
Serial Peripheral Interface (SPI) Module
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
Freescale Semiconductor
169
When CPHA = 1, the slave begins to drive its MISO output when SS1 goes to active low, but the data is
not dened until the rst SPSCK edge. The rst SPSCK edge shifts the rst bit of data from the shifter
onto the MOSI output of the master and the MISO output of the slave. The next SPSCK edge causes both
the master and the slave to sample the data bit values on their MISO and MOSI inputs, respectively. At the
third SPSCK edge, the SPI shifter shifts one bit position which shifts in the bit value that was just sampled,
and shifts the second data bit value out the other end of the shifter to the MOSI and MISO outputs of the
master and slave, respectively. When CHPA = 1, the slave’s SS input is not required to go to its inactive
high level between transfers.
Figure 13-6 shows the clock formats when CPHA = 0. At the top of the gure, the eight bit times are shown
for reference with bit 1 starting as the slave is selected (SS IN goes low), and bit 8 ends at the last SPSCK
edge. The MSB rst and LSB rst lines show the order of SPI data bits depending on the setting in LSBFE.
Both variations of SPSCK polarity are shown, but only one of these waveforms applies for a specic
transfer, depending on the value in CPOL. The SAMPLE IN waveform applies to the MOSI input of a
slave or the MISO input of a master. The MOSI waveform applies to the MOSI output pin from a master
and the MISO waveform applies to the MISO output from a slave. The SS OUT waveform applies to the
slave select output from a master (provided MODFEN and SSOE = 1). The master SS output goes to active
low at the start of the rst bit time of the transfer and goes back high one-half SPSCK cycle after the end
of the eighth bit time of the transfer. The SS IN waveform applies to the slave select input of a slave.
Figure 13-6. SPI Clock Formats (CPHA = 0)
BIT TIME #
(REFERENCE)
MSB FIRST
LSB FIRST
SPSCK
(CPOL = 0)
SPSCK
(CPOL = 1)
SAMPLE IN
(MISO OR MOSI)
MOSI
(MASTER OUT)
MISO
(SLAVE OUT)
SS OUT
(MASTER)
SS IN
(SLAVE)
BIT 7
BIT 0
BIT 6
BIT 1
BIT 2
BIT 5
BIT 1
BIT 6
BIT 0
BIT 7
12
6
7
8
...