// 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()
{
}