> -----Original Message-----
> From: Roman Black [SMTP:fastvid@EZY.NET.AU]
> Sent: Thursday, February 08, 2001 2:28 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: high res DA challenge
>
> Bob Ammerman wrote:
> >
> > > PS. The preferred way to generate good psuedo random
> > > numbers is to generate numbers between 0 and 255 (for
> > > eaxample) and place in a 256 unit lookup table where
> > > there is only ONE of each number, but the postions
> > > are random. Then you can read 256 random numbers
> > > in sequence and have perfect weighting.
> >
> > This is really unneeded.
> >
> > A well designed PRNG (say one based on a shift register with feedback) _
> > will, _by its very design_, generate every possible code in its range.
> >
> > Bob Ammerman
>
>
> Hi Bob, maybe I misunderstand you, but I still think
> the point was valid. If you generate a random number
> every time you update the pwm and the number is TRULY
> random it could be a disaster. You could generate the
> same number every time for an unknown amount of times.
> This is the real problem with truly random numbers.
>
> A psuedo random system (like I suggested) cannot generate
> the same number twice, nor can it generate the same
> number at all until all possible numbers have been read.
> This gives an equal weighting to all numbers IN ANY
> GIVEN TIME PERIOD which is never achieved with a true
> random number. A truly random system could generate
> any number at any time, it could generate the same number
> X times in a row and then not again for millions of
> generations. Very bad if it is controlling your pwm. :o)
> -Roman
>
What you suggest is absoluetly correct, but not exactly a major problem as
generating a such a truly random number with just a PIC would be pretty much
impossible.  A shift register PRNG, is both fast, easy to implement and seem
to have the ideal characteristics for this application.

Mike

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