	; temporary storage for XOR feedback calculation 

; -------------------------------------------------------------------------- 
; Reset vektor
; -------------------------------------------------------------------------- 
	ORG  	0
LCOLD
	MOVWF	OSCCAL		; calibrate internal oscillator
	GOTO   MAIN

; -------------------------------------------------------------------------- 
; Subroutines
; -------------------------------------------------------------------------- 


; -------------------------------------------------------------------------- 
; Main program
; -------------------------------------------------------------------------- 
	ORG 	100H
MAIN

;-- Initialisation
      	CLRWDT             ; Clear WDT and prescaler
        
	MOVLW	0C7H	      ; init OPTION register:
                                ; GPWU 1 (disabled)
                                ; GPPU 1 (disabled)
                                ; T0CS 0 (Timer0 as timer)
                                ; T0SE 0 (positive edge)
                                ; PSA  0 (Prescaler assigned to timer0)
                                ; PS2  1 (Prescaler = 256)
                                ; PS1  1
                                ; PS0  1
	OPTION
                                                           
	MOVLW	38H		; set TRIS register: xx111000 = GP0..GP2 as outputs
     	TRIS   GPIO

	BCF	PA0		; set page 0
	CLRF	FSR		; clear RAM pointer


	MOVLW	0AFH		; preset shift register with
	MOVWF	RND0		; 0x340550AF
	MOVLW	050H		; (any other value != 0 will also
	MOVWF	RND1		; do the job)
	MOVLW	005H
	MOVWF	RND2
	MOVLW	034H
	MOVWF	RND3

	

LSTART
	BTFSS	ENBL		; wait for ENBL = high
	GOTO	LSTART


; calculate feedback value =  Q27 XOR Q30 and put in carry flag:
	SWAPF	RND3,W		; Q27 --> TEMP.7
	MOVWF	TEMP
	RLF	RND3,W		; Q30 --> W.7	
	XORWF	TEMP,F
	RLF	TEMP,F		; CY = Q27 XOR Q30

	RLF	RND0,F		; shift random number, CY --> Q0
	RLF	RND1,F
	RLF	RND2,F
	RLF	RND3,F

	MOVF	RND0,W		; output at GPIO,0..2
	MOVWF	GPIO

	GOTO	LSTART		; repeat forever 


	END