MC68HC08XL36 Datasheet, PDF (196/362 Pages) Freescale Semiconductor, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
SPI
Only a master SPI module can initiate transmissions. Software begins
the transmission from a master SPI module by writing to the transmit
data register. If the shift register is empty, the byte immediately transfers
to the shift register, setting the SPI transmitter empty bit, SPTE. The byte
begins shifting out on the MOSI pin under the control of the serial clock.
(See Figure 3. Full-Duplex Master-Slave Connections.)
The SPR1 and SPR0 bits control the baud rate generator and determine
the speed of the shift register. (See SPI Status and Control Register on
page 221.) Through the SPSCK pin, the baud rate generator of the
master also controls the shift register of the slave peripheral.
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, 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 with
SPRF set and then reading the SPI data register. Writing to the SPI data
register clears the SPTE bit.
When the DMAS bit is set, the SPI status and control register does not
have to be read to clear the SPRF bit. A read of the SPI data register by
either the CPU or the DMA clears the SPRF bit. A write to the SPI data
register by the CPU or by the DMA clears the SPTE bit.
MASTER MCU
SHIFT REGISTER
BAUD RATE
GENERATOR
MISO
MOSI
SPSCK
SS
SLAVE MCU
MISO
MOSI
SPSCK
SHIFT REGISTER
V_DD
SS
Figure 3. Full-Duplex Master-Slave Connections
MC68HC08XL36
196
Serial Peripheral Interface Module (SPI)
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6-spi_c
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