MC68HC05C9E Datasheet, PDF (64/106 Pages) Freescale Semiconductor, Inc – Microcontrollers

Serial Peripheral Interface (SPI)
In the master mode, the SCK pin is an output. It idles high or low, depending on the CPOL bit in the SPCR,
until data is written to the shift register, at which point eight clocks are generated to shift the eight bits of
data and then SCK goes idle again.
In a slave mode, the slave select start logic receives a logic low at the SS pin and a clock at the SCK pin.
Thus, the slave is synchronized with the master. Data from the master is received serially at the MOSI
line and loads the 8-bit shift register. After the 8-bit shift register is loaded, its data is parallel transferred
to the read buffer. During a write cycle, data is written into the shift register, then the slave waits for a clock
train from the master to shift the data out on the slave’s MISO line.
Figure 10-3 illustrates the MOSI, MISO, SCK, and SS master-slave interconnections.
PD3/MOSI
SPI SHIFT REGISTER
76543210
SPDR ($000C)
PD2/MISO
PD5/SS
I/O PORT
SPI SHIFT REGISTER
76543210
SPDR ($000C)
PD4/SCK
MASTER MCU
SLAVE MCU
Figure 10-3. Serial Peripheral Interface Master-Slave Interconnection
10.5 SPI Registers
This subsection describes the three registers in the SPI which provide control, status, and data storage
functions. These registers are:
• Serial peripheral control register (SPCR)
• Serial peripheral status register (SPSR)
• Serial peripheral data I/O register (SPDR)
10.5.1 Serial Peripheral Control Register
The SPI control register (SPCR), shown in Figure 10-4, controls these functions:
• Enables SPI interrupts
• Enables the SPI system
• Selects between standard CMOS or open drain outputs for port D
• Selects between master mode and slave mode
• Controls the clock/data relationship between master and slave
• Determines the idle level of the clock pin
Address:
Read:
Write:
Reset:
$000A
Bit 7
6
5
4
3
2
SPIE
SPE
DWOM MSTR
CPOL
CPHA
0
0
0
0
0
1
U = Undetermined
Figure 10-4. SPI Control Register (SPCR)
1
SPR1
U
Bit 0
SPR0
U
MC68HC05C9E Advance Information Data Sheet, Rev. 0.1
64
Freescale Semiconductor