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Deserial Serial Peripheral Interface (DSPI)
MPC5644A Microcontroller Reference Manual, Rev. 6
1292
Freescale Semiconductor
The SPI configuration supports two module-specific modes: master mode and slave mode. The FIFO operations are similar for
both modes. The main difference is that in master mode the DSPI initiates and controls the transfer according to the fields in the
SPI command field of the TX FIFO entry. In slave mode the DSPI only responds to transfers initiated by a bus master external to
the DSPI and the SPI command field space is used for the 16 most significant bitS of the transmit data.
30.9.2.1
Master mode
In SPI master mode the DSPI initiates the serial transfers by controlling the Serial Communications Clock
(SCK) and the Peripheral Chip Select (PCS) signals. The SPI command field in the executing TX FIFO
entry determines which of the DSPI_CTAR registers will be used to set the transfer attributes and which
PCS signal to assert. The command field also contains various bits that help with queue management and
transfer protocol. See
Section 30.8.2.7, DSPI PUSH TX FIFO Register (DSPI_PUSHR)
” for details on the
SPI command fields. The data field in the executing TX FIFO entry is loaded into the shift register and
shifted out on the Serial Out (SOUT) pin. In SPI master mode, each SPI frame to be transmitted has a
command associated with it allowing for transfer attribute control on a frame by frame basis.
30.9.2.2
Slave mode
In SPI slave mode the DSPI responds to transfers initiated by a SPI bus master. The DSPI does not initiate
transfers. Certain transfer attributes such as clock polarity, clock phase and frame size must be set for
successful communication with a SPI master. The SPI slave mode transfer attributes are set in the
DSPI_CTAR0.
30.9.2.3
FIFO disable operation
The FIFO disable mechanisms allow SPI transfers without using the TX FIFO or RX FIFO. The DSPI
operates as a double-buffered simplified SPI when the FIFOs are disabled. The FIFOs are disabled
separately; setting the DSPI_MCR[DIS_TXF] bit disables the TX FIFO, and setting the
DSPI_MCR[DIS_RXF] bit disables the RX FIFO.
The FIFO Disable mechanisms are transparent to the user and to host software; Transmit data and
commands are written to the DSPI_PUSHR and received data is read from the DSPI_POPR.
When the TX FIFO is disabled the TFFF, TFUF and TXCTR fields in DSPI_SR behave as if there is a
one-entry FIFO but the contents of the DSPI_TXFR registers and TXNXTPTR are undefined. Likewise,
when the RX FIFO is disabled, the RFDF, RFOF and RXCTR fields in the DSPI_SR behave as if there is
a one-entry FIFO, but the contents of the DSPI_RXFR registers and POPNXTPTR are undefined.
30.9.2.4
Transmit first-in first-out (TX FIFO) buffering mechanism
The TX FIFO functions as a buffer of SPI data and SPI commands for transmission. The TX FIFO holds
from 1 to 16 words, each consisting of a command field and a data field. The number of entries in the TX
FIFO is device specific. SPI commands and data are added to the TX FIFO by writing to the DSPI PUSH
TX FIFO Register (DSPI_PUSHR). TX FIFO entries can only be removed from the TX FIFO by being
shifted out or by flushing the TX FIFO.
The TX FIFO Counter field DSPI_SR[TXCTR] indicates the number of valid entries in the TX FIFO.
Field DSPI_SR[TXCTR] is updated every time the DSPI _PUSHR is written or SPI data is transferred into
the shift register from the TX FIFO.
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