Serial Peripheral Interface (SPI)
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
18-45
18.9.3
External Signal Configuration
The SPI supports a four-wire interface—transmit, receive, clock, and slave select. See Chapter
Figure 3-1., MSC8144 External Signals for detailed signal descriptions. After reset, the signals
are assigned as GPIO17 to GPIO20. They must be configured as SPI signals by writing the
correct values to the GPIO configuration registers. Refer to Table 3-11 on page 3-41 and
Chapter 22, GPIO for programming information.
The SPI can be configured as a slave or as a master in single- or multiple-master environments.
The master SPI generates the transfer clock SPI_CK, using the SPI baud rate generator (BRG).
The SPI BRG input is QUICC Engine clk /2. The selection as slave or master determines the
signal direction (input or output) to select when configuring the signals using the GPIO
configuration registers.
SPI_CK is a gated clock, active only during data transfers. Four combinations of SPI_CK phase
and polarity can be configured by using SPMODE[CI, CP]. SPI signals can also be configured as
open-drain to support a multimaster configuration in which a shared SPI signal is driven by the
processor or an external SPI device.
The SPI master-in slave-out SPI_MISO signal acts as an input for master devices and as an
output for slave devices. Conversely, the master-out slave-in SPI_MOSI signal is an output for
master devices. The dual functionality of these signals allows the SPIs in a multimaster
environment to communicate with one another using a common hardware configuration.
When the SPI is a master, SPI_CK is the clock output signal that shifts received data in
from SPI_MISO and transmitted data out to SPI_MOSI. SPI masters must output a slave
select signal to enable SPI slave devices by using a separate general-purpose I/O signal.
Assertion of SPI_SL while it is a master causes an error.
When the SPI is a slave, SPI_CK is the clock input that shifts received data in from
SPI_MOSI and transmitted data out through SPI_MISO. SPI_SL is the input enable to the
SPI slave. In a multi-master environment, SPI_SL (always an input) is also used to detect
an error when more than one master is operating.
18.9.4
SPI Transmission and Reception Process
Because the SPI is a character-oriented communication unit, the core processor must pack and
unpack the receive/transmit frames. A frame consists of all of the characters transmitted or
received during a completed SPI transmission session, from the first character written to the
internal SPI transmit data register (SPITD) to the last character transmitted following the setting
of the LST bit in the SPI command (SPCOM) register.
The core processor receives data by reading the SPI receive data register (SPIRD) when the SPI
event register (SPIE) not-empty bit (SPIE[NE]) is set. The core processor transmits data by
writing it into the SPITD. When the next character to transmit is the final one in the current
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