MPC8280 datasheet(1288/1386 Pages) FREESCALE

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Serial Peripheral Interface (SPI)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

39-4

Freescale Semiconductor

39.3.2

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.

39.3.3

The SPI in Multimaster 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 Figure 39-3. Only one SPI device can act as master at a time—all

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.

MPC8280 Datasheet(PDF) 1288 Page - Freescale Semiconductor, Inc