Universal Asynchronous Transceiver (UART) is a Universal Serial Data Bus, hardware communication protocol using asynchronous serial communication in a configurable manner rate. In most embedded systems, microcontrollers and computers, UART plays an important communication role as a hardware communication protocol between devices to ensure the efficient transmission of communication data. The understanding and use of the UART protocol is crucial in our design process. In order to better understand. basic principle this protocol and realize the use of URAT, this paper introduces serial communication, UART frame format explanation, the working principle of UART, simulation of simple UART3. Depending on the source clock, also known as the Baud Clock, each heterogeneous device in a complete system may be able to create a distinct baud rate and connect with other devices over a UART interface. The fundamental tenet of the UART protocol stipulates that for proper communication, the transmitter and receiver must be set at the same baud rate. It also introduces several advanced designs and applications of UART with baud rate, namely UART with automatic frequency generator and frequency divider and Determination of UART Receiver Baud Rate Tolerance.