35.3.1 Full Duplex Mode

When both TXR and RXR are set, the SPI host is in Full Duplex mode. In this mode, data transfer triggering is affected by the BMODE bit.

When BMODE = 1, data transfers will occur whenever the receive FIFO is not full and data is present in the transmit FIFO. In practice, as long as the receive FIFO is not full, data will be transmitted/received as soon as the SPIxTXB register is written to, matching the functionality of SPI (MSSP) modules on older 8-bit Microchip 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. The following figure shows an example of a communication using this mode.

Figure 35-3. SPI Host Operation - Data Exchange, RXR = 1, TXR = 1

When BMODE = 0, the transfer counter (SPIxTCNT) must also be written to before transfers will occur. 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.