NXP Semiconductors MKW01Z128, Reference Manual

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SS is the input or output for the master mode, and it is always the input for the slave
mode.
The bidirectional mode does not affect SPSCK and SS functions.
Note
In bidirectional master mode, with the mode fault feature
enabled, both data pins MISO and MOSI can be occupied by
the SPI, though MOSI is normally used for transmissions in
bidirectional mode and MISO is not used by the SPI. If a mode
fault occurs, the SPI is automatically switched to slave mode. In
this case, MISO becomes occupied by the SPI and MOSI is not
used. Consider this scenario if the MISO pin is used for another
purpose.
31.4.10 Error conditions
The SPI module has one error condition: the mode fault error.
31.4.10.1 Mode fault error
If the SS input becomes low while the SPI is configured as a master, it indicates a system
error where more than one master may be trying to drive the MOSI and SPSCK lines
simultaneously. This condition is not permitted in normal operation, and it sets the
MODF bit in the SPI status register automatically provided that C2[MODFEN] is set.
In the special case where the SPI is in master mode and C2[MODFEN] is cleared, the SS
pin is not used by the SPI. In this special case, the mode fault error function is inhibited
and MODF remains cleared. If the SPI system is configured as a slave, the SS pin is a
dedicated input pin. A mode fault error does not occur in slave mode.
If a mode fault error occurs, the SPI is switched to slave mode, with the exception that
the slave output buffer is disabled. So the SPSCK, MISO and MOSI pins are forced to be
high impedance inputs to avoid any possibility of conflict with another output driver. A
transmission in progress is aborted and the SPI is forced into idle state.
If the mode fault error occurs in the bidirectional mode for an SPI system configured in
master mode, the output enable of MOMI (MOSI in bidirectional mode) is cleared if it
was set. No mode fault error occurs in the bidirectional mode for the SPI system
configured in slave mode.
Chapter 31 Serial Peripheral Interface (SPI)
MKW01Z128 MCU Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc. 609