24.3.2.2.2 SPI Frame Formats
The serial frame in SPI mode is defined to be one character of eight data bits. The USART in master SPI mode has two valid frame formats:
- 8-bit data, MSb first
- 8-bit data, LSb first
The data order is selected by writing to the Data Order bit (UDORD) in the Control C register (USARTn.CTRLC).
After a complete frame is transmitted, a new frame can directly follow it, or the communication line can return to the idle (high) state.