MC68HC08AZ60A Datasheet, PDF (256/480 Pages) Motorola, Inc – Microcontrollers

Serial Peripheral Interface (SPI)
frequency of the SPSCK for an SPI configured as a slave can be any
frequency less than or equal to the bus speed.
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 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 Transmission Formats on page 256.
If the write to the data register is late, the SPI transmits the data already
in the shift register from the previous transmission.
NOTE: SPSCK must be in the proper idle state before the slave is enabled to
prevent SPSCK from appearing as a clock edge.
17.6 Transmission Formats
During an SPI transmission, data is simultaneously transmitted (shifted
out serially) and received (shifted in serially). A serial clock line
synchronizes shifting and sampling on the two serial data lines. A slave
select line allows individual selection of a 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 a multiple-master bus contention.
17.6.1 Clock Phase and Polarity Controls
Software can select any of four combinations of serial clock (SCK) 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.
Advance Information
256
Serial Peripheral Interface (SPI)
MC68HC08AZ60A — Rev 0.0
MOTOROLA