sendSerial

void sendSerial() {

    uint32_t nBitsSent = 0;

    /*  Padding: we need to pre-pad
        We will send 15 12-bit values (180 bits) which is 22.5 8-bit SPI 8-bit bytes
        For every TLC5490 we actually need to send 16 12-bit values (192 bits in total)
        because that's what the TLC5490 serial comms expect.
        Since we are using only 15 of the output lines,
        the 16th value is unnecessary (we are not using OUT15)
        We also need to pad for the extra nibble (22.5 SPI above)
        So if we start by sending the empty 12-bit value, we are OK
        This padding needs to be done for every chip
    */
    //~ padSPI(0);
    SPDR = 0;                   // First 8 bits of the empty 12-bit data
    while(!(SPSR & (1<<SPIF))) {
        // Wait for the transfer to finish
    }
    SPDR = 0;                   // Last 4 bits of the empty 12-bit data
                                // plus the empty 4 high bits of the first intensity data
    while(!(SPSR & (1<<SPIF))) {
        // Wait for the transfer to finish
    }
    nBitsSent += 16;
    uint8_t idxNextSendType = 2;    // The next send type is type 2
                                        // so we indicate that

    // Different curColDatas are not implemented yet
    //~ uint8_t * curColData = nColumnData[idxUnit];

    uint8_t idxChannel = 0;
    uint8_t nPixelsSent = 0;
    uint8_t dataByte = (idxChannel==0)?0xFF:0x00;
    uint8_t toSPDR = dataByte;

    for (;;) { // Send bytes loop
        SPDR = toSPDR;          // Send the byte
        nBitsSent += 8;

        while(!(SPSR & (1<<SPIF))) {
            // Wait for the transfer to finish
        }

#define PADDING
#ifdef PADDING
        if (nBitsSent % 192 == 0) {
            if (nBitsSent >= 192 *3)
                break;


            SPDR = 0;                   // First 8 bits of the empty 12-bit data
            while(!(SPSR & (1<<SPIF))) {
                // Wait for the transfer to finish
            }
            SPDR = 0;                   // Last 4 bits of the empty 12-bit data
                                        // plus the empty 4 high bits of the first intensity data
            while(!(SPSR & (1<<SPIF))) {
                // Wait for the transfer to finish
            }
            idxNextSendType = 1;        // It will get incremented first
                                        // so the first byte will actually be
                                        // idxNextSendType = 2

            nBitsSent += 16;
        }
#endif

        // Update the send type
        if (++idxNextSendType>2) { idxNextSendType = 0; }
        //~ ++idxNextSendType %= 4;

        // Prepare the next one for loading
        if (idxNextSendType==1) {
                // Case 1: Need to send the low nibble + padding
                toSPDR = dataByte<<4;
        } else {
            // Either way we need another new data in dataByte
            //
            if (idxChannel==0) {
                // Load next palette index
                nPixelsSent++;
            }
            dataByte = (idxChannel==0)?0xFF:0x00;

            if (++idxChannel>2) idxChannel = 0;
            //~ This fails: ++idxChannel %= 3;

            if (idxNextSendType) {
                // idxSendType == 2
                // Case 2: Need to send pure data
                toSPDR = dataByte;
            } else {
                // idxSendType == 0
                // Case 0: Need to send padding + high nibble
                toSPDR = dataByte >> 4;
            }
        }

        while(!(SPSR & (1<<SPIF)))  // Wait for the transfer to finish
            ;

    } // End of send bytes loop
}