LFSR

One of the basic building blocks used here is the Linear Feedback Shift Register. This is used to generate random values. The one used here is a 16 bit shift register. The values at certain taps are xor'ed together and feed back into the shift register. The number of unique states should be 2^n -1. In this case 2^16 -1 = 65535.

This is the same LFSR as is described in the wikipedia.

https://en.wikipedia.org/wiki/Linear_feedback_shift_register

Internally, the code is REALLY simple Define your lfsr:

unsigned int lfsr  = 1;


void clockLfsr () {
  //calculate new state
  boolean outputBit = bitRead(lfsr, 10) ^ bitRead(lfsr, 12)  ^ bitRead(lfsr, 13) ^ bitRead(lfsr, 15);
  lfsr = lfsr << 1;
  lfsr |= outputBit;
}

Each time you want to clock the LFSR, you just call the above code

grab the number of bits you want by masking it off

byte reg = lfsr & B11111111;

I generally shift it 4x to get a new number. You can do 2 and 4, but if you do 3x, you actually reduce the number of total states.