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Chapter 12 Serial Peripheral Interface (S12SPIV4)
MC9S12XDP512 Data Sheet, Rev. 2.17
Freescale Semiconductor
537
12.4.6
Error Conditions
The SPI has one error condition:
Mode fault error
12.4.6.1
Mode Fault Error
Ifthe SS inputbecomes low while the SPI is configured as a master, it indicatesa system error where more
than one master may be trying to drive the MOSI and SCK lines simultaneously. This condition is not
permitted in normal operation, the MODF bit in the SPI status register is set automatically, provided the
MODFEN bit is set.
In the special case where the SPI is in master mode and MODFEN bit 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. In case
the SPI system is configured as a slave, the SS pin is a dedicated input pin. Mode fault error doesn’t 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 SCK, 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.
IfthemodefaulterroroccursinthebidirectionalmodeforaSPIsystemconfiguredinmastermode,output
enable of the MOMI (MOSI in bidirectional mode) is cleared if it was set. No mode fault error occurs in
the bidirectional mode for SPI system configured in slave mode.
The mode fault flag is cleared automatically by a read of the SPI status register (with MODF set) followed
by a write to SPI control register 1. If the mode fault flag is cleared, the SPI becomes a normal master or
slave again.
NOTE
Ifamodefaulterroroccursandareceiveddatabyteispendinginthereceive
shift register, this data byte will be lost.
12.4.7
Low Power Mode Options
12.4.7.1
SPI in Run Mode
In run mode with the SPI system enable (SPE) bit in the SPI control register clear, the SPI system is in a
low-power, disabled state. SPI registers remain accessible, but clocks to the core of this module are
disabled.