Serial Peripheral Interface Module (SPI)
Transmission Formats
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Serial Peripheral Interface Module (SPI) 255
When CPHA = 0 for a slave, the falling edge of SS indicates the
beginning of the transmission. This causes the SPI to leav e its idle state
and begin driving the MISO pin with the MSB of its data. Once the
transmission begins, no new data is allowed 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.
13.6.3 Transmission Format When CPHA = 1
Figure 13-6 shows an SPI transmission in which CPHA is logic 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 d ire ctly con nected
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 b e
high or must be reconfigured as general-purpose I/O not affecting the
SPI. See 13.7.2 Mode Fault Error. When CPHA = 1, the master begins
driving its MOSI pin on the first SPSCK edge. Ther efo re , th e slave uses
Figure 13-6. Transmission Format (CPHA = 1)
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 LS B
MSB
12345678
SPSCK CYCLE #
FOR REFERENCE
SPSCK, CPOL = 0
SPSCK, CPOL = 1
MOSI
FROM MASTER
MISO
FROM SLAVE
SS, TO SLAVE
CAPTURE STROBE