The following code implements a linear feedback register type of
pseudo-random number generator.  This algorithm is frequently implemented in
hardware for spread-spectrum radio applications.

It will generate all possible bit combinations EXCEPT all zeroes, for
various lengths, if the value is Poly is chosen correctly, and Variable is
initialized with any non-zero number.  In other words, it will produce a
"maximal length sequence", in which each of 2^n-1 nonzero bit combinations
appear once and only once.  If Variable is initialized with 0, then it just
produces lots of zeroes.

This macro was used in an application where a pseudo-random stream of bits
was needed, hence its output of a bit in CY.  If you want up to an 8-bit
value, it will be contained in Variable after executing the macro.

Suitable values for Poly:
for 6 bits (sequence length 63): 0x21, 0x33, 0x36
for 7 bits (sequence length 127): 0x41, 0x44...
for 8 bits (sequence length 255): 0x8e, 0xb4...

For more suitable Poly values see: Dixon, Robert C., Spread Spectrum Systems
with Commercial Applications; (Table 3.7)
;------------------------------------------------------------
;  GenerateLRC Macro -- generates up to 8-bit LRC
;
;  Input:   Variable -- needs to be initialized (usually all ones to # of
bits)
;
;  Output:  Next output bit is in CY, Variable is updated for next cycle
;
;------------------------------------------------------------

GenerateLRC Macro Poly, Variable
   clrc
   rlf   Variable,f
   movlw Poly
   skpnc
   xorwf Variable,f
   endm

> -----Original Message-----
> From: Ron Anthony [mailto:ronant@OPTONLINE.NET]
> Sent: Saturday, August 18, 2001 10:37 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: Pseudo-Random numbers
>
>
> Hello all.  I need to generate numbers that appear to be
> ranomized, it makes
> no difference if they are truly random.  I can seed it with a
> value culled
> from ram before i wipe the ram, but it's not necessary.
>
> Any simple code to generate a seemingly random number, from 0 to 255?
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
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>

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