NXP Semiconductors freescale K51 Series Reference Manual

46.4.2.3 FIFO Disable OperationThe FIFO disable mechanisms allow SPI transfers without using the TX FIFO or RXFIFO. The DSPI operates as a double-buffered simplified SPI when the FIFOs aredisabled. The FIFOs are disabled separately; setting the MCR[DIS_TXF] bit disables theTX FIFO, and setting the MCR[DIS_RXF] bit disables the RX FIFO.The FIFO Disable mechanisms are transparent to the user and to host software; Transmitdata and commands are written to the PUSHR and received data is read from the POPR.When the TX FIFO is disabled the TFFF, TFUF and TXCTR fields in SR behave as ifthere is a one-entry FIFO but the contents of the TXFR registers and TXNXTPTR areundefined. Likewise, when the RX FIFO is disabled, the RFDF, RFOF and RXCTRfields in the SR behave as if there is a one-entry FIFO, but the contents of the RXFRregisters and POPNXTPTR are undefined.46.4.2.4 Transmit First In First Out (TX FIFO) Buffering MechanismThe TX FIFO functions as a buffer of SPI data and SPI commands for transmission. TheTX FIFO holds 4 words, each consisting of a command field and a data field. Thenumber of entries in the TX FIFO is device-specific. SPI commands and data are addedto the TX FIFO by writing to the DSPI PUSH TX FIFO Register (PUSHR). TX FIFOentries can only be removed from the TX FIFO by being shifted out or by flushing theTX FIFO.The TX FIFO Counter field (TXCTR) in the DSPI Status Register (SR) indicates thenumber of valid entries in the TX FIFO. The TXCTR is updated every time the DSPI_PUSHR is written or SPI data is transferred into the shift register from the TX FIFO.The TXNXTPTR field indicates which TX FIFO Entry will be transmitted during thenext transfer. The TXNXTPTR contains the positive offset from TXFR0 in number of32-bit registers. For example, TXNXTPTR equal to two means that the TXFR2 containsthe SPI data and command for the next transfer. The TXNXTPTR field is incrementedevery time SPI data is transferred from the TX FIFO to the shift register. The maximumvalue of the field is equal to the maximum implemented TXFR register number and itrolls over after reaching the maximum.Functional DescriptionK51 Sub-Family Reference Manual, Rev. 6, Nov 20111154 Freescale Semiconductor, Inc.