36.2.6.2 Variable Transfer Size Mode (BMODE = 1)
In this mode, SPIxTWIDTH specifies the width of every individual piece of the data transfer in bits. SPIxTCNT specifies the number of transfers of this bit length. If SPIxTWIDTH = 0, each piece is a full byte of data. If SPIxTWIDTH ≠ 0, then only that specified number of bits from the transmit FIFO are shifted out, with the unused bits ignored.
Received data are padded with zeros in the unused bit areas when transferred into the receive FIFO. The LSBF bit determines whether the Most Significant or Least Significant bits of the transfers are ignored or padded.
In this mode, the transfer counter being zero only stops messages from being sent or received when in Receive Only mode.
With BMODE = 1, it is possible for the transfer counter (SPIxTCNT) to decrement below zero, although when in Host Receive Only mode, transfer clocks will cease when the transfer counter reaches zero.