Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 834

Serial Peripheral Interface (SPI)
The SPI then generates programmable clock pulses on SPICLK for each character and simultaneously
shifts Tx data out on SPIMOSI and Rx data in on SPIMISO. Received data is written into a Rx buffer using
the next available RxBD. The SPI keeps sending and receiving characters until the whole buffer is sent or
an error occurs. The CPM then clears TxBD[R] and RxBD[E] and issues a maskable interrupt to the CPM
interrupt controller (CPIC).
When multiple TxBDs are ready, TxBD[L] determines whether the SPI keeps transmitting without
SPCOM[STR] being set again. If the current TxBD[L] is cleared, the next TxBD is processed after data
from the current buffer is sent. Typically there is no delay on SPIMOSI between buffers. If the current
TxBD[L] is set, sending stops after the current buffer is sent. In addition, the RxBD is closed after
transmission stops, even if the Rx buffer is not full; therefore, Rx buffers need not be the same length as
Tx buffers.
30.3.2 The SPI as a Slave Device
In slave mode, the SPI receives messages from an SPI master and sends a simultaneous reply. The slave's
SPISEL must be asserted before Rx clocks are recognized; once SPISEL is asserted, SPICLK becomes an
input from the master to the slave. SPICLK can be any frequency from DC to BRGCLK/2 (12.5 MHz for
a 25-MHz system).
To prepare for data transfers, the slave's core writes data to be sent into a buffer, configures a TxBD with
TxBD[R] set, and configures one or more RxBDs. The core then sets SPCOM[STR] to activate the SPI.
Once SPISEL is asserted, the slave shifts data out from SPIMISO and in through SPIMOSI. A maskable
interrupt is issued when a full buffer finishes receiving and sending or after an error. The SPI uses
successive RxBDs in the table to continue reception until it runs out of Rx buffers or SPISEL is negated.
Transmission continues until no more data is available or SPISEL is negated. If it is negated before all data
is sent, it stops but the TxBD stays open. Transmission continues once SPISEL is reasserted and SPICLK
begins toggling. After the characters in the buffer are sent, the SPI sends ones as long as SPISEL remains
asserted.
30.3.3 The SPI in Multi-master Operation
The SPI can operate in a multimaster environment in which SPI devices are connected to the same bus. In
this configuration, the SPIMOSI, SPIMISO, and SPICLK signals of all SPIs are shared; the SPISEL inputs
are connected separately, as shown in
others must be slaves. When an SPI is configured as a master and its SPISEL input is asserted, a
multimaster error occurs because more than one SPI device is a bus master. The SPI sets SPIE[MME] in
the SPI event register and a maskable interrupt is issued to the core. It also disables SPI operation and the
output drivers of SPI signals. The core must clear SPMODE[EN] before the SPI is used again. After
correcting the problems, clear SPIE[MME] and reenable the SPI.
30-4
Figure 30-3. Only one SPI device can act as master at a time—all
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor