driving WS2812B (neopixels) with I2S without SCK/LRCK

Hi, I'm trying do drive WS2812B LEDs (aka neopixels) with the I2S peripheral, using method similar to this https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/driving-ws2812b-leds-using-i2s-on-the-nordic-nrf52

I got the right signal out of SDOUT pin, provided I also configure LRCK and SCK pins (which I don't need). When I set either or both of the LRCK/SCK pins to 0xffffffff, no signal comes out of the SDOUT pin.

Is it possible to leave the SCK/LRCK disconnected and have the I2S still operate?

The code is below. It also has detects the end of transmission correctly (first TXPTRUPD triggers after the hardware picks up the pointer, so we set the MAXCNT to one more byte and wait for that one to be picked up). 

#include "NRF52SPI.h"
#include "hal/nrf_i2s.h"
#include "codal-core/inc/types/Event.h"
#include "ErrorNo.h"
#include "CodalFiber.h"

namespace codal
{

// based on https://electronut.in/nrf52-i2s-ws2812/

static int numIrq;

extern "C" void I2S_IRQHandler()
{
    if (NRF_I2S->EVENTS_TXPTRUPD != 0)
    {
        NRF_I2S->EVENTS_TXPTRUPD = 0;
        volatile uint32_t dummy = NRF_I2S->EVENTS_TXPTRUPD;
        if (numIrq == 0)
        {
            numIrq++;
            NRF_I2S->RXTXD.MAXCNT = 1;
        }
        else
        {
            NRF_I2S->ENABLE = 0;
            Event(DEVICE_ID_SPI, 2000);
        }
    }
}

void neopixel_send_buffer(Pin *pin, const uint8_t *data, unsigned size)
{
    if (NRF_I2S->ENABLE)
        return;

    int32_t iptr = 0, optr = 6;
    uint32_t len = optr + size + optr;
    uint32_t *expBuf = new uint32_t[len];
    memset(expBuf, 0, len * 4);

    while (iptr < (int)size)
    {
        uint32_t outp = 0x88888888;
        uint8_t inp = data[iptr];
        for (int i = 0; i < 8; ++i)
        {
            if (inp & (1 << i))
                outp |= 0xe << (i * 4);
        }
        outp = (outp >> 16) | (outp << 16);
        expBuf[optr++] = outp;
        iptr++;
    }

    NRF_I2S->CONFIG.RXEN = 0;
    NRF_I2S->CONFIG.TXEN = 1;
    NRF_I2S->CONFIG.MCKEN = 1;
    NRF_I2S->CONFIG.MCKFREQ = I2S_CONFIG_MCKFREQ_MCKFREQ_32MDIV10; // 3.2MHz
    NRF_I2S->CONFIG.RATIO = I2S_CONFIG_RATIO_RATIO_32X;
    NRF_I2S->CONFIG.SWIDTH = I2S_CONFIG_SWIDTH_SWIDTH_16Bit;
    NRF_I2S->CONFIG.ALIGN = I2S_CONFIG_ALIGN_ALIGN_Left;
    NRF_I2S->CONFIG.FORMAT = I2S_CONFIG_FORMAT_FORMAT_I2S;
    NRF_I2S->CONFIG.CHANNELS = I2S_CONFIG_CHANNELS_CHANNELS_Stereo;

#if 0
    NRF_I2S->PSEL.MCK = -1;
    NRF_I2S->PSEL.SCK = -1;
    NRF_I2S->PSEL.LRCK = -1;
    NRF_I2S->PSEL.SDOUT = (uint32_t)pin->name;
    NRF_I2S->PSEL.SDIN = -1;
#endif

    NRF_I2S->PSEL.MCK = -1;  // D9
    NRF_I2S->PSEL.SCK = 27;  // D10
    NRF_I2S->PSEL.LRCK = 6;  // D11
    NRF_I2S->PSEL.SDOUT = 8; // D12
    NRF_I2S->PSEL.SDIN = -1; // disconnect


    NRF_I2S->TXD.PTR = (uint32_t)expBuf;
    NRF_I2S->RXTXD.MAXCNT = len;

    NRF_I2S->INTEN = I2S_INTEN_TXPTRUPD_Msk;

    numIrq = 0;

    NVIC_SetPriority(I2S_IRQn, 1);
    NVIC_ClearPendingIRQ(I2S_IRQn);
    NVIC_EnableIRQ(I2S_IRQn);

    NRF_I2S->ENABLE = 1;
    NRF_I2S->TASKS_START = 1;

    fiber_wait_for_event(DEVICE_ID_SPI, 2000);

    delete expBuf;
}

} // namespace codal

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