Serial Peripheral Interface Module (SPI)
Technical Data MC68HC908AB32 — Rev. 1.0
286 Serial Peripheral Interface Module (SPI) MOTOROLA
The clock phase (CPHA) control bit selects one of two fundamentally
different transmission formats. The clock phase and polarity should be
identical for the master SPI device and the communicating slave device.
In some cases, the phase and polarity are changed between
transmissions to allow a master device to communicate with peripheral
slaves having different requirements.
NOTE:
Before writing to the CPOL bit or the CPHA bit, disable the SPI by
clearing the SPI enable bit (SPE).
16.6.2 Transmission Format When CPHA = 0
Figure 16-4 shows an SPI transmission in which CPHA is logic 0. The
figure should not be used as a replacement for data sheet parametric
information.
Two waveforms are shown for SPSCK: one for CPOL = 0 and another
for CPOL = 1. The diagram may be interpreted as a master or slave
timing diagram since the serial clock (SPSCK), master in/slave out
(MISO), and master out/slave in (MOSI) pins are directly connected
between the master and the slave. The MISO signal is the output from
the slave, and the MOSI signal is the output from the master. The SS
line
is the slave select input to the slave. The slave SPI drives its MISO
output only when its slave select input (SS
) is at logic 0, so that only the
selected slave drives to the master. The SS
pin of the master is not
shown but is assumed to be inactive. The SS
pin of the master must be
high or must be reconfigured as general-purpose I/O not affecting the
SPI. (See 16.8.2 Mode Fault Error.) When CPHA = 0, the first SPSCK
edge is the MSB capture strobe. Therefore, the slave must begin driving
its data before the first SPSCK edge, and a falling edge on the SS
pin is
used to start the slave data transmission. The slave’s SS
pin must be
toggled back to high and then low again between each byte transmitted
as shown in Figure 16-5.
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