ST STM32L4+ Series Reference Manual page 1871

RM0432
synchronize the slave with the beginning of each data sequence. NSS can be managed by
both software and hardware (see
When the BSY bit is set it signifies an ongoing data frame transaction. When the dedicated
frame transaction is finished, the RXNE flag is raised. The last bit is just sampled and the
complete data frame is stored in the RXFIFO.
Procedure for disabling the SPI
When SPI is disabled, it is mandatory to follow the disable procedures described in this
paragraph. It is important to do this before the system enters a low-power mode when the
peripheral clock is stopped. Ongoing transactions can be corrupted in this case. In some
modes the disable procedure is the only way to stop continuous communication running.
Master in full-duplex or transmit only mode can finish any transaction when it stops
providing data for transmission. In this case, the clock stops after the last data transaction.
Special care must be taken in packing mode when an odd number of data frames are
transacted to prevent some dummy byte exchange (refer to
the SPI is disabled in these modes, the user must follow standard disable procedure. When
the SPI is disabled at the master transmitter while a frame transaction is ongoing or next
data frame is stored in TXFIFO, the SPI behavior is not guaranteed.
When the master is in any receive only mode, the only way to stop the continuous clock is to
disable the peripheral by SPE=0. This must occur in specific time window within last data
frame transaction just between the sampling time of its first bit and before its last bit transfer
starts (in order to receive a complete number of expected data frames and to prevent any
additional "dummy" data reading after the last valid data frame). Specific procedure must be
followed when disabling SPI in this mode.
Data received but not read remains stored in RXFIFO when the SPI is disabled, and must
be processed the next time the SPI is enabled, before starting a new sequence. To prevent
having unread data, ensure that RXFIFO is empty when disabling the SPI, by using the
correct disabling procedure, or by initializing all the SPI registers with a software reset via
the control of a specific register dedicated to peripheral reset (see the SPIiRST bits in the
RCC_APBiRSTR registers).
Standard disable procedure is based on pulling BSY status together with FTLVL[1:0] to
check if a transmission session is fully completed. This check can be done in specific cases,
too, when it is necessary to identify the end of ongoing transactions, for example:
•
When NSS signal is managed by software and master has to provide proper end of
NSS pulse for slave, or
•
When transactions' streams from DMA or FIFO are completed while the last data frame
or CRC frame transaction is still ongoing in the peripheral bus.
The correct disable procedure is (except when receive only mode is used):
1. Wait until FTLVL[1:0] = 00 (no more data to transmit).
2. Wait until BSY=0 (the last data frame is processed).
3. Disable the SPI (SPE=0).
4. Read data until FRLVL[1:0] = 00 (read all the received data).
The correct disable procedure for certain receive only modes is:
Section 52.4.5: Slave select (NSS) pin
RM0432 Rev 6
Serial peripheral interface (SPI)
management).
Data packing section). Before
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