ST9 datasheet(92/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

4.6 SERIAL COMMUNICATIONS INTERFACE

4.6.1 Description

The SCI is the association of a UART and a complex logic that handles tasks such as char-

acter recognition and DMA. It can also work as a simple serial expansion port that then resem-

bles the SPI. The example given here appears as a classical UART application, viewed exter-

nally, but it is assisted by the built-in DMA controller to provide effortless transfers of data in

and out of the application. As serial communication is a feature that is very often used, it will

be found in all three applications.

The SCI has four operating modes:

– Asynchronous mode where data and clock can be asynchronous. Each data is sampled 16

times per clock period. The baud rate should be set to the 16 division mode and the frequen-

cy of the input clock is set to suit.

– Asynchronous mode with synchronous clock where data and clock are synchronous but the

transmit and receive clock are asynchronous. The receive clock must be given by the exter-

nal peripheral.

– Serial Expansion mode where data and clock are synchronous and the clock is supplied by

the transmitter (ST9 in transmission and externally in reception).

– Synchronous mode where data and clock are synchronous, the transmit data clock is sup-

plied by the ST9 and the receive data clock is received by the external peripheral. In this

mode there are no start and stop bits.

These four operating modes allow you to connect the ST9 to any external interface.

4.6.1.1 UART

The UART offers all the usual functions for asynchronous transfer. You can set it to handle

word lengths of 5 to 8 bits, with or without parity, even or odd. It offers also the possibility to ap-

pend a signalling bit (called Address bit or 9th bit -- regardless of the word length) at the end

of the transmitted data, just before the stop bit. This can be used to help design a multidrop

network. The UART can be set to interrupt the processor only when this bit is high, indicating

that the current character is an address or other identifier. This prevents the processor from

being disturbed by the traffic on the network unless an address byte is received, so that the

processor can check if it is concerned or not by the incoming data.

The UART is set to 8 bits, 1 stop bit and no parity, by writing the appropriate value in the

CHCR register.

A timer is part of the SCI block. It is used as a Baud Rate Generator (BRG). It allows you to di-

vide either the internal clock or the frequency available at the RXCLK input pin, by an arbitrary

value. This output can be fed to the receive and transmit sections, though each section can

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