> From: robin bussell <robin.bussell@lucent.com>
>
> > >on the chip. What i'm looking for is a *TRUE* random number generator (that
> > >is, as true as posible!)
> > > Thanks in advance.
> >
>  If you're using a pic with A/D conversion capabilities then the best
> you can do is
> to hook up a reverse biased diode as a noise source and sample it when
> you need
> a random number.. that way they end up proper quantum physics type
> random!

... or if you don't have ADC, try timing the interval between two
geiger counter pulses (cosmic rays will do, no need for radioactive
material).

Regarding diodes as noise sources, this should be a zener diode in
reverse breakdown, or a transistor BE junction in same condition
(about 8V typically).

For true uniform distribution, the output of the above techniques
should be fed into a polynomial generator, as in CRC generator, or into
a linear congruential generator i.e. r = r*a + b.  See Knuth,
Seminumerical Algorithms, for more details.  Merely sampling the level
of white noise will give you more or less Gaussian distribution.
Sampling geiger counter intervals will give Poisson distribution,
I think (my memory of uni physics is fading fast).

Regards,
SJH
Canberra, Australia