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.