ST92F124 datasheet(215/429 Pages) STMICROELECTRONICS

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MULTIPROTOCOL SERIAL COMMUNICATIONS INTERFACE (SCI-M)

MULTIPROTOCOL SERIAL COMMUNICATIONS INTERFACE (Cont’d)

10.5.4.3 Serial Expansion Mode

This mode is used to communicate with an exter-

nal synchronous peripheral.

The transmitter only provides the clock waveform

during the period that data is being transmitted on

the CLKOUT pin (the Data Envelope). Data is

latched on the rising edge of this clock.

Whenever the SCI is to receive data in serial port

expansion mode, the clock must be supplied ex-

ternally, and be synchronous with the transmitted

data. The SCI latches the incoming data on the ris-

ing edge of the received clock, which is input on

the RXCLK pin.

10.5.4.4 Synchronous Mode

This mode is used to access an external synchro-

nous peripheral, dummy start/stop bits are not in-

cluded in the data frame. Polarity, stand-by level

and active edges of I/O signals are fully and sepa-

rately programmable for both inputs and outputs.

It's necessary to set the SMEN bit of the Synchro-

nous Input Control Register (SICR) to enable this

mode and all the related extra features (otherwise

disabled).

The transmitter will provide the clock waveform

only during the period when the data is being

transmitted via the CLKOUT pin, which can be en-

abled by setting both the XTCLK and OCLK bits of

the Clock Configuration Register. Whenever the

SCI is to receive data in synchronous mode, the

clock waveform must be supplied externally via

the RXCLK pin and be synchronous with the data.

For correct receiver operation, the XRX bit of the

Clock Configuration Register must be set.

Two external signals, Request-To-Send and Data-

Carrier-Detect (RTS/DCD), can be enabled to syn-

chronise the data exchange between two serial

units. The RTS output becomes active just before

the first active edge of CLKOUT and indicates to

the target device that the MCU is about to send a

synchronous frame; it returns to its stand-by state

following the last active edge of CLKOUT (MSB

transmitted).

The DCD input can be considered as a gate that

filters RXCLK and informs the MCU that a trans-

mitting device is transmitting a data frame. Polarity

of RTS/DCD is individually programmable, as for

clocks and data.

The data word is programmable from 5 to 8 bits, as

for the other modes; parity, address/9th, stop bits

and break cannot be inserted into the transmitted

frame. Programming of the related bits of the SCI

control registers is irrelevant in Synchronous

Mode: all the corresponding interrupt requests

must, in any case, be masked in order to avoid in-

correct operation during data reception.

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