Random Function

Projects Programming
1

Can anyone tell me what sort of algorithm the arduino uses to create the numbers in the random() function?

Thanks

2

http://cvs.savannah.gnu.org/viewvc/avr-libc/avr-libc/libc/stdlib/random.c?revision=1.2.2.1&view=markup

ATTRIBUTE_CLIB_SECTION
static long
do_random(unsigned long *ctx)
{
	/*
	 * Compute x = (7^5 * x) mod (2^31 - 1)
	 * wihout overflowing 31 bits:
	 *      (2^31 - 1) = 127773 * (7^5) + 2836
	 * From "Random number generators: good ones are hard to find",
	 * Park and Miller, Communications of the ACM, vol. 31, no. 10,
	 * October 1988, p. 1195.
	 */
	long hi, lo, x;

	x = *ctx;
	/* Can't be initialized with 0, so use another value. */
	if (x == 0)
		x = 123459876L;
	hi = x / 127773L;
	lo = x % 127773L;
	x = 16807L * lo - 2836L * hi;
	if (x < 0)
		x += 0x7fffffffL;
	return ((*ctx = x) % ((unsigned long)RANDOM_MAX + 1));
}

static unsigned long next = 1;

ATTRIBUTE_CLIB_SECTION
long
random(void)
{
	return do_random(&next);
}
3

This could be a bit offtopic, but I found this site about randomness, the provided example seems to improve the overall random generator inside arduino; at setup() write:

unsigned long seed=0, count=32;
while (--count)
  seed = (seed<<1) | (analogRead(0)&1);
randomSeed(seed);

Note: this is only effective if A0 is not used by anything else.

4

We've tried the "read analog input" trick before. It works fairly well but is certainly not random. There is a mega thread about this on the forum somewhere, complete with scatter graphs.

Rob

5

I said "seems to improve"..
Also pay attention that using the previous block of code is far better than using "randomSeed( analogRead(0) );".

6

@fuh
your code indeed generates a better seed than just one analogRead(); wrote about it some time ago and used the internal temp sensor (not supported by all duinos)

Same problems, but it keeps your A0 line available