MC68HC908GZ8 Datasheet, PDF (280/344 Pages) Motorola, Inc – Microcontrollers

Serial Peripheral Interface (SPI) Module
NOTE: Before writing to the CPOL bit or the CPHA bit, disable the SPI by clearing the SPI
enable bit (SPE).
21.5.2 Transmission Format When CPHA = 0
Figure 21-4 shows an SPI transmission in which CPHA is logic 0. 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 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 be high or must be
reconfigured as general-purpose I/O not affecting the SPI. (See 21.7.2 Mode Fault
Error.) When CPHA = 0, the first SPSCK edge is the MSB capture strobe.
Therefore, the slave must begin driving its data before the first SPSCK edge, and
a falling edge on the SS pin is used to start the slave data transmission. The slave’s
SS pin must be toggled back to high and then low again between each byte
transmitted as shown in Figure 21-5.
SPSCK CYCLE #
FOR REFERENCE
SPSCK; CPOL = 0
1
2
3
4
5
6
7
8
SPSCK; CPOL =1
MOSI
FROM MASTER
MISO
FROM SLAVE
SS; TO SLAVE
CAPTURE STROBE
MSB 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
Figure 21-4. Transmission Format (CPHA = 0)
MISO/MOSI
MASTER SS
SLAVE SS
CPHA = 0
SLAVE SS
CPHA = 1
BYTE 1
BYTE 2
BYTE 3
Figure 21-5. CPHA/SS Timing
Data Sheet
280
Serial Peripheral Interface (SPI) Module
MC68HC908GZ8
Freescale Semiconductor