
MOTOROLA
Chapter 31. Serial Peripheral Interface
31-3
Part V. The Communications Processor Module
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When the SPI is a master, SPICLK is the clock output signal that shifts received data
in from SPIMISO and transmitted data out to SPIMOSI. SPI masters must output a
slave select signal to enable SPI slave devices by using a separate general-purpose
I/O signal. Assertion of an SPIÕs SPISEL, while it is in master mode, causes an error.
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When the SPI is a slave, SPICLK is the clock input that shifts received data in from
SPIMOSI and transmitted data out through SPIMISO. SPISEL is the enable input to
the SPI slave.
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In a multimaster environment, SPISEL (always an input) is also used to detect when
more than one master is operating, which is an error condition.
As described in Chapter 34, ÒParallel I/O Ports,Ó SPIMISO, SPIMOSI, SPICLK, and
SPISEL are multiplexed with port B[28Ð31] signals, respectively. They are conÞgured as
SPI signals through the port B signal assignment register (PBPAR) and the Port B data
direction register (PBDIR), speciÞcally by setting PBPAR[DDn] and PBDIR[DRn].
31.3 ConÞguring the SPI Controller
The SPI can be programmed to work in a single- or multiple-master environment. This
section describes SPI master and slave operation in a single-master conÞguration and then
discusses the multi-master environment.
SPI transmission and reception will always be enabled simultaneously. If the transmit or
receive function is not needed, the user can point the associated channel of a non-ready
TxBD or RxBD, and simply ignore the resultant Tx underrun or Rx busy errors.
31.3.1 The SPI as a Master Device
In master mode, the SPI sends a message to the slave peripheral, which sends back a
simultaneous reply. A single master MPC860 with multiple slaves can use general-purpose
parallel I/O signals to selectively enable slaves, as shown in Figure 31-2. To eliminate the
multimaster error in a single-master environment, the masterÕs SPISEL input can be forced
inactive by selecting port B[31] for general-purpose I/O (PBPAR[DD31] = 0).
Summary of Contents for MPC860 PowerQUICC
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