What is the difference between UART and USART, and when would you use synchronous mode?

UART (Universal Asynchronous Receiver-Transmitter) supports only asynchronous communication — no clock signal is shared between devices, so both sides must independently agree on the baud rate. USART (Universal Synchronous/Asynchronous Receiver-Transmitter) supports both asynchronous mode (identical to UART) and a synchronous mode where the master outputs a clock signal on a dedicated pin.

In synchronous mode, the USART behaves somewhat like a simplified SPI: the master drives a clock, and data is sampled on the clock edge. This eliminates baud rate mismatch errors entirely, since the receiver locks to the transmitted clock. It also enables higher data rates because there is no oversampling overhead — each clock edge transfers one bit.

Synchronous mode is rarely used in practice because SPI is more widely supported and better standardized for the same use case. However, it is useful when: (1) you need a simple clocked serial link between two MCUs and have run out of SPI peripherals, (2) you want the framing features of UART (start/stop bits, parity) combined with clock synchronization, or (3) a specific peripheral (some smart card interfaces use synchronous UART) requires it. Most modern STM32 MCUs label their serial peripherals as USART, but the vast majority of applications use them in asynchronous (UART) mode.