RM0090: Reference manual - STMicroelectronics

Serial peripheral interface (SPI) RM0090
Procedure
1. Set the DFF bit to define 8- or 16-bit data frame format
2. Select the CPOL and CPHA bits to define one of the four relationships between the
data transfer and the serial clock (see Figure 272). For correct data transfer, the CPOL
and CPHA bits must be configured in the same way in the slave device and the master
device. This step is not required when the TI mode is selected through the FRF bit in
the SPI_CR2 register.
3. The frame format (MSB-first or LSB-first depending on the value of the LSBFIRST bit in
the SPI_CR1 register) must be the same as the master device. This step is not required
when TI mode is selected.
4. In Hardware mode (refer to Slave select (NSS) pin management on page 794), the
NSS pin must be connected to a low level signal during the complete byte transmit
sequence. In NSS software mode, set the SSM bit and clear the SSI bit in the SPI_CR1
register. This step is not required when TI mode is selected.
5. Set the FRF bit in the SPI_CR2 register to select the TI mode protocol for serial
communications.
6. Clear the MSTR bit and set the SPE bit (both in the SPI_CR1 register) to assign the
pins to alternate functions.
In this configuration the MOSI pin is a data input and the MISO pin is a data output.
Transmit sequence
The data byte is parallel-loaded into the Tx buffer during a write cycle.
The transmit sequence begins when the slave device receives the clock signal and the most
significant bit of the data on its MOSI pin. The remaining bits (the 7 bits in 8-bit data frame
format, and the 15 bits in 16-bit data frame format) are loaded into the shift-register. The
TXE flag in the SPI_SR register is set on the transfer of data from the Tx Buffer to the shift
register and an interrupt is generated if the TXEIE bit in the SPI_CR2 register is set.
Receive sequence
For the receiver, when data transfer is complete:
● The Data in shift register is transferred to Rx Buffer and the RXNE flag (SPI_SR
register) is set
● An Interrupt is generated if the RXNEIE bit is set in the SPI_CR2 register.
After the last sampling clock edge the RXNE bit is set, a copy of the data byte received in
the shift register is moved to the Rx buffer. When the SPI_DR register is read, the SPI
peripheral returns this buffered value.
Clearing of the RXNE bit is performed by reading the SPI_DR register.
SPI TI protocol in slave mode
In slave mode, the SPI interface is compatible with the TI protocol. The FRF bit of the
SPI_CR2 register can be used to configure the slave SPI serial communications to be
compliant with this protocol.
The clock polarity and phase are forced to conform to the TI protocol requirements whatever
the values set in the SPI_CR1 register. NSS management is also specific to the TI protocol
which makes the configuration of NSS management through the SPI_CR1 and SPI_CR2
registers (such as SSM, SSI, SSOE) transparent for the user.
797/1416 Doc ID 018909 Rev 3