Asynchronous communication is provided by the Universal Asynchronous Receiver-Transmitter (UART). These kind of interfaces, commonly simply known as serial ports, are called asynchronous because they do not need to share a clock signal to synchronize the sender and the receiver, but rather work on pre-defined clock rates that can be aligned while the communication is ongoing. Microcontrollers may contain multiple UARTs that can be attached to a specific set of pins upon request. Asynchronous communication is provided by UART as a full-duplex channel, through two independent wires, connecting the RX pin of each endpoint to the TX pin on the opposite side.

To properly understand each other, the systems at the two endpoints must set up the UART using the same parameters. This includes the framing of the bytes on the wire, and the frame rate. All of these parameters have to be known in advance by both endpoints in order to correctly establish a communication channel. Despite being simpler than the other types of serial communication, UART-based serial communication is still widely used in electronic devices, particularly as an interface toward modems and GPS receivers. Furthermore, using TTL-to-USB serial converters, it is easy to connect a UART to a console on the host machine, which is often handy for providing log messages.