Chapter 15 Serial Peripheral Interface (S08SPIV3)
MC9S08SG32 Data Sheet, Rev. 7
Freescale Semiconductor
235
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 one-half SPSCK cycle before the start of the transfer and goes back high at
the end of the eighth bit time of the transfer. The SS IN waveform applies to the slave select input of a
slave.
Figure 15-10. SPI Clock Formats (CPHA = 1)
When CPHA = 1, the slave begins to drive its MISO output when SS 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 15-11 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
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
...