
Serial Peripheral Interface Module (SPI)
Transmission Formats
MC68HC(7)08LN56 — Rev. 2.0
General Release Specication
MOTOROLA
Serial Peripheral Interface Module (SPI)
171
NONDISCLOSURE
AGREEMENT
REQUIRED
11.6 Transmission Formats
During an SPI transmission, data is simultaneously transmitted (shifted
out serially) and received (shifted in serially). A serial clock synchronizes
shifting and sampling on the two serial data lines. A slave select line
allows selection of an individual slave SPI device; slave devices that are
not selected do not interfere with SPI bus activities. On a master SPI
device, the slave select line can optionally be used to indicate
multiple-master bus contention.
11.6.1 Clock Phase and Polarity Controls
Software can select any of four combinations of serial clock (SPSCK)
phase and polarity using two bits in the SPI control register (SPCR). The
clock polarity is specified by the CPOL control bit, which selects an
active high or low clock and has no significant effect on the transmission
format.
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) .
11.6.2 Transmission Format When CPHA = 0
Figure 11-4 shows an SPI transmission in which CPHA is logic zero.
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