A 16 LFSR with start state detection and pooling from the current system uptime

16_bit_lfsr.c

///////////////////////////////////////////////////////////////////////////////
//
// randomNum takes void arguments and gets its random seed from the current
// uptime of the system
//
///////////////////////////////////////////////////////////////////////////////
int randomNum(void) {
    static uint16_t start_state = 0;
    static uint16_t lfsr = 0;
    if (lfsr == start_state) {
      start_state = 0;
      lfsr = 0;
    }
    /* Any nonzero start state will work. */
    start_state = (start_state == 0) ? start_state = (uint16_t) sys_uptime() : start_state;
    lfsr = (lfsr == 0) ? lfsr = start_state : lfsr;
    uint16_t bit;       /* Must be 16bit to allow bit<<15 later in the code */
    unsigned period = 0;

    do
    {
        /* taps: 16 14 13 11; feedback polynomial: x^16 + x^14 + x^13 + x^11 + 1 */
        bit  = ((lfsr >> 0) ^ (lfsr >> 2) ^ (lfsr >> 3) ^ (lfsr >> 5) ) & 1;
        lfsr =  (lfsr >> 1) | (bit << 15);
        ++period;
    } while (lfsr > MAX);
    return lfsr;
}