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PIC Microcontoller Math Methods

Quick little 8 bit random number generators

Andrew Warren [fastfwd at ix.netcom.com] of Fast Forward Engineering - San Diego, California says

Load a register called "RANDOM" with any non-zero value, then call this routine each time you'd like a new pseudo-random value:
    LFSR:   RLF     RANDOM,W
            RLF     RANDOM,W
            BTFSC   RANDOM,4
            XORLW   1
            BTFSC   RANDOM,5
            XORLW   1
            BTFSC   RANDOM,3
            XORLW   1
            MOVWF   RANDOM
            RETLW   0

Scott Dattalo says:

[with the double RLF at the start of this routine,] Andy is implementing 'roll left' where the most significant bit of RANDOM will get copied to least significant position. This is how it works. The first RLF will copy the most significant bit of RANDOM into the carry. What ever was in the carry prior to the first RLF will get copied into the least significant bit position - but we don't care. Also, since the destination is the W register, the variable RANDOM is unaffected. The second RLF repeats the same rotate operation, but this time the carry has been initialized to the MS bit of random. So this second rotate will copy the MS bit into the least significant bit. All of the other bits are of course shifted left one bit position. See?

Or, if you prefer, you can use this routine (written by Marv Isaacman) instead:

    MARV:   MOVLW   01DH
            CLRC
            RLF     RANDOM
            SKPNC
            XORWF   RANDOM
            RETLW   0

From Robert LaBudde and Nikolai Golovchenko

Randomize
;Rnew = Rold * 221 + 53
;221 = 256 - 32 - 4 + 1
;256 can be eliminated
;so we need to calculate Rnew = Rold * (1 - 32 - 4) + 53 using
;truncating arithmetic
;or Rnew = Rold * (-32 - 3) + 53
                clrc
                rlf     Number, 1
                swapf   Number, 0
                andlw   0xE0
                rrf     Number, 1
                addwf   Number, 0
                addwf   Number, 0
                addwf   Number, 0
                sublw   0x35
                movwf   Number


See piclist post 2000\12\17\211923a

Code:

Comments:

Questions:

See also:


file: /Techref/microchip/rand8bit.htm, 4KB, , updated: 2010/4/29 09:42, local time: 2017/11/24 19:50,
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