> >Take a look at the following app note.  "Random" is relative; it depends
> >on the number of bits in the shift register before the pattern repeats.
>
> It's not random at all.
> It's completely and absolutely predictable.

True.  Hence the word "random" in quotes.  Actually, it is typically known
as a
psuedo-random sequence.

> OTOH, if I know the register configuration (tap points) and what state it
> is now, or what state it was at any time previous, then I can predict the
> next state exactly.
>

Absolutely.  A common usage is to send test patterns (such as the well
known 511 pattern) down a modem link to determine the bit error rate
of the link.  The receiver must know what the result should be in order
to determine how many errors occured.

Ken
klumia@adelphia.net


----- Original Message ----- 
From: "Dave VanHorn" <dvanhorn@dvanhorn.org>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>;
"Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Tuesday, October 12, 2004 12:07 AM
Subject: Re: [PIC] Twinkling LED's and other randomized Effects


>
> >
> >Take a look at the following app note.  "Random" is relative; it depends
> >on the number of bits in the shift register before the pattern repeats.
>
> It's not random at all.
> It's completely and absolutely predictable.
> Radioactive decay is random.
> Although it's gaussian in large groups, the time between decays is totally
> unpredictable.
> OTOH, if I know the register configuration (tap points) and what state it
> is now, or what state it was at any time previous, then I can predict the
> next state exactly.
>
> The interesting thing is that if you only look at a small subset of the
> bits, then it appears random. If the register is long enough, and it
> doesn't take that much, it's repeat time can exceed the age of the
universe.
>
> This makes it very useful for secure radio links.
>
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