mc68hc908gr8vp Freescale Semiconductor, Inc, mc68hc908gr8vp Datasheet - Page 216

• Manufacturer Part Number: mc68hc908gr8vp
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

Serial Peripheral Interface (SPI)
In a master SPI with the mode fault enable bit (MODFEN) set, the mode fault flag (MODF) is set if SS
goes to logic 0. A mode fault in a master SPI causes the following events to occur:
When configured as a slave (SPMSTR = 0), the MODF flag is set if SS goes high during a transmission.
When CPHA = 0, a transmission begins when SS goes low and ends once the incoming SPSCK goes
back to its idle level following the shift of the eighth data bit. When CPHA = 1, the transmission begins
when the SPSCK leaves its idle level and SS is already low. The transmission continues until the SPSCK
returns to its idle level following the shift of the last data bit. See
In a slave SPI (MSTR = 0), the MODF bit generates an SPI receiver/error CPU interrupt request if the
ERRIE bit is set. The MODF bit does not clear the SPE bit or reset the SPI in any way. Software can abort
the SPI transmission by clearing the SPE bit of the slave.
To clear the MODF flag, read the SPSCR with the MODF bit set and then write to the SPCR register. This
entire clearing mechanism must occur with no MODF condition existing or else the flag is not cleared.
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If ERRIE = 1, the SPI generates an SPI receiver/error CPU interrupt request.
The SPE bit is cleared.
The SPTE bit is set.
The SPI state counter is cleared.
The data direction register of the shared I/O port regains control of port drivers.
To prevent bus contention with another master SPI after a mode fault error,
clear all SPI bits of the data direction register of the shared I/O port before
enabling the SPI.
Setting the MODF flag does not clear the SPMSTR bit. The SPMSTR bit
has no function when SPE = 0. Reading SPMSTR when MODF = 1 shows
the difference between a MODF occurring when the SPI is a master and
when it is a slave.
When CPHA = 0, a MODF occurs if a slave is selected (SS is low) and later
unselected (SS is high) even if no SPSCK is sent to that slave. This
happens because SS low indicates the start of the transmission (MISO
driven out with the value of MSB) for CPHA = 0. When CPHA = 1, a slave
can be selected and then later unselected with no transmission occurring.
Therefore, MODF does not occur since a transmission was never begun.
A high on the SS pin of a slave SPI puts the MISO pin in a high impedance
state. Also, the slave SPI ignores all incoming SPSCK clocks, even if it was
already in the middle of a transmission.
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
NOTE
NOTE
NOTE
20.5 Transmission
Freescale Semiconductor
Formats.