i need to have a random number between 0 and 7 generated. i am using an attiny2313. can any one give me a little snippet of code.

## random number generation

**mmmlinux wrote:**

i need to have a random number between 0 and 7 generated. i am using an attiny2313. can any one give me a little snippet of code.

From the C FAQ:

13.16: How can I get random integers in a certain range?A: The obvious way,

rand() % N /* POOR */

(which tries to return numbers from 0 to N-1) is poor, because

the low-order bits of many random number generators are

distressingly *non*-random. (See question 13.18.) A better

method is something like(int)((double)rand() / ((double)RAND_MAX + 1) * N)

If you're worried about using floating point, you could use

rand() / (RAND_MAX / N + 1)

Both methods obviously require knowing RAND_MAX (which ANSI

#defines in), and assume that N is much less than

RAND_MAX.(Note, by the way, that RAND_MAX is a *constant* telling you

what the fixed range of the C library rand() function is. You

cannot set RAND_MAX to some other value, and there is no way of

requesting that rand() return numbers in some other range.)If you're starting with a random number generator which returns

floating-point values between 0 and 1, all you have to do to get

integers from 0 to N-1 is multiply the output of that generator

by N.References: K&R2 Sec. 7.8.7 p. 168; PCS Sec. 11 p. 172.

--

brunk

this is a good fast integer generator

http://en.wikipedia.org/wiki/Lin...

I use this specific line for integers from 0 to 250

rand=(rand*109+89)%251;

init rand with some value. I start with 89. I think even 0 works. requires 16bit operations. might work with %256 too for 0-255 range. I just use primes.

short sequence of 40 numbers seems to have nice even distribution

Kunne ogsÃ¥ bare gÃ¸res sÃ¥dan

Dim I As Word ' dim variable

Do

I = Rnd(40) 'get random number (0-39)

Print I 'print the value

Wait 1 'wait 1 second

Loop

It would be nice to post in English language.

BTW, Bing translates it as

"Could also just be such"

oh.. all people not speak norwegian!

If you want a non-deterministic random generator (i.e. one not based on mathematical algorithms), just do some ADC samples of an open port line and use the low bit of consecutive samples to construct your random number.

-- Thilo

I also don't speak norwegian but can still understand good enough to translate "can also be made like this".

Some algorithms are better than others, but depending on what you are planning to do with the random number, we cannot know what is overkill and what is stupid way.

For normal dice games etc it is good enough to do something like this (pseudocode):

repeat random=random+1; if random==8 then random=0; until key_pressed;

That's danish, not norwegian.

A number from 0 to 7?

How random can 3 bits be?

There has been a pretty recent post about using the lower bit(s) of the ADC as input for a random number and a few years ago a guy wrote a nice article with several different sources for random bits in AVR processors. The ADC was one of the sources, phase noise between the avr main chrystal and a 32kHz watch chrystal was another. The article (in the pojects section?) also had (has) code examples and some analysis of the randomness of those different sources. Good read and highly recomended for danisch, norwegian and other people.

**Paulvdh wrote:**

A number from 0 to 7?

How random can 3 bits be?

That's a very good question. Here's a sequence of 20 independent random numbers from 0 to 7

3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3

No, I'm not kidding, it really is. It's just that 3 have a 99.99% probability to be drawn and thus a very low probability for the other numbers to share.

Here's another sequence from 0 to 7 generated with a completely different method

3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3

In this case I have a start number choosen with equal probability and next number be the same as the previous with 99.99% probability and with 0.01% probability be a new one choosen uniformly from 0 to 7.

These two methods will give completely different sequences in the long run, with completely different statistical properties. But they're both random sequences of 3 bit random numbers.

So how random can 3 bits be? Well they can be random in very many ways. Usually one want something more than just "random".

@sniggelen:

You're complely wrong of course, it's not "3" but it's "4".

int getRandomNumber() { return 4; // Chosen by fair dice roll. // Guaranteed to be random. }

Reference:

http://xkcd.com/221/

Anyway, I always did suck at statistics at school.

Usually one want something more than just "random".

There are a variety of well-studied techniques for this. Google "pseudorandom number generation" or "pseudorandom number generator".

I believe that modern Intel chips have an instruction or somesuch that actually produces true random numbers (it uses two interfering oscillators plugged into a voltage meter, or something) but I do not know whether any programming libraries actually tap into this.

I believe that modern Intel chips have an instruction or somesuch that actually produces true random numbers (it uses two interfering oscillators plugged into a voltage meter, or something) but I do not know whether any programming libraries actually tap into this.

What relevance would that have to an avr-gcc programmer anyway?

In the meantime the Tutorial forum here has at least two good articles about random numbers on AVRs.

One I tried in my younger days was to read the program's code byte by byte. It was terrible.

It was terrible.

Not only not random but presumably totally deterministic. At least if you have a number of variables in SRAM I suppose there'd be a chance if you pointed to the area (maybe the SFRs too?) then hashed/CRCd the whole lot you mioght get something a bit random?

Thanks worked a treat. I know it's not purely random, but it's varied enough to suit my needs.

**TPE wrote:**

oh.. all people not speak norwegian!

What a surprise.

Hi people!

This is what I use. May have some bugs but worked for me (atTiny2313).

void initRand(void) { uint8_t state; static uint8_t EEMEM sstate; state = eeprom_read_byte(&sstate); // Check if it's unwritten EEPROM (first time). Use something funny // in that case. if (state == 0xff) state = 0xAA; srand(state); eeprom_write_byte(&sstate, rand()); } int main(void){ initRand(); #define SLEEPTIME 5 #define MAX_DICE 5 while(1){ //some blinking, as if rolling for(uint8_t i=0;i<=20;i++){ showSequence(SLEEPTIME); sleep(SLEEPTIME); } //show result uint8_t dice = rand() / (RAND_MAX / MAX_DICE + 1); while(1){ showDice(dice+1); } }

Hope it is ok to bump an old thread, but this show up on google.

The code is from my lame dice game, to test charlieplexing. Shameless plug:

https://raw.github.com/marcolinu...

EDIT: code tags. Thanks for the hint js!

Shameless hint here http://www.avrfreaks.net/index.p... :wink:

Here is one I did that uses entropy on floating input pin (PD6 in this case) in combination with the linear generator from widipedia:

#define RANDOM_PORT PORTD

#define RANDOM_PIN PIND

#define RANDOM_DDR DDRD

#define RANDOM_BIT PD6

uint16_t _fx_rand()

{

static uint16_t lfsr = 0xACE1u;

static uint16_t bit;

bit = ((lfsr >> 0) ^ (lfsr >> 2) ^ (lfsr >> 3) ^ (lfsr >> 5) ) & 1;

return lfsr = (lfsr >> 1) | (bit << 15);

}

void fx_random(char *buf, uint16_t size){

// set pin as input

RANDOM_DDR &= ~_BV(RANDOM_BIT);

RANDOM_PORT &= ~_BV(RANDOM_BIT);

while(size){

size--;

buf[size] = 0;

for(int c = 0; c < 8 * sizeof(char); c++){

uint16_t timeout = 0xff;

uint16_t x = 1;

uint8_t d = RANDOM_PIN & _BV(RANDOM_BIT);

while((d == (RANDOM_PIN & _BV(RANDOM_BIT))) && (timeout--)){

x = _fx_rand();

}

buf[size] |= ((x & 1) << c);

}

}

}

Oh.. forgot to mention..

Usage:

random int:

int random = 0; fx_random((char*)&random, sizeof(int));

random buffer:

char buf[20]; fx_random(buf, sizeof(buf));