What is UART and how does it differ from SPI and I2C?

UART (Universal Asynchronous Receiver-Transmitter) is an asynchronous, full-duplex serial protocol that uses two data wires — TX and RX — with no shared clock. Both endpoints must independently agree on the same baud rate before communication begins. This is fundamentally different from SPI and I2C, which are synchronous protocols where the master supplies a clock signal that slaves use for timing.

UART is strictly point-to-point: one transmitter talks to one receiver on each wire. There is no concept of addressing, bus arbitration, or multi-device sharing — if you need to talk to three peripherals, you need three separate UART ports (or a mux). SPI and I2C both support multiple devices on a shared bus. The trade-off is simplicity: UART requires only two wires (plus ground), has no clock skew issues, and is the easiest serial protocol to debug with an oscilloscope or logic analyzer because the frame format is self-contained.

UART is also the only one of the three that works over long distances when paired with a differential physical layer (RS-232, RS-485). SPI and I2C are limited to on-board or short-cable distances because their single-ended signals degrade with length.