XC912BC32CFU8 Motorola Semiconductor Products, XC912BC32CFU8 Datasheet - Page 215

• Manufacturer Part Number: XC912BC32CFU8
• Description: M68HC12B Family Data Sheet
• Manufacturer: Motorola Semiconductor Products

14.3.1 SPI Baud Rate Generation
14.3.2 SPI Operation
M68HC12B Family — Rev. 8.0
MOTOROLA
SCK (CPOL = 0)
SCK (CPOL = 1)
SAMPLE I
CHANGE O
MOSI PIN
CHANGE O
MISO PIN
SEL SS (O)
MASTER ONLY
SEL SS (I)
MOSI/MISO
MSB first (LSBF = 0) :
LSB first (LSBF = 1) :
TRANSFER
The P clock is input to a divider series and the resulting SPI clock rate may be
selected to be P divided by 2, 4, 8, 16, 32, 64, 128, or 256. Three bits in the SP0BR
register control the SPI clock rate. This baud rate generator is activated only when
SPI is in the master mode and serial transfer is taking place. Otherwise, this divider
is disabled to save power.
In the SPI system, the 8-bit data register in the master and the 8-bit data register
in the slave are linked to form a distributed 16-bit register. When a data transfer
operation is performed, this 16-bit register is serially shifted eight bit positions by
the SCK clock from the master so the data is effectively exchanged between the
master and the slave. Data written to the SP0DR register of the master becomes
the output data for the slave and data read from the SP0DR register of the master
after a transfer operation is the input data from the slave.
A clock phase control bit (CPHA) and a clock polarity control bit (CPOL) in the SPI
control register 1 (SP0CR1) select one of four possible clock formats to be used by
the SPI system. The CPOL bit simply selects non-inverted or inverted clock. The
CPHA bit is used to accommodate two fundamentally different protocols by shifting
the clock by one half cycle or no phase shift.
t
L
Figure 14-12. SPI Clock Format 0 (CPHA = 0)
MSB
LSB
BEGIN
Bit 6
Bit 1
Bit 5
Bit 2
Serial Interface
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
END
Serial Peripheral Interface (SPI)
MSB
LSB
Minimum 1/2 SCK
t
T
for t
Serial Interface
T
t
, t
I
l
, t
L
t
L
Data Sheet
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