| Universal
Asynchronous Receiver Transmitter |
Introduction The
Universal Asynchronous Receiver Transmitter (UART) is the most widely used serial
data communication circuit ever. UARTs allow full duplex communication over serial
communication links as RS232. UARTs are available as inexpensive standard products
from many semiconductor suppliers, making it unlikely that this specific design
is useful by itself. The basic
functions of a UART are a microprocessor interface, double buffering of transmitter
data, frame generation, parity generation, parallel to serial conversion, double
buffering of receiver data, parity checking, serial to parallel conversion. The
data is transmitted asynchronously one bit at a time and there is no clock line.
The frame format of used by UARTs is a low start bit, 5-8 data bits, optional
parity bit, and 1 or 2 stop bits. Universal Asynchronous Receive/Transmit consists
of baud rate generator, transmitter and receiver. The number of bits transmitted
per second is called baud rate and the baud rate generator generates the transmitter
and receiver clocks separately. UART synchronizes the incoming bit stream with
the local clock. Transmitter interfaces
to the data bus with the transmitter data register empty (TDRE) and write signals.
When transmitting, UART takes eight bits of parallel data and converts it into
serial bit stream and transmit them serially.
Receiver interfaces to the data
bus with the receiver ready and the read signals. When UART detects the start
bit, it receives the data serially and converts it into parallel form and when
stop bit (logic high) is detected, data is recognized as a valid data.
UART
Transmitter
The UART transmitter
mainly consists of two eight bit registers the Transmit Data Register (TDR) and
Transmit Shift Register (TSR) along with the Transmitter Control. The transmitter
control generates the TDRE and TSRE signals which controls the data transmission
through the UART transmitter. The write operation into the TDR is based on the
signals generated from the microprocessor.
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