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Deep Dive into UART protocol

Deep Dive into UART protocol

Deep Dive into UART protocol 

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The Universal Asynchronous Receiver/Transmitter (UART) protocol is a fundamental building block for serial communication in electronics.  It's a robust and straightforward method for transferring data between devices, and its simplicity makes it a popular choice for microcontrollers and embedded systems. This blog delves into the technical depths of UART, exploring its inner workings,  strengths, and applications.

UART Communication

UART communication stands in contrast to parallel communication, where data bits are transmitted simultaneously. In contrast, UART utilizes a serial approach, sending data one bit at a time over a single wire (simplex), two wires (half-duplex), or four wires (full-duplex). This minimizes the number of connections needed, making it ideal for resource-constrained environments.

The "asynchronous" aspect of UART refers to the absence of a dedicated clock signal to synchronize the data transfer.  Each device relies on its internal clock and configuration to maintain the same baud rate (data transmission speed) for successful communication.

Breaking Down the UART Frame

Data in UART communication is packaged into frames for efficient transmission. Each frame comprises several crucial elements-

Start Bit

This single bit, always a logical 0 (low voltage level), signals the beginning of a data transmission. It acts as a wake-up call for the receiving device, alerting it to prepare for incoming data.

Data Bits

These bits carry the actual information being transmitted.  The number of data bits per frame can vary depending on the configuration, typically ranging from 5 to 8 bits.  For instance, a 7-bit frame could transmit 128 unique characters (2^7). First to be sent is the least significant bit (LSB), then the most significant bit (MSB).

Parity Bit

This bit is used for rudimentary error detection.  By calculating the total number of 1s in the frame (including data bits), the parity bit can be set to either achieve even parity (an even number of 1s) or odd parity (an odd number of 1s).  The receiving device performs the same calculation and compares it to the received parity bit.  A mismatch indicates a potential transmission error.

Stop Bit(s)

These bits, always logical 1s (high voltage level), mark the end of the data transmission.  The number of stop bits can be 1, 1.5, or 2, depending on the configuration.  They allow the receiver to synchronize its internal clock and prepare for the next incoming frame.

A Guide to Configuring UART

The Universal Asynchronous Receiver/ Transmitter (UART) protocol serves as a workhorse for serial communication in embedded systems. While conceptually simple, effective UART communication hinges on proper configuration. This guide delves into the technical depths of UART configuration, empowering you to establish reliable data exchange between devices.

Baud Rate Generator

Microcontroller UART modules typically integrate a baud rate generator (BRG) circuit that produces the clock signal for data transmission based on the configured baud rate.  Consulting the microcontroller's datasheet is vital for understanding the BRG configuration registers.

Clock Source

The BRG requires a clock source to function. This clock can be derived from the microcontroller's internal or external crystal oscillator. The chosen clock source frequency and the BRG configuration determine the achieved baud rate.

Interrupt-Driven Communication

UART communication can be implemented using polling or interrupts. Polling involves continuously checking the UART status register for incoming data, while interrupts notify the microcontroller when data is received or transmission is complete. Interrupts improve efficiency by allowing the microcontroller to attend to other tasks while waiting for UART events.

Introduction to the Configuration Landscape

UART configuration encompasses several crucial parameters that must be identical on both transmitting and receiving devices to ensure successful communication. These parameters can typically be configured through software registers within the microcontroller's integrated UART module.

Number of Data Bits

This defines the number of bits used to represent each data character, typically ranging from 5 to 8. A common choice is 8 data bits (8N1), which offers a balance between data capacity and efficiency.


An optional error-checking mechanism. You can choose-

  • No Parity (Even or Odd)- Disables parity checking.
  • Even Parity- Adds an extra bit to ensure an even number of 1s in the frame (including data bits).
  • Odd Parity- Adds an extra bit to ensure an odd number of 1s in the frame.

Stop Bits

These bits signal the end of a data transmission. You can select

  • 1 Stop Bit- Most common configuration for shorter distances.
  • 1.5 Stop Bits- Offers a trade-off between noise immunity and efficiency.
  • 2 Stop Bits- Enhances noise immunity but reduces transmission speed.

Baud Rate  

A critical parameter determining the data transmission speed, measured in bits per second (bps).  Common baud rates include 9600 bps, 57600 bps, and 115200 bps.  Both communicating devices must be configured with the same baud rate to avoid data corruption.

Integrating UART in Microcontroller Projects

The Universal Asynchronous Receiver/Transmitter (UART) protocol reigns supreme for serial communication in microcontrollers. Its simplicity and efficiency make it ideal for transmitting and receiving data between microcontrollers and peripherals. This guide delves into the practicalities of integrating UART in your microcontroller projects, equipping you with the technical knowledge to establish robust communication channels.

Unveiling the Hardware

Most microcontrollers incorporate a dedicated UART module, eliminating the need for external components. This module handles the nitty-gritty of serial communication, including-

Serial-to-Parallel Conversion

Converts parallel data received from the microcontroller's processor into a serial bit stream for transmission.


Adds start, data, parity (if enabled), and stop bits to the data stream according to the configured parameters.

Bit Timing

Ensures accurate transmission and reception by controlling the timing of each bit based on the baud rate.

Error Detection

Optionally utilizes parity checking to identify potential transmission errors.

Parallel-to-Serial Conversion (Receiving)

Reconstructs the original parallel data from the received serial bit stream.

Pin Connections

The UART module typically exposes a set of pins for interfacing with external devices.  These pins commonly include-

TX (Transmit)

Carries the data stream to be transmitted to another device.

RX (Receive)

Receives the incoming serial data stream from another device.

GND (Ground)

Provides a common ground reference for electrical signals.

Software Integration

While the UART module handles the low-level communication tasks, software running on the microcontroller interacts with the module to configure communication parameters, send/receive data, and manage errors.  This software interaction typically involves-

UART Library

Most microcontroller development platforms provide a pre-written UART library that simplifies communication. The library offers functions for

  • Initializing the UART module with the desired baud rate, data format (number of data bits, parity, stop bits), and interrupt configuration.
  • Sending data bytes or strings through the TX pin.
  • Receiving data bytes or strings from the RX pin.
  • Checking for errors like framing errors or overrun/underrun conditions (buffer overflow/underflow).

Interrupt-Driven Communication

UART communication can be implemented using polling or interrupts. Polling involves continuously checking the UART status register for incoming data, while interrupts notify the microcontroller when data is received or transmission is complete. Interrupts are generally preferred for efficiency, allowing the microcontroller to attend to other tasks while waiting for UART events.

UART's Role in Robot Communication

Robots often comprise numerous components- sensors, actuators, motors, and external devices- that need to exchange data for coordinated operation. UART facilitates communication between these elements in several ways-

Sensor Data Transmission

UART enables robots to transmit sensor data (e.g., temperature, distance, light) from sensors like LiDARs, encoders, and cameras to the main microcontroller for processing and decision-making.

Motor Control

UART communication can be used to send control commands from the main microcontroller to motor controllers, dictating motor speed, direction, and position for precise robot movement.

Serial Communication with External Devices

Robots often interact with external devices like computers, tablets, or human-machine interfaces (HMIs) for control, data visualisation, or user interaction. UART provides a reliable channel for serial communication between these external devices and the robot's microcontroller.

Debugging and Programming

UART serves as a vital debugging and programming tool.  Microcontroller development environments often utilize UART connections to upload firmware to the robot, monitor system logs, and troubleshoot communication issues.


The Universal Asynchronous Receiver/Transmitter (UART) protocol has carved a lasting legacy in the realm of serial communication. Its simplicity, efficiency, and minimal resource requirements make it a ubiquitous choice for microcontrollers across diverse applications, including the ever-evolving field of robotics.

While I2C and SPI offer advantages in specific scenarios, UART's asynchronous nature and ability to function over longer distances solidify its position as a cornerstone for basic and essential communication needs. Understanding the technical aspects of UART configuration, error handling, and advanced features empowers engineers and roboticists to leverage its capabilities effectively. As the world of embedded systems continues to evolve, UART's enduring relevance ensures its continued role as a fundamental building block for reliable and efficient data exchange.

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