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
+These are implementations of the Galois LFSR (Linear Feedback Shift Register) 8-bit random generator (tested) The next one is not a funtion. Place it in the code where the random number is needed or you can put it in a function yourself. The prime LFSR polynom is 0xB4. BCF STATUS,C RRCF LFSRVALUEL,W BTFSC STATUS,C XORLW 0xB4 MOVWF LFSRVALUEL 16-bit random generator (tested) The 0xA1 is part of a prime polynom value 0xA1A1 which generates a full LFSR Random: BCF STATUS,C RRCF LFSRVALUEH,F RRCF LFSRVALUEL,F BTFSS STATUS,C RETURN MOVLW 0xA1 XORWF LFSRVALUEH XORWF LFSRVALUEL RETURN 24-bit random generator (tested) The prime polynom is 0xD7D7D7 Random: global Random BCF STATUS,C RRCF LFSRVALUEU,F RRCF LFSRVALUEH,F RRCF LFSRVALUEL,F BTFSS STATUS,C RETURN MOVLW 0xD7 XORWF LFSRVALUEU XORWF LFSRVALUEH XORWF LFSRVALUEL RETURN 32-bit random generator (tested) The prime polynom is 0xA6A6A6A6 (Yes this is a prime polynom! It went through a hell to find it) Random: BCF STATUS,C RRCF LFSRVALUEV,F RRCF LFSRVALUEU,F RRCF LFSRVALUEH,F RRCF LFSRVALUEL,F BTFSS STATUS,C RETURN MOVLW 0xA6 XORWF LFSRVALUEV XORWF LFSRVALUEU XORWF LFSRVALUEH XORWF LFSRVALUEL RETURN
|file: /Techref/microchip/rand8bit.htm, 4KB, , updated: 2010/4/29 10:42, local time: 2017/9/24 01:40,
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<A HREF="http://www.piclist.com/techref/microchip/rand8bit.htm"> PIC Microcontoller Math Methods - Quick little 8 bit random number generators</A>
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