mc68hc912bc32 Freescale Semiconductor, Inc, mc68hc912bc32 Datasheet - Page 202

• Manufacturer Part Number: mc68hc912bc32
• Description: M68hc12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Serial Interface
14.3.1 SPI Baud Rate Generation
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.
14.3.2 SPI Operation
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.
202
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
t
L
Figure 14-12. SPI Clock Format 0 (CPHA = 0)
MSB
LSB
M68HC12B Family Data Sheet, Rev. 9.1
BEGIN
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
END
MSB
LSB
Freescale Semiconductor
Minimum 1/2 SCK
t
T
for t
T
t
, t
I
l
, t
L
t
L