RM0090 Serial peripheral interface (SPI)
Doc ID 018909 Rev 4 816/1422
In slave receive-only mode (MSTR=0, BIDIMODE=0, RXONLY=1) or
bidirectional receive mode (MSTR=0, BIDIMODE=1, BIDOE=0)
1. You can disable the SPI (write SPE=1) at any time: the current transfer will complete
before the SPI is effectively disabled
2. Then, if you want to enter the Halt mode, you must first wait until BSY = 0 before
entering the Halt mode (or disabling the peripheral clock).
27.3.9 SPI communication using DMA (direct memory addressing)
To operate at its maximum speed, the SPI needs to be fed with the data for transmission and
the data received on the Rx buffer should be read to avoid overrun. To facilitate the transfers,
the SPI features a DMA capability implementing a simple request/acknowledge protocol.
A DMA access is requested when the enable bit in the SPI_CR2 register is enabled.
Separate requests must be issued to the Tx and Rx buffers (see Figure 284 and
Figure 285):
● In transmission, a DMA request is issued each time TXE is set to 1. The DMA then
writes to the SPI_DR register (this clears the TXE flag).
● In reception, a DMA request is issued each time RXNE is set to 1. The DMA then reads
the SPI_DR register (this clears the RXNE flag).
When the SPI is used only to transmit data, it is possible to enable only the SPI Tx DMA
channel. In this case, the OVR flag is set because the data received are not read.
When the SPI is used only to receive data, it is possible to enable only the SPI Rx DMA
channel.
In transmission mode, when the DMA has written all the data to be transmitted (flag TCIF is
set in the DMA_ISR register), the BSY flag can be monitored to ensure that the SPI
communication is complete. This is required to avoid corrupting the last transmission before
disabling the SPI or entering the Stop mode. The software must first wait until TXE=1 and
then until BSY=0.
Note: During discontinuous communications, there is a 2 APB clock period delay between the
write operation to SPI_DR and the BSY bit setting. As a consequence, it is mandatory to
wait first until TXE=1 and then until BSY=0 after writing the last data.
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