Transmission Formats - Motorola MC68HC908GP32 Technical Data Manual

NOTE:

20.6 Transmission Formats

20.6.1 Clock Phase and Polarity Controls
MC68HC908GP32 MC68HC08GP32
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MOTOROLA
Freescale Semiconductor, Inc.
When the master SPI starts a transmission, the data in the slave shift
register begins shifting out on the MISO pin. The slave can load its shift
register with a new byte for the next transmission by writing to its transmit
data register. The slave must write to its transmit data register at least
one bus cycle before the master starts the next transmission. Otherwise,
the byte already in the slave shift register shifts out on the MISO pin.
Data written to the slave shift register during a transmission remains in
a buffer until the end of the transmission.
When the clock phase bit (CPHA) is set, the first edge of SPSCK starts
a transmission. When CPHA is clear, the falling edge of SS starts a
transmission. (See 20.6 Transmission
SPSCK must be in the proper idle state before the slave is enabled to
prevent SPSCK from appearing as a clock edge.
During an SPI transmission, data is simultaneously transmitted (shifted
out serially) and received (shifted in serially). A serial clock synchronizes
shifting and sampling on the two serial data lines. A slave select line
allows selection of an individual slave SPI device; slave devices that are
not selected do not interfere with SPI bus activities. On a master SPI
device, the slave select line can optionally be used to indicate multiple-
master bus contention.
Software can select any of four combinations of serial clock (SPSCK)
phase and polarity using two bits in the SPI control register (SPCR). The
clock polarity is specified by the CPOL control bit, which selects an
active high or low clock and has no significant effect on the transmission
format.
Rev. 6
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Serial Peripheral Interface Module (SPI)
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Serial Peripheral Interface Module (SPI)
Transmission Formats
Formats.)
Technical Data
309