MC68HC08AS32A Datasheet, PDF (219/296 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Serial Peripheral Interface (SPI)
Functional Description
15.4.1 Master Mode
NOTE:
The SPI operates in master mode when the SPI master bit, SPMSTR (SPCR
$0010), is set.
Configure the SPI modules as master and slave before enabling them. Enable the
master SPI before enabling the slave SPI. Disable the slave SPI before disabling
the master SPI. (See 15.13.1 SPI Control Register.)
Only a master SPI module can initiate transmissions. Software begins the
transmission from a master SPI module by writing to the SPI data register. If the
shift register is empty, the byte immediately transfers to the shift register, setting
the SPI transmitter empty bit, SPTE (SPSCR $0011). The byte begins shifting out
on the MOSI pin under the control of the serial clock. (See Figure 15-4.)
The SPR1 and SPR0 bits control the baud rate generator and determine the speed
of the shift register. (See 15.13.2 SPI Status and Control Register.) Through the
SPSCK pin, the baud rate generator of the master also controls the shift register of
the slave peripheral.
MASTER MCU
SLAVE MCU
SHIFT REGISTER
MISO
MOSI
SPSCK
MISO
MOSI
SPSCK
SHIFT REGISTER
BAUD RATE
GENERATOR
SS
SS
V_DD
Figure 15-4. Full-Duplex Master-Slave Connections
As the byte shifts out on the MOSI pin of the master, another byte shifts in from the
slave on the master’s MISO pin. The transmission ends when the receiver full bit,
SPRF (SPSCR), becomes set. At the same time that SPRF becomes set, the byte
from the slave transfers to the receive data register. In normal operation, SPRF
signals the end of a transmission. Software clears SPRF by reading the SPI status
and control register and then reading the SPI data register. Writing to the SPI data
register clears the SPTE bit.
15.4.2 Slave Mode
The SPI operates in slave mode when the SPMSTR bit (SPCR $0010) is clear. In
slave mode the SPSCK pin is the input for the serial clock from the master MCU.
Before a data transmission occurs, the SS pin of the slave MCU must be at logic 0.
SS must remain low until the transmission is complete. See 15.6.2 Mode Fault
Error.
MC68HC08AS32A — Rev. 1
MOTOROLA
Serial Peripheral Interface (SPI)
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Data Sheet
219