
Technical Data
MC68HC11E Family
—
Rev. 4
170
Serial Peripheral Interface (SPI)
MOTOROLA
Serial Peripheral Interface (SPI)
8.6.1 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.
8.6.2 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.
8.6.3 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.
8.6.4 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