Serial Peripheral Interface Module (SPI)
Transmission Formats
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Serial Peripheral Interface Module (SPI) 253
NOTE: If the write to the data register is late, the SPI transmits the data alread y
in the shift register from the previous transmission.
SPSCK must be in the proper idle state before the slave is enabled to
prevent SPSCK from appearing as a clock edge.
13.6 Transmission FormatsDuring an SPI transmission, data is simultaneously transmi tte d ( shi fte d
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 tha t are
not selected do not interfere with SPI b us activities. On a master SPI
device, the slave select line can optionally be used to indicate
multiple-master bus contention.
13.6.1 Clock Phase and Polarity Controls
Software can select any of four combinations of serial clock (SPSCK)
phase and polarity using two bits in the SPI control r egister (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.
The clock phase (CPHA) control bit selects one of two fundamentally
different transmission formats. The clock phase and polarity should be
identical for the master SPI device and the communicating slave device.
In some cases, the phase and polarity are changed between
transmissions to allow a master device to communicate with peripheral
slaves having different requirements.
NOTE: Before writing to the CPOL bit or the CPHA bit, disable the SPI by
clearing the SPI enable bit (SPE).
13.6.2 Transmission Format When CPHA = 0
Figure 13-4 shows an SPI transmission in which CPHA is logic 0. The
figure should not be used as a replacement for data sheet parametric
information.Two waveforms are shown for SPSCK: one for CPOL = 0