Texas Instruments TMS320F2837 D Series Workshop Manual And Lab Manual page 290

Serial Peripheral Interface (SPI)
Since data is shifted out of the SPIDAT register MSB first, transmission characters of less than 16
bits must be left-justified by the CPU software prior to be written to SPIDAT.
Received data is shifted into SPIDAT from the left, MSB first. However, the entire sixteen bits of
SPIDAT is copied into SPIBUF after the character transmission is complete such that received
characters of less than 16 bits will be right-justified in SPIBUF. The non-utilized higher
significance bits must be masked-off by the CPU software when it interprets the character. For
example, a 9 bit character transmission would require masking-off the 7 MSB's.
Programmable data

length of 1 to 16 bits
Transmitted data of less

than 16 bits must be left
justified
MSB transmitted first
Received data of less

than 16 bits are right
justified
User software must

mask-off unused MSB's
12 - 6
SPI Data Character Justification
TMS320F2837xD Microcontroller Workshop - Communications
SPIDAT - Processor #1
11001001XXXXXXXX
SPIDAT - Processor #2
XXXXXXXX11001001