Question

What are the stop and start bit roles in an UART? How the transmitter and receiver are synchronized in an UART?

Answer 1

UART stands for Universal Asynchronous Receiver / Transmitter.

It is Full Duplex mode of serial communication with only two wires required ,i.e, Tx and Rx.

Since, in Asynchronous communication there is no involvement of clock to synchronize the output of transmitting device to the samples received by receiver device , hence there is a need to use Start and Stop bit. These start and stop bit are added to the data packet to be transmitted which marks the beginning and end of the data transfer. The receiver device starts reading data after it encounters a start bit and ends reading data after encountering the stop bit. The receiver later discards the start and stop bits and keeps the actual data transmitted.

Since, there is no clock for synchronization, hence in UART, the concept of BAUD RATE is introduced. Baud rate is a measure of the speed of data transfer, expressed in bits per second (bps).

For successful data transfer both devices must operate at the same Baud Rate. Hence Transmitter and Receiver are synchronized in UART by keeping same Baud Rate.