UART - Universal Asynchronous Receiver/Transmitter:

Communications:

Data flows from the Tx pin of the transmitting UART to the Rx pin of the receiving UART.

There is also a third ground reference wire.

Communication is asynchronous which means there is no universal clock information being transferred across a wire. The clock is internal to each microcontroller device.

UART communication doesn’t work if the transmitter and receiver have been configured for different data-transmission frequencies. The internal clock signals must be sufficiently accurate relative to the expected frequency and sufficiently stable over time and temperature. UART, in a sense, is robust, and simple, yet susceptible to large environmental issues.

UART Communication:

Transmission Speed (baud rate)

Data Length

Start and Stop bits (The UART signal is always high, thus the start bit is always low, and the end bit is high again)