Arduino random seed



If you only need to seed the internal random number function on your arduino (or other AVR based PRNG), the following example sketch demonstrates a small method to achieve a good seed value with a reasonably uniform 32-bit (long) distribution. 
// TrueRandomSeed.ino
// This example sketch shows how to provide a truly random seed value to the built in
// library pseudo random number generator. This ensures that your sketch will be
// using a different sequence of random numbers every time it runs. Unlike the
// usually suggested randomSeed(analogRead(0)) this method will provide a much more
// uniform and varied seed value. For more information about the basic technique used
// here to produce a random number or if you need more than one such number you can
// find a library, Entropy from the following web site along with documentation of how
// the library has been tested to provide TRUE random numbers on a variety of AVR
// chips and arduino environments.
//
// https://sites.google.com/site/astudyofentropy/project-definition/
// timer-jitter-entropy-sources/entropy-library
//
// Copyright 2014 by Walter Anderson, wandrson01 at gmail dot com
//
 
#include <avr/interrupt.h>
#include <avr/wdt.h>
#include <util/atomic.h>
// The following addresses a problem in version 1.0.5 and earlier of the Arduino IDE
// that prevents randomSeed from working properly.
// https://github.com/arduino/Arduino/issues/575
#define randomSeed(s) srandom(s)
 
volatile uint32_t seed; // These two variables can be reused in your program after the
volatile int8_t nrot; // function CreateTrulyRandomSeed()executes in the setup()
                     // function.
 
void CreateTrulyRandomSeed()
{
seed = 0;
nrot = 32; // Must be at least 4, but more increased the uniformity of the produced
// seeds entropy.
// The following five lines of code turn on the watch dog timer interrupt to create
// the seed value
cli();
MCUSR = 0;
_WD_CONTROL_REG |= (1<<_WD_CHANGE_BIT) | (1<<WDE);
_WD_CONTROL_REG = (1<<WDIE);
sei();
 
while (nrot > 0); // wait here until seed is created
 
// The following five lines turn off the watch dog timer interrupt
cli();
MCUSR = 0;
_WD_CONTROL_REG |= (1<<_WD_CHANGE_BIT) | (0<<WDE);
_WD_CONTROL_REG = (0<< WDIE);
sei();
}
 
ISR(WDT_vect)
{
nrot--;
seed = seed << 8;
seed = seed ^ TCNT1L;
}
 
void setup()
{
CreateTrulyRandomSeed();
randomSeed(seed);
// The preceeding two function calls will take approximately 0.5 second to execute if
// nrot is set to 32
... the rest of your setup code should FOLLOW from here.
}
 
void loop()
{
}

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