What Is UART?
UART (Universal Asynchronous Receiver/Transmitter) is the most common asynchronous serial communication interface, carrying data between two devices without a shared clock line. For a general comparison see the what are UART, SPI, I2C article.
How Does It Work?
It uses only two lines: TX (transmit) and RX (receive). There's no clock line; instead, the two sides agree on the same baud rate. Data is sent in a frame of a start bit, data bits and a stop bit.
Baud Rate
It's the number of bits transferred per second (e.g. 9600, 115200). The two devices' baud rates must match; if different, data is read corrupted. Since no clock is shared, this agreement is critical.
| Property | Value |
|---|---|
| Line count | 2 (TX, RX) |
| Clock | None (asynchronous) |
| Distance | Short (direct) |
Difference From SPI and I2C
SPI and I2C work with a shared clock (synchronous) and support many devices; UART is clockless and usually between two devices. Its simplicity is its biggest advantage. For noisier/longer-distance environments buses like CAN Bus are used, and between different voltage levels a level shifter.
Where Is It Used?
The serial port between a microcontroller and a computer, GPS/Bluetooth/GSM modules, a debug console and RS-232/RS-485 conversion.
Common Mistakes
- Baud mismatch: if the two sides are at different rates, data can't be read.
- TX-RX crossing: one device's TX must connect to the other's RX.
- Level matching: a level shifter may be needed between 3.3 V and 5 V devices.
Frequently Asked Questions
Is UART the same as RS-232?+
Can many devices be connected?+
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