MC68HC908GR8 Datasheet, PDF (210/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Serial Peripheral Interface (SPI)
SPSCK CYCLE #
FOR REFERENCE
SPSCK; CPOL = 0
1
2
3
4
5
6
7
8
SPSCK; CPOL =1
MOSI
FROM MASTER
MISO
FROM SLAVE
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
SS; TO SLAVE
CAPTURE STROBE
Figure 20-4. Transmission Format (CPHA = 0)
MISO/MOSI
BYTE 1
BYTE 2
BYTE 3
MASTER SS
SLAVE SS
CPHA = 0
SLAVE SS
CPHA = 1
Figure 20-5. CPHA/SS Timing
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 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 20.7.2 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 20-5.
When CPHA = 0 for a slave, the falling edge of SS indicates the beginning of the transmission. This
causes the SPI to leave 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.
20.5.3 Transmission Format When CPHA = 1
Figure 20-6 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 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
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
210
Freescale Semiconductor