Spi Special Features; Nss Pulse Mode - STMicroelectronics STM32F05 series Reference Manual

Serial peripheral interface / inter-IC sound (SPI/I2S)
When an overrun condition occurs, the newly received value does not overwrite the previous
one in the RXFIFO. The newly received value is discarded and all data transmitted
subsequently is lost. Clearing the OVR bit is done by a read access to the SPI_DR register
followed by a read access to the SPI_SR register.
Mode fault (MODF)
Mode fault occurs when the master device has its internal NSS signal (NSS pin in NSS
hardware mode, or SSI bit in NSS software mode) pulled low. This automatically sets the
MODF bit. Master mode fault affects the SPI interface in the following ways:
●
The MODF bit is set and an SPI interrupt is generated if the ERRIE bit is set.
●
The SPE bit is cleared. This blocks all output from the device and disables the SPI
interface.
●
The MSTR bit is cleared, thus forcing the device into slave mode.
Use the following software sequence to clear the MODF bit:
1. Make a read or write access to the SPIx_SR register while the MODF bit is set.
2. Then write to the SPIx_CR1 register.
To avoid any multiple slave conflicts in a system comprising several MCUs, the NSS pin
must be pulled high during the MODF bit clearing sequence. The SPE and MSTR bits can
be restored to their original state after this clearing sequence. As a security, hardware does
not allow the setting of the SPE and MSTR bits while the MODF bit is set. In a slave device
the MODF bit cannot be never set except the result of a previous multimaster conflict.
CRC error (CRCERR)
This flag is used to verify the validity of the value received when the CRCEN bit in the
SPIx_CR1 register is set. The CRCERR flag in the SPIx_SR register is set if the value
received in the shift register does not match the receiver SPIx_RXCRCR value. The flag is
cleared by the software.
TI mode frame format error (FRE)
A TI mode frame format error is detected when an NSS pulse occurs during an ongoing
communication when the SPI is operating in slave mode and configured to conform to the TI
mode protocol. When this error occurs, the FRE flag is set in the SPIx_SR register. The SPI
is not disabled when an error occurs, the NSS pulse is ignored, and the SPI waits for the
next NSS pulse before starting a new transfer. The data may be corrupted since the error
detection may result in the loss of two data bytes.
The FRE flag is cleared when SPIx_SR register is read. If the ERRIE bit is set, an interrupt
is generated on the NSS error detection. In this case, the SPI should be disabled because
data consistency is no longer guaranteed and communications should be reinitiated by the
master when the slave SPI is enabled again.
26.4

SPI special features

26.4.1

NSS pulse mode

This mode is activated by the NSSP bit in the SPIx_CR1 register and it takes effect only if
the SPI interface is configured as Motorola SPI master with capture on the first edge
(SPIx_CR1 CPHA = 0). When activated, an NSS pulse is generated between two
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