USARTs

A USART, or Universal Synchronous/Asynchronous Receiver/Transmitter, is a serial communication peripheral used in many microcontrollers. It can operate in asynchronous mode, behaving like a traditional UART, or in synchronous mode, where a clock is used to coordinate data transfer.

In asynchronous mode, data is framed with a start bit, a configurable number of data bits (commonly 8), optional parity, and one or more stop bits. The baud rate is generated by a divider tied to a clock source, requiring the transmitter and receiver to run at the same rate. The peripheral typically provides a transmit and a receive data register, status flags, and may generate interrupts on data arrival, data register empty, or error conditions. Some devices include programmable data length and parity settings.

In synchronous mode, a shared clock coordinates sampling and shifting of data, enabling higher throughput and removing or reducing start/stop overhead. Synchronous operation is used for protocols that provide their own timing or a master clock. Some USARTs can operate in half-duplex or full-duplex configurations and may support mode switching at runtime.

Common features and considerations include hardware flow control lines (such as RTS/CTS) on some devices, parity options, error detection, and optional data buffering or FIFO. In practice, USARTs connect to external interfaces through level translators or transceivers (for example RS-232 or RS-485) to match line voltages. They are widely used for debugging consoles, PC communication, and embedded sensor networks.

Variants and naming conventions vary by vendor; many microcontrollers refer to the peripheral as EUSART, LPUART, or simply UART/USART. The UART is the asynchronous subset of the USART.