ST92196A datasheet(195/268 Pages) STMICROELECTRONICS

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- SERIAL COMMUNICATIONS INTERFACE (SCI)

SERIAL COMMUNICATIONS INTERFACE (Cont’d)

8.11.4 Clocks And Serial Transmission Rates

The communication bit rate of the SCI transmitter

and receiver sections is provided from the internal

Baud Rate Generator divided by 16. x

Baud Rate Generator. The internal Baud Rate

Generator consists of a 16-bit programmable di-

vide by “N” counter which can be used to generate

the transmitter and/or receiver clocks. The mini-

mum baud rate divisor is 2 and the maximum divi-

sor is 216-1.

With INTCLK running at 20MHz or 10MHz, a max-

imum bit rate of 625 KBaud or 312.5K Baud re-

spectively is possible.

WARNING: Programming the baud rate divider to

0 or 1 will stop the divider.

The output of the Baud Rate generator has a pre-

cise 50% duty cycle. INTCLK (and therefore the

MCU Xtal) should be chosen to provide a suitable

frequency for division by the Baud Rate Generator

to give the required transmit and receive bit rates.

Suitable INTCLK frequencies and the respective

divider values for standard Baud rates are shown

in Table 41.

8.11.5 SCI Initialization Procedure

Writing to either of the two Baud Rate Generator

Registers immediately disables and resets the SCI

baud rate generator, as well as the transmitter and

receiver circuitry.

After writing to the second Baud Rate Generator

enabled. The Baud Rate Generator will load the

new value and start counting.

To initialize the SCI, the user should first initialize

the most significant byte of the Baud Rate Gener-

ator Register; this will reset all SCI circuitry. The

user should then initialize all other SCI registers

(SICR/SOCR included) for the desired operating

mode and then, to enable the SCI, he should ini-

tialize the least significant byte Baud Rate Gener-

ator Register.

'On-the-Fly' modifications of the control registers'

content during transmitter/receiver operations, al-

though possible, can corrupt data and produce un-

desirable spikes on the I/O lines. Furthermore,

modifying the control registers' content without

reinitialising the SCI circuitry (during stand-by cy-

cles, waiting to transmit or receive data) must be

kept carefully under control by software to avoid

spurious data being transmitted or received.

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