Serial peripheral interface / integrated interchip sound (SPI/I2S) RM0440
1742/2126 RM0440 Rev 4
Figure 577. Hardware/software slave select management
39.5.6 Communication formats
During SPI communication, receive and transmit operations are performed simultaneously.
The serial clock (SCK) synchronizes the shifting and sampling of the information on the data
lines. The communication format depends on the clock phase, the clock polarity and the
data frame format. To be able to communicate together, the master and slaves devices must
follow the same communication format.
Clock phase and polarity controls
Four possible timing relationships may be chosen by software, using the CPOL and CPHA
bits in the SPIx_CR1 register. The CPOL (clock polarity) bit controls the idle state value of
the clock when no data is being transferred. This bit affects both master and slave modes. If
CPOL is reset, the SCK pin has a low-level idle state. If CPOL is set, the SCK pin has a
high-level idle state.
If the CPHA bit is set, the second edge on the SCK pin captures the first data bit transacted
(falling edge if the CPOL bit is reset, rising edge if the CPOL bit is set). Data are latched on
each occurrence of this clock transition type. If the CPHA bit is reset, the first edge on the
SCK pin captures the first data bit transacted (falling edge if the CPOL bit is set, rising edge
if the CPOL bit is reset). Data are latched on each occurrence of this clock transition type.
The combination of CPOL (clock polarity) and CPHA (clock phase) bits selects the data
capture clock edge.
1
0
NSS Input
SSM control bit
SSI control bit
SSOE control bit
NSS Output
NSS
pin
(used in Master mode and NSS
HW management only)
NSS
Output
Control
Master
mode
Slave mode
Non activeOKVdd
NSS
Inp.
ActiveConflictVss
NSS external logic
NSS internal logic
GPIO
logic
MSv35526V6
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