MC68HC711PH8 Motorola, MC68HC711PH8 Datasheet - Page 125

• Manufacturer Part Number: MC68HC711PH8
• Description: High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit
• Manufacturer: Motorola

7.2.1
Software can select one of four combinations of serial clock phase and polarity using two bits in
the SPI control register (SPCR). The clock polarity is specified by the CPOL control bit, which
selects an active high or active low clock, and has no significant effect on the transfer format. The
clock phase (CPHA) control bit selects one of two different transfer formats. The clock phase and
polarity should be identical for the master SPI device and the communicating slave device. In
some cases, the phase and polarity are changed between transfers to allow a master device to
communicate with peripheral slaves having different requirements.
When CPHA equals zero, the SS line must be deasserted and reasserted between each
successive serial byte. Also, if the slave writes data to the SPI data register (SPDR) while SS is
low, a write collision error results.
When CPHA equals one, the SS line can remain low between successive transfers.
7.3
The following paragraphs contain descriptions of the four SPI signals: master in slave out (MISO),
master out slave in (MOSI), serial clock (SCK), and slave select (SS).
Any SPI output line must have its corresponding data direction bit in DDRD register set. If the DDR
bit is clear, that line is disconnected from the SPI logic and becomes a general-purpose input. All
SPI input lines are forced to act as inputs regardless of the state of the corresponding DDR bits in
DDRD register.
MC68HC11PH8
Data out (CPHA=0)
Data out (CPHA=1)
SCK (CPOL=0)
SCK (CPOL=1)
(for reference)
Sample input
Sample input
SS (to slave)
SCK cycle #
Note: this Þgure shows the LSBF=0 (default) case. If LSBF=1, data is transferred in the reverse order (LSB Þrst).
Clock phase and polarity controls
SPI signals
MSB
MSB
1
SERIAL PERIPHERAL INTERFACE
6
Figure 7-2 SPI transfer format
2
6
5
3
5
4
4
4
3
5
3
2
6
2
1
7
1
LSB
8
LSB
MOTOROLA
TPG
7-3
7