Acronym · Interfaces & SerDes
UART (Universal Asynchronous Receiver Transmitter)
An asynchronous serial interface sending framed characters with start and stop bits at an agreed baud rate, needing no shared clock. It remains the default debug console for embedded systems.
- Also known as
- Serial port
- Type
- Acronym
- Domain
- Interfaces & SerDes, Peripheral and embedded bus
Notes
Both ends must be configured for the same baud rate and framing, which is why a garbled console usually means a clock mismatch.
Overview
A UART (universal asynchronous receiver-transmitter) is a hardware block that sends and receives serial data one character at a time over a single wire in each direction, without a shared clock, by framing every character with start and stop bits at an agreed bit rate. It is one of the oldest and simplest peripherals on microcontrollers and SoCs. Its most familiar job today is the debug console that engineers use to talk to a board during bring-up.
How it works
The line idles at the high level. To send a character, the transmitter pulls the line low for one bit time (the start bit), then shifts out typically five to eight data bits, least significant bit first, an optional parity bit, and one or more stop bits at the high level. The receiver detects the falling edge of the start bit and samples each following bit near its centre. Because there is no clock wire, both ends must be set to the same baud rate, data length, parity and stop bits, such as the common 115200 8N1, and their clocks must be close enough that sampling stays within each bit until the stop bit.
Inside a typical UART, a programmable divider produces an internal clock at a multiple of the baud rate, often 16 times, and the receiver samples mid-bit using it. Texas Instruments' KeyStone UART, for example, follows the industry-standard 16550 design, divides its input clock by a 16-bit value, offers 16 times or 13 times oversampling and has 16-byte transmit and receive FIFOs. Optional hardware flow control uses request-to-send and clear-to-send lines, and interrupts or DMA move data to and from memory.
The UART defines only the logic-level framing. Electrical signalling is set by a separate transceiver or level, such as raw logic levels on a board, RS-232 on older serial ports or RS-485. Throughput is modest, but the simplicity makes UARTs easy to use and debug.
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