ST92F120JR9T datasheet(189/320 Pages) STMICROELECTRONICS

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SERIAL COMMUNICATIONS INTERFACE (SCI)

SERIAL COMMUNICATIONS INTERFACE (Cont’d)

10.5.4 Serial Frame Format

Characters sent or received by the SCI can have

some or all of the features in the following format,

depending on the operating mode:

START: the START bit indicates the beginning of

a data frame in Asynchronous modes. The START

condition is detected as a high to low transition.

A dummy START bit is generated in Serial Expan-

sion mode. The START bit is not generated in

Synchronous mode.

DATA: the DATA word length is programmable

from 5 to 8 bits, for both Synchronous and Asyn-

chronous modes. LSB are transmitted first.

PARITY: The Parity Bit (not available in Serial Ex-

pansion mode and Synchronous mode) is option-

al, and can be used with any word length. It is used

for error checking and is set so as to make the total

number of high bits in DATA plus PARITY odd or

even, depending on the number of “1”s in the

DATA field.

ADDRESS/9TH: The Address/9th Bit is optional

and may be added to any word format. It is used in

both Serial Expansion and Asynchronous modes

to indicate that the data is an address (bit set).

The ADDRESS/9TH bit is useful when several mi-

crocontrollers are exchanging data on the same

serial bus. Individual microcontrollers can stay idle

on the serial bus, waiting for a transmitted ad-

dress. When a microcontroller recognizes its own

address, it can begin Data Reception, likewise, on

the transmit side, the microcontroller can transmit

another address to begin communication with a

different microcontroller.

The ADDRESS/9TH bit can be used as an addi-

tional data bit or to mark control words (9th bit).

STOP: Indicates the end of a data frame in Asyn-

chronous modes. A dummy STOP bit is generated

in Serial Expansion mode. The STOP bit can be

programmed to be 1, 1.5, 2, 2.5 or 3 bits long, de-

pending on the mode. It returns the SCI to the qui-

escent marking state (i.e., a constant high-state

condition) which lasts until a new start bit indicates

an incoming word. The STOP bit is not generated

in Synchronous mode.

Figure 92. SCI Character Formats

(1) LSB First

(2) Not available in Synchronous mode

(3) Not available in Serial Expansion mode

and Synchronous mode

START(2)

DATA(1)

PARITY(3)

ADDRESS(2)

STOP(2)

# bits

5, 6,7,8

0, 1

1, 1.5, 2, 2.5,

1, 2, 3

16X

states

NONE

ODD

EVEN

OFF

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