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ST62T35B/E35B

4.5 U. A. R. T. (Universal Asynchronous Receiver/Transmitter)

The UART provides the basic hardware for asyn-

chronous serial communication which, combined

with an appropriate software routine, gives a serial

interface providing communication with common

baud rates (up to 38,400 Baud with an 8MHz ex-

ternal oscillator) and flexible character formats.

Operating in Half-Duplex mode only, the UART

uses 11-bit characters comprising 1 start bit, 9 data

bits and 1 Stop bit. Parity is supported by software

only for transmit and for checking the received par-

ity bit (bit 9). Transmitted data is sent directly, while

received data is buffered allowing further data

characters to be received while the data is being

read out of the receive buffer register. Data trans-

mit has priority over data being received.

The UART is supplied with an MCU internal clock

that is also available in WAIT mode ofthe processor.

4.5.1 PORTS INTERFACING

RXD reception line and TXD emission line are

sharing the same external pins as two I/O lines.

Therefore, UART configuration requires to set

these two I/O lines through the relevant ports reg-

isters. The I/O line common with RXD line must be

defined as input mode (with or without pull-up)

while the I/O line common with TXD line must be

defined as output mode (Push-pull or open drain).

The transmitted data is inverted and can therefore

use a single transistor buffering stage. Defined as

input, the RXD line can be read at any time as an

I/O line during the UART operation. The TXD pin

follows I/O port registers value when UARTOE bit

is cleared, which means when no serial transmis-

sion is in progress. As a consequence, a perma-

nent high level has to be written onto the I/O port

in order to achieve a proper stop condition on the

TXD line when no transmission is active.

Figure 31. UART Block Diagram

STA RT

DETE CTOR

DATA SHIFT

D8 D7 D6 D5 D4 D3 D2 D1 D0

CONTROL REGISTER

BAUD RATE

RECEIVE BUFFE R

PROGRAMMABLE

DIVI DER

DIN

DOUT

BAUD RATE x 8

WRITE

READ

RXD1

TXD1

UARTOE

RX and TX

INTERRUPTS

TXD

MUX

fOSC

VR02009

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