AVR Atmega128 random number generator from range

Question

I'm building project for studies on ATmega128 with my friend and we have a problem with random number generator (from 0 to 5), because the function always show the same result. We can't add time.h, because AVR Studio didn't accept this.

Code below:

uint8_t randomNumber(uint8_t r){
r = rand()%5;
return r;
}

other try

uint8_t randomNumber(uint8_t min, uint8_t max){
uint8_t = result;
result = min + rand() % (max+1 - min);
return result;
}

Any ideas? Thanks, Sebastian

Enable one of the hardware timers and use its value for `srand()`? — David Collins, Jun 21 '18 at 17:59
@DavidCollins Won't help. It will be deterministic. Need RTC. Or use external source of entropy, such as temperature sensor. — Eugene Sh., Jun 21 '18 at 18:04
This q talks about some ways to seed the RNG: [How do I generate random numbers in a microcontroller efficiently?](//stackoverflow.com/q/1558321) — 001, Jun 21 '18 at 18:15
Did you have any ideas how to write it? The problem is that the whole documentation is in Chinese and we have trouble with reading this. — Sebastian Jarosz, Jun 21 '18 at 18:27
Almost duplicate: https://stackoverflow.com/questions/10864668/is-it-possible-to-generate-random-numbers-using-physical-sensors/10871077#10871077 — JimmyB, Jul 10 '18 at 09:39

bathMarm0t · Accepted Answer · 2018-06-21T19:07:00.970

Wow this question sent me down the rabbit hole.

Pseudorandom numbers are relatively easy to generate.
Truly random numbers are VERY hard to generate.
The quality of your random number (whether biases appear in it) depends completely on your seed value.
Seed values for random number generators must be (wait for it) random, otherwise people can guess which numbers you are using, defeating the randomness of your generator.

Where to get a random seed value from?

Options proposed by the internet:

Natural noise from the environment (read an adc, or... https://www.fourmilab.ch/hotbits/ (i know it's not practical for an arduino project, but interesting none the less)).
Time user inputs (humans are by default not precise).
Timing differences between crystals. [https://en.wikipedia.org/wiki/Clock_drift]

Mild disclaimer: 1/3 have been proven unsafe in commercial environments, and it's easy to see how #2 could be gamed by using a computer instead of a human.

So the quickest way is probably to use a floating ADC. Before you think this is a good idea: https://skemman.is/bitstream/1946/10689/1/ardrand.pdf

Remember: Larger pools of seeds increases the randomness (aka using a 32bit random seed value is better than using a boolean random seed value).

ADC's on the 128 have 1024 values, realistically, a floating point value will trend to far less than that (I've read you should treat it like 32).

To improve your chances of getting random numbers, take the lowest bit from the adc reading multiple times (aka read the adc 16 times to get a 16 bit "random" number).

Assuming you have your adc set up etc.

UNTESTED PSEUDO CODE

/* srand example */
#include <stdio.h>      /* printf, NULL */
#include <stdlib.h>     /* srand, rand */
#include <avr/io.h>

//pseudo code.  you must implement init_adc() and read_adc()
int main ()
{
  //Init and seed.
  uint16_t u_rand_val = 0;
  uint16_t u_seed_rand_val = 0;

  init_adc();
  //Note we're assuming the channel that you are reading from is FLOATING or hooked up to something very noisy.  
  //Gather bits from the adc, pushing them into your pseudorandom seed.
  for(uint8_t i=0; i<16; i++){
      u_seed_rand_val = u_seed_rand_val<<1 | (read_adc()&0b1);
      }
  srand (u_seed_rand_val);

  while(1){
       //Do whatever you were going to do.
       //Note that calls to rand() use the seed set up by srand above.
       u_rand_val = rand()%5;
       print("Cur val:%u", u_rand_val);
       }
  return 0;
}

Thanks! But when i want to add this i get 2 errors from AVR Studio. undefined reference to 'init_adc' and the same to 'read_adc' — Sebastian Jarosz, Jun 21 '18 at 20:12
Get used to reading code before running it. I put a comment in there first thing saying you need to implement (read: write) init_adc and read_adc. Initializing and reading from an adc are must-knows for microcontroller work and are dependent on whatever chip you're using. I left out the implementations so the code was more readable/portable. Search around for how to use the ADC on an ATMEGA128 — bathMarm0t, Jun 21 '18 at 22:26

AVR Atmega128 random number generator from range

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