How Does UART Transmit a Byte?

Basic
Article
Ages 14+
4 min read
Updated November 1, 2026

A UART is the circuit block that carries out asynchronous serial communication: data is sent without an external clock signal. Each data word, often a byte, is packed into a frame of bits, and the two devices must agree on the baud rate. This is how many sensors and modules talk to a microcontroller.

Contents

Two UART signal lines join the devices, one for each direction. The figure also shows a shared ground reference; check compatible logic levels.

First device
Second device
Shared ground reference

What you will learn

Connect the TX and RX pins correctly between two devices.
Describe the parts of a UART frame.
Explain what the baud rate is.

Before you start

It helps to know what GPIO is and what high and low signal levels are.

How does UART connect two devices?

In a typical bidirectional UART connection, each device uses a receiver pin, RX, and a transmitter pin, TX. A universal asynchronous receiver/transmitter (UART) is a block of circuitry responsible for implementing serial communication.

The RX and TX labels are with respect to the device itself. So the RX of one device should go to the TX of the other, and vice versa. For a direct logic-level connection, the devices need a common ground reference and compatible voltage levels; confirm these requirements before connecting. Asynchronous means that data is transferred without support from an external clock signal.

One device's transmit pin goes to the other's receive pin. The figure also shows a shared ground reference; check compatible logic levels.

First device
Second device
Line from TX to RX
Line from RX to TX
Shared ground reference

How is a byte framed?

Each block of data, usually a byte, is sent in a packet or frame of bits. A frame has synchronization bits around the data; some configurations also add an optional parity bit.

A frame has one start bit; common configurations use one or two stop bits.

A start bit, the data bits, then a stop bit return the line to idle.

Start bit
Data bits
Stop bit

Parity is a form of very simple, low-level error checking. It is optional.

What is the baud rate?

The baud rate specifies how fast data is sent over a serial line. Inverting the baud rate gives the time it takes to transmit a single bit.

The higher a baud rate goes, the faster data is sent and received, but there are limits to how fast data can be transferred.

The higher the baud rate, the shorter each bit.

Low baud, long bits
High baud, short bits
Long time per bit
Short time per bit

Real-world example

With an adult, look at a sensor module with four pins. Two of them are often TX and RX. When joined to a microcontroller, the module's TX goes to the microcontroller's RX.

Common misunderstandings

Joining TX to TX is correct. The RX of one device should go to the TX of the other, and vice versa.

Two devices know each other's speed on their own. There is no external clock, so both sides have to agree on the baud rate, and on the exact same protocol settings.

Any number of devices can share one TX line. In basic point-to-point UART wiring, multiple devices transmitting on one TX line can cause bus contention.

A logic-level module can plug straight into any serial port. A TTL serial device cannot be interfaced directly with an RS-232 bus; the signals have to be shifted.

A UART carries out asynchronous serial communication over RX and TX signal lines, crossed between two devices.
Each data word, often a byte, is sent in a frame with a start bit, the data, stop bits and possibly a parity bit.
The baud rate says how fast data is sent, and its inverse is the time for one bit.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

Universal asynchronous receiver/transmitterUART: A circuit block that implements asynchronous serial communication.
TX and RX pinsChân TX và RX: The transmit (TX) and receive (RX) pins of a serial device.
AsynchronousKhông đồng bộ: Transferring data without an external clock signal.
FrameKhung dữ liệu: A packet of bits with data and synchronization bits; some configurations add an optional parity bit.
Baud rateTốc độ baud: A value that tells how fast data is sent over a serial line.
Parity bitBit chẵn lẻ: An optional bit added to a frame for very simple low-level error checking.

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