28.3.2.2.2 Buffer Mode

The SPI peripheral can be configured in Buffered mode by writing a ‘1’ to the Buffer Mode Enable (BUFEN) bit in the Control B (SPIn.CTRLB) register to avoid data collisions.

This mode will enable two receive buffers and one transmit buffer. Both will have separate interrupt flags, transmit complete and receive complete. Figure 28-1 shows the extra buffers.

When Buffer mode is enabled it can work in two different ways. The Buffer Mode Wait for Receive (BUFWR) bit in the Control B (SPIn.CTRLB) register controls how the Buffer mode works. The details of how they work including timing diagrams are described below.

Note: When operating as a client in Buffered mode and the SPI clock is close to maximum frequency, the client may not be able to set up data in time for the first sample edge during back-to-back transfers. Refer to the Electrical Characteristics - SPI section for details.

Client Buffer Mode with Wait for Receive Bit Written to ‘0’

In Client mode, if the Buffer mode Wait for Receive (BUFWR) bit in SPIn.CTRLB is written to ‘0’, a dummy byte will be sent before the transmission of user data starts. Figure 28-3 shows a transmission sequence with this configuration. Notice how the value 0x45 is written to the Data (SPIn.DATA) register but never transmitted.

Figure 28-3. SPI Timing Diagram in Buffer Mode with BUFWR in SPIn.CTRLB Written to ‘0’

When the Wait for Receive (BUFWR) bit in SPIn.CTRLB is written to ‘0’, all writes to the Data (SPIn.DATA) register go to the Transmit Data Buffer register. The figure above shows that the value 0x43 is written to the Data (SPIn.DATA) register but not immediately transferred to the shift register, so the first byte sent will be a dummy byte. The value of the dummy byte equals the values that were in the shift register at the same time. After the first dummy transfer is completed, the value 0x43 is transferred to the shift register. Then 0x44 is written to the Data (SPIn.DATA) register and goes to the Transmit Data Buffer register. A new transfer is started, and 0x43 will be sent. The value 0x45 is written to the Data (SPIn.DATA) register, but the Transmit Data 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 (SPIn.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 DREIF goes low every time the Transmit Data Buffer register is written and goes high after a transfer when the previous value in the Transmit Data Buffer register is copied into the shift register. The Receive Complete Interrupt Flag (RXCIF) in SPIn.INTFLAGS is set one cycle after the DREIF 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 in the Transmit Data Buffer register has been sent.

Client Buffer Mode with Wait for Receive Bit Written to ‘1’

In Client mode, if the Buffer Mode Wait for Receive (BUFWR) bit in SPIn.CRTLB is written to ‘1’, the transmission of user data starts as soon as the SS pin is driven low. Figure 28-4 shows a transmission sequence with this configuration. Notice how the value 0x45 is written to the Data (SPIn.DATA) register but never transmitted.

Figure 28-4. SPI Timing Diagram in Buffer Mode with CTRLB.BUFWR Written to ‘1’

All writes to the Data (SPIn.DATA) register go to the Transmit Data Buffer register. The figure above shows that the value 0x43 is written to the Data (SPIn.DATA) register, and since the SS pin is high, it is copied to the shift register in the next cycle. The next write (0x44) will go to the Transmit Data Buffer register. During the first transfer, the value 0x43 will be shifted out. In the figure above, the value 0x45 is written to the Data (SPIn.DATA) register, but the Transmit Data Buffer register is not updated since the DREIF is low. After the transfer is completed, the value 0x44 from the Transmit Data Buffer register is copied to the shift register. The value 0x46 is written to the Transmit Data Buffer register. During the next two transfers, 0x44 and 0x46 are shifted out. The flags behave identically to the Buffer Mode Wait for Receive (BUFWR) bit in SPIn.CTRLB set to ‘0’.