The TXNXTPTR field indicates the TX FIFO Entry that will be transmitted during the
next transfer. The TXNXTPTR field is incremented every time SPI data is transferred
from the TX FIFO to the shift register. The maximum value of the field is equal to the
maximum implemented TXFR number and it rolls over after reaching the maximum.
44.5.2.4.1 Filling the TX FIFO
Host software or other intelligent blocks can add (push) entries to the TX FIFO and CMD
FIFO by writing to the PUSHR. When the TX FIFO is not full, the TX FIFO Fill Flag
(TFFF) in the SR is set. The TFFF bit is cleared when TX FIFO is full and the DMA
controller indicates that a write to PUSHR is complete. Writing a '1' to the TFFF bit also
clears it. The TFFF can generate a DMA request or an interrupt request. See
FIFO Fill Interrupt or DMA Request
The module ignores attempts to push data to a full TX FIFO, and the state of the TX
FIFO does not change and no error condition is indicated.
44.5.2.4.2 Draining the TX FIFO
The TX FIFO entries are removed (drained) by shifting SPI data out through the shift
register. Entries are transferred from the TX FIFO to the shift register and shifted out as
long as there are valid entries in the TX FIFO. Every time an entry is transferred from the
TX FIFO to the shift register, the TX FIFO Counter decrements by one. At the end of a
transfer, the TCF bit in the SR is set to indicate the completion of a transfer. The TX
FIFO is flushed by writing a '1' to the CLR_TXF bit in MCR.
If an external bus master initiates a transfer with a module slave while the slave's TX
FIFO is empty, the Transmit FIFO Underflow Flag (TFUF) in the slave's SR is set. See
Transmit FIFO Underflow Interrupt Request
for details.
44.5.2.5 Receive First In First Out (RX FIFO) buffering mechanism
The RX FIFO functions as a buffer for data received on the SIN pin. The RX FIFO holds
4 received SPI data frames. The number of entries in the RX FIFO is device-specific. SPI
data is added to the RX FIFO at the completion of a transfer when the received data in the
shift register is transferred into the RX FIFO. SPI data are removed (popped) from the
RX FIFO by reading the module POP RX FIFO Register (POPR). RX FIFO entries can
only be removed from the RX FIFO by reading the POPR or by flushing the RX FIFO.
The RX FIFO Counter field (RXCTR) in the module's Status Register (SR) indicates the
number of valid entries in the RX FIFO. The RXCTR is updated every time the POPR is
read or SPI data is copied from the shift register to the RX FIFO.
Chapter 44 Serial Peripheral Interface (SPI)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
1205
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