> Sorry but which LFSR? The simple LFSR in the PIClist
> archive does not seem to do this.
> 
> 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

It does. Each time you call this routine it shifts the (in this case
16-bit) value on step and generates one new pseudo-random bit, which is
inserted into the just vacated bit. If you want it to generate for
instance a random byte you must call it 8 times, otherwise there will be
a very strong correlation between successive bytes.

OMHO 16 bit is often a bit short, I would opt for a 24-bit SR. The Jal
library contains one:

var byte _random_b1
var byte _random_b2
var byte _random_b3 

-- make sure there is at least one bit set!
asm bsf _random_b1, 0

procedure _random_shift is
   -- from the art of electronics, p657:
   -- 24 bits LFSR needs xor feedback from taps at 17, 22 and 23
   assembler
           movlw 0
      bank btfsc _random_b3, 1
              xorlw 0xFF
           btfsc _random_b3, 6
              xorlw 0xFF
           btfsc _random_b3, 7
              xorlw 0xFF
           addlw 1 
      bank rlf   _random_b1, f
      bank rlf   _random_b2, f
      bank rlf   _random_b3, f
   end assembler
end procedure

Wouter van Ooijen

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