Re: [avr-gcc-list] rand()

[Matthias Granberry]

In-Reply-To: <address@hidden> (Torsten Hahn's message
of "Wed, 10 Jul 2002 10:20:05 +0200")
References: <address@hidden>

Last time I checked, it was there.  The AVR micros have the timers and
some even have an input capture.  You could seed it by detecting a
level change on an external RC oscillator or some similar scheme.  It
looks like it returns an unsigned 32-bit int using a linear
congruential scheme, so it might have some period problems, but it
might not be too bad.  One thing that you might want to look into is
the randomness of the low-order bits.

Here is the actual calculation:

static int
do_rand(unsigned long *ctx)
{
        return ((*ctx = *ctx * 1103515245 + 12345) % ((unsigned long
int)RAND_MAX + 1));
}


I haven't done any calculations or tests on it, but it is possible
that if you try and get a random number by calling (n=rand()%10;) or
something similar, the numbers will quickly fall into a loop, giving
you fairly poor "random" numbers.

Matthias Granberry
University of Texas at Dallas

Torsten Hahn <address@hidden> writes:

> Hi,
>
> Has somebody still implemented an random number generator on the avr
? May be, 
> it would be nice such a function in the avr-libc. Random number routines
are 
> not the problem i think but how to generate a "seed". On a pc you can
use the 
> time() function. Has somebody an idea how to do it on avr ?
>
> Cu,
> Torsten.
> -- 
> Torsten Hahn 
> TU Bergakademie Freiberg - Institut fr Experimentelle Physik 
> Silbermannstrae 1, 09596 Freiberg
> mail: address@hidden
> phone: +49 3731 392670 
> avr-gcc-list at http://avr1.org


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