neopixel crashing the the board?

I'd propose the fix the code like this, however I didn't touch C for ages.

Line 1: set start = 0 for the first run (cache loading)
Line 23: wait if ccount>(0xFFFFFFFF-1000)
Line 24: set start = ccount, in the range [0 .. (0xFFFFFFFF-1000)]

I selected the number 1000 just to show the case, in reality it should correspond to the number of ticks needed to transfer a single bit (tOne + tLow (incl. program overhead)). Maybe 300 is more reasonable.

  start =  0;                     //fast pre-roll
  tZero = 63;                  //T0H = 400ns for WS2812B
  tOne = 121;                  //T1H = 800ns for WS2812B
  tLow =  181;                //PERIOD = 1200ns for WS2812B
  
  ets_intr_lock();                // disable most interrupts
  while(1) {
    GPIO_REG_WRITE(GPIO_OUT_W1TS_ADDRESS, pinMask);  // Set high
    uint32_t t;
    if (pix & mask) t = tOne;
    else            t = tZero;
    while (_getCycleCount()-start < t) ;             // busy-wait (high)
    GPIO_REG_WRITE(GPIO_OUT_W1TC_ADDRESS, pinMask);  // Set low
    start = _getCycleCount();
    while (_getCycleCount()-start < tLow) ;          // busy-wait (low)
    mask = mask>>1;                                  // Next bit
    if (mask == 0) {                                 // Next byte
      if (p >= end) break;                           // at end, we're done
      pix = *p++;
      mask = 0x80;
    }
    pinMask = _BV(pin);                              // correct pinMask after pre-roll
    while (_getCycleCount() > 0xFFFFFC17) ;          // ccount overflow handler
    start = _getCycleCount();                        // get start time of this bit
  }
  ets_intr_unlock();