David Vanhorn's Random.asm
A 19 bit Pseudo Noise generator
A 19 bit Pseudo Noise generator
| CODE |
;*************************************************************************** ; ; File Name :'random.asm" ; Title : ; Date : ; Version : ; Support telephone :765 287 1987 David B. VanHorn ; Support fax :765 287 1989 ; Support Email :dvanhorn@cedar.net ; Target MCU :AT90S8515 ; ; DESCRIPTION ; ; DEFINITIONS ; ; ;***************************************************************************; ; M O D I F I C A T I O N H I S T O R Y ; ; ; rev. date who why ; ---- -------- --- ------------------------------------------ ; 0.01 97.09.30 dvh Creation ; 0.02 98.08.05 dvh Making sure it works, plus tightening up. ; 6uS at 8 Mhz. ; 0.03 98.08.23 dvh Removed some un-needed CLC instructions and ; replaced roX with lsX's ; Random: ;Maximal legnth 19 bit shift register sequence, push R16 ;Make workspace push R17 ; ; RAND3 RAND2 RAND1 ;22222111 11111110 00000000 ;43210987 65432109 87654321 mov R16,RAND1 ;Make copy andi R16,$13 ;Bits 5 and 2 and 1 mov R17,RAND3 ;Make copy andi R17,$04 ;Bit 19 ;clc ;not needed, because we will overwrite bit 0 later rol RAND1 ;Shift the bits D7->Carry rol RAND2 ;Carry->D0 D7->Carry rol RAND3 ;Carry->D0 D7->Carry, but that will be fixed. ;Now we have to Xor the bits to see what ;goes in to bit 1 rol R17 ;Move bit 19->20 andi R17,$08 ;Important that D1,0 be zero andi R16,$13 ;Mask out irrelevants, protecting 19 or R17,R16 ;Get bits 5->21 2->18 1->17 andi R17,$18 ;Nuke bits 18,17 lsr R17 ;19->19 5->20 lsr R17 ;19->18 5->19 lsr R17 ;19->17 5->18 eor R17,R16 ;First xor, result in R17 (5x2 in 02 and 19x1 in 01) mov R16,R17 ;Make a copy ror R16 ;Move the 5x2 result to 01 andi R17,$01 ;Mask off everything else andi R16,$01 ;in both eor R17,R16 ; brne D_One ;If one, do that, else D_Zero: ;Set a zero in the lsb of the low byte mov R16,RAND1 ;Get the low byte andi R16,$FE ;Make the LSB zero mov RAND1,R16 ;Put it back Rjmp D_Exit ;Bye bye D_One: ;Set a one in the lsb of the low byte mov R16,RAND1 ;Get the low byte ori R16,$01 ;Make the LSB one mov RAND1,R16 ;Put it back D_Exit: pop R17 ;Put everything back where I got it. pop R16 ; ret |