Buffer Mode

To avoid data collisions, the SPI peripheral can be configured in buffered mode by writing a '1' to the Buffer Mode Enable bit in the Control B register (BUFEN in SPIn.CTRLB). In this mode, the SPI has additional interrupt flags and extra buffers. The extra buffers are shown in Figure 1. There are two different modes for the Buffer mode, selected with the Buffer mode Wait for Receive bit (BUFWR). The two different modes are described below with timing diagrams.

Figure 1. SPI Timing Diagram in Buffer Mode with BUFWR in SPIn.CTRLB Set to Zero
All writes to the Data register goes to the Transmit Buffer register. The figure above shows that the value 0x43 is written to the Data register, but it is not immediately transferred to the shift register so the first byte sent will be a dummy byte. The value of the dummy byte is whatever was in the shift register at the time, usually the last received byte. After the first dummy transfer is completed the value 0x43 is transferred to the Shift register. Then 0x44 is written to the Data register and goes to the Transmit Buffer register. A new transfer is started and 0x43 will be sent. The value 0x45 is written to the Data register, but the Transmit Buffer register is not updated since it is already full containing 0x44 and the Data Register Empty Interrupt Flag (DREIF in SPIn.INTFLAGS) is low. The value 0x45 will be lost. After the transfer, the value 0x44 is moved to the Shift register. During the next transfer, 0x46 is written to the Data register and 0x44 is sent out. After the transfer is complete, 0x46 is copied into the Shift register and sent out in the next transfer.

The Data Register Empty Interrupt Flag (DREIF in SPIn.INTFLAGS) goes low every time the Transmit Buffer register is written and goes high after a transfer when the previous value in the Transmit Buffer register is copied into the Shift register. The Receive Complete Interrupt Flag (RXCIF in SPIn.INTFLAGS) is set one cycle after the Data Register Empty Interrupt Flag goes high. The Transfer Complete Interrupt Flag is set one cycle after the Receive Complete Interrupt Flag is set when both the value in the shift register and the Transmit Buffer register have been sent.

Figure 2. SPI Timing Diagram in Buffer Mode with CTRLB.BUFWR Set to One
All writes to the Data register goes to the transmit buffer. The figure above shows that the value 0x43 is written to the Data register and since the Slave Select pin is high it is copied to the Shift register the next cycle. Then the next write (0x44) will go to the Transmit Buffer register. During the first transfer, the value 0x43 will be shifted out. In the figure, the value 0x45 is written to the Data register, but the Transmit Buffer register is not updated since the Data Register Empty Interrupt Flag is low. After the transfer is completed, the value 0x44 from the Transmit Buffer register is copied over to the Shift register. The value 0x46 is written to the Transmit Buffer register. During the next two transfers, 0x44 and 0x46 are shifted out. The flags behave the same as with Buffer mode Wait for Receive Bit (BUFWR in SPIn.CTRLB) set to zero.