
Serial Peripheral Interface (SPI) Module
Transmission Formats
MC68HC908GZ60 MC68HC908GZ48 MC68HC908GZ32 — Rev. 1.0
Data Sheet
MOTOROLA
Serial Peripheral Interface (SPI) Module
267
into the shift register from the transmit data register. Therefore, the SPI data
register of the slave must be loaded with transmit data before the falling edge of
SS. Any data written after the falling edge is stored in the transmit data register and
transferred to the shift register after the current transmission.
Figure 16-5. Transmission Format (CPHA = 0)
Figure 16-6. CPHA/SS Timing
16.4.3 Transmission Format When CPHA = 1
Figure 16-7 shows an SPI transmission in which CPHA = 1. 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 low, 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
begins driving its MOSI pin on the first SPSCK edge. Therefore, the slave uses the
first SPSCK edge as a start transmission signal. The SS pin can remain low
between transmissions. This format may be preferable in systems having only one
master and only one slave driving the MISO data line.
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
MSB
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
MSB
123
4
5
6
7
8
SPSCK CYCLE #
FOR REFERENCE
SPSCK; CPOL = 0
SPSCK; CPOL =1
MOSI
FROM MASTER
MISO
FROM SLAVE
SS; TO SLAVE
CAPTURE STROBE
BYTE 1
BYTE 3
MISO/MOSI
BYTE 2
MASTER SS
SLAVE SS
CPHA = 0
SLAVE SS
CPHA = 1