Two methods of harvesting entropy from an Arduino microcontroller board. Pseudo-Publication here: https://gist.github.com/bloc97/b55f684d17edd8f50df8e918cbc00f94

arduino_entropy_rng.c

#include <MPU6050.h>

uint8_t randomByte(uint8_t analogPin) {		
  
  int firstValue = analogRead(analogPin);

  const uint8_t minTemporalEntropyScale = 8; //Resolution in microseconds of the onboard micros() timer
  
  int bitsCaptured = 0;

  uint8_t randomBits = 0x00;
  uint8_t lastRandomBit = 0x00;

  while (bitsCaptured < 8) {
    int newValue = analogRead(analogPin);
    if (firstValue != newValue) {
      firstValue = newValue;
      uint8_t microsNoise = (micros() / minTemporalEntropyScale) & 1;
      uint8_t previousNoise = lastRandomBit;
      lastRandomBit = (microsNoise ^ previousNoise) & 1);
      randomBits = randomBits | (lastRandomBit << bitsCaptured);
      bitsCaptured++;
    }
  }
  
  return randomBits;
}


uint8_t randomByteFromAccel(MPU6050 mpu) {
  
  Vector rawAccel = mpu.readRawAccel();
  
  const uint8_t minEntropyScale = 4; //Resolution of the MPU sensor, the one I used outputed integers that are multiples of 4.

  int16_t xBits = rawAccel.XAxis;
  int16_t yBits = rawAccel.YAxis;
  int16_t zBits = rawAccel.ZAxis;

  uint8_t real4xBits = (xBits / minEntropyScale) & 0xF;
  uint8_t real4yBits = (yBits / minEntropyScale) & 0xF;
  uint8_t real4zBits = (zBits / minEntropyScale) & 0xF;

  uint8_t random8Bits = ((real4xBits & 0x3) << 6) ^ (real4zBits << 4) ^ (real4yBits << 2) ^ real4xBits ^ (real4zBits >> 2);

  return random8Bits;
}