35.3.2 Transmit Only Mode

When TXR is set and RXR is clear, the SPI host is in Transmit Only mode. In this mode, data transfer triggering is affected by the BMODE bit.

When BMODE = 1, data transfers will occur whenever the transmit FIFO is not empty. Data will be transmitted as soon as the SPIxTXB register is written to, matching the functionality of the SPI (MSSP) modules on previous 8-bit devices. The SPIxTCNT will decrement with each transfer. However, when SPIxTCNT is zero, the next transfer is not inhibited and the corresponding SPIxTCNT decrement will cause the count to roll over to the maximum value. Any data received in this mode is not stored in the receive FIFO. The following figure shows an example of sending a command and then sending a byte of data using this mode.

Figure 35-4. SPI Host Operation - Command+Write Data, TXR = 1, RXR = 0

When BMODE = 0, the transfer counter (SPIxTCNT) must also be written to before transfers will occur, and transfers will cease when the transfer counter reaches ‘0’.

For example, if SPIxTXB is written twice and then SPIxTCNTL is written with ‘3’, the transfer will start with the SPIxTCNTL write. The two bytes in the TXFIFO will be sent after which the transfer will suspend until the third and last byte is written to SPIxTXB.