DocID025202 Rev 7
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RM0365
Serial peripheral interface / inter-IC sound (SPI/I2S)
959
Communication diagrams
Some typical timing schemes are explained in this section. These schemes are valid no
matter if the SPI events are handled by polling, interrupts or DMA. For simplicity, the
LSBFIRST=0, CPOL=0 and CPHA=1 setting is used as a common assumption here. No
complete configuration of DMA streams is provided.
The following numbered notes are common for
through
.
1.
The slave starts to control MISO line as NSS is active and SPI is enabled, and is
disconnected from the line when one of them is released. Sufficient time must be
provided for the slave to prepare data dedicated to the master in advance before its
transaction starts.
At the master, the SPI peripheral takes control at MOSI and SCK signals (occasionally
at NSS signal as well) only if SPI is enabled. If SPI is disabled the SPI peripheral is
disconnected from GPIO logic, so the levels at these lines depends on GPIO setting
exclusively.
2. At the master, BSY stays active between frames if the communication (clock signal) is
continuous. At the slave, BSY signal always goes down for at least one clock cycle
between data frames.
3. The TXE signal is cleared only if TXFIFO is full.
4. The DMA arbitration process starts just after the TXDMAEN bit is set. The TXE
interrupt is generated just after the TXEIE is set. As the TXE signal is at an active level,
data transfers to TxFIFO start, until TxFIFO becomes full or the DMA transfer
completes.
5. If all the data to be sent can fit into TxFIFO, the DMA Tx TCIF flag can be raised even
before communication on the SPI bus starts. This flag always rises before the SPI
transaction is completed.
6. The CRC value for a package is calculated continuously frame by frame in the
SPIx_TxCRCR and SPIx_RxCRCR registers. The CRC information is processed after
the entire data package has completed, either automatically by DMA (Tx channel must
be set to the number of data frames to be processed) or by SW (the user must handle
CRCNEXT bit during the last data frame processing).
While the CRC value calculated in SPIx_TxCRCR is simply sent out by transmitter,
received CRC information is loaded into RxFIFO and then compared with the
SPIx_RxCRCR register content (CRC error flag can be raised here if any difference).
This is why the user must take care to flush this information from the FIFO, either by
software reading out all the stored content of RxFIFO, or by DMA when the proper
number of data frames is preset for Rx channel (number of data number of
CRC frames) (see the settings at the example assumption).
7. In data packed mode, TxE and RxNE events are paired and each read/write access to
the FIFO is 16 bits wide until the number of data frames are even. If the TxFIFO is ¾
full FTLVL status stays at FIFO full level. That is why the last odd data frame cannot be
stored before the TxFIFO becomes ½ full. This frame is stored into TxFIFO with an 8-
bit access either by software or automatically by DMA when LDMA_TX control is set.
8. To receive the last odd data frame in packed mode, the Rx threshold must be changed
to 8-bit when the last data frame is processed, either by software setting FRXTH=1 or
automatically by a DMA internal signal when LDMA_RX is set.