Serial Peripheral Interface (SPI)
Data Sheet
M68HC11E Family — Rev. 5
136
Serial Peripheral Interface (SPI)
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MOTOROLA
Any SPI output line must have its corresponding data direction bit in DDRD register
set. If the DDR bit is clear, that line is disconnected from the SPI logic and becomes
a general-purpose input. All SPI input lines are forced to act as inputs regardless
of the state of the corresponding DDR bits in DDRD register.
8.5.1 Master In/Slave Out
MISO is one of two unidirectional serial data signals. It is an input to a master
device and an output from a slave device. The MISO line of a slave device is placed
in the high-impedance state if the slave device is not selected.
8.5.2 Master Out/Slave In
The MOSI line is the second of the two unidirectional serial data signals. It is an
output from a master device and an input to a slave device. The master device
places data on the MOSI line a half-cycle before the clock edge that the slave
device uses to latch the data.
8.5.3 Serial Clock
SCK, an input to a slave device, is generated by the master device and
synchronizes data movement in and out of the device through the MOSI and MISO
lines. Master and slave devices are capable of exchanging a byte of information
during a sequence of eight clock cycles.
Four possible timing relationships can be chosen by using control bits CPOL and
CPHA in the serial peripheral control register (SPCR). Both master and slave
devices must operate with the same timing. The SPI clock rate select bits,
SPR[1:0], in the SPCR of the master device, select the clock rate. In a slave device,
SPR[1:0] have no effect on the operation of the SPI.
8.5.4 Slave Select
The slave select (SS) input of a slave device must be externally asserted before a
master device can exchange data with the slave device. SS must be low before
data transactions and must stay low for the duration of the transaction.
The SS line of the master must be held high. If it goes low, a mode fault error flag
(MODF) is set in the serial peripheral status register (SPSR). To disable the mode
fault circuit, write a 1 in bit 5 of the port D data direction register. This sets the SS
pin to act as a general-purpose output rather than the dedicated input to the slave
select circuit, thus inhibiting the mode fault flag. The other three lines are dedicated
to the SPI whenever the serial peripheral interface is on.
The state of the master and slave CPHA bits affects the operation of SS. CPHA
settings should be identical for master and slave. When CPHA = 0, the shift clock
is the OR of SS with SCK. In this clock phase mode, SS must go high between
successive characters in an SPI message. When CPHA = 1, SS can be left low
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