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Deep dive into i2c protocol

Deep dive into i2c protocol

Deep dive into i2c protocol

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The I2C protocol, short for Inter-Integrated Circuit, is a ubiquitous yet powerful tool in the realm of embedded systems design.  Invented by Philips Semiconductors (now NXP) in 1982, it facilitates serial communication between integrated circuits (ICs) on a single board. This blog delves into the technical depths of I2C, exploring its core functionalities, communication mechanisms, and the intricacies that make it a workhorse for intra-board data exchange.

Introduction To I2C

I2C, short for Inter-Integrated Circuit, is a synchronous serial communication protocol designed for short-distance communication between integrated circuits (ICs) within a single device or on a printed circuit board (PCB).

Key Characteristics

Multi-Master Capable

I2C allows multiple masters (devices initiating communication) to share the bus. This enables efficient communication within a complex system with various ICs needing to exchange data.

Synchronous Communication

I2C utilizes a shared clock signal to synchronize data transmission between the master and slave (devices receiving data) devices. This synchronous nature ensures data arrives reliably at the intended recipient.

Two-Wire Interface

I2C requires only two wires for communication: Serial Data Line (SDA) and Serial Clock Line (SCL). This minimizes cabling complexity and simplifies PCB design.

I2C Communication Flow

Start Condition

The master initiates communication by generating a start condition on the SDA line (transition from high to low while SCL is high).

Device Addressing

The master transmits the slave address followed by a Read/Write bit, specifying the intended data transfer direction.

Data Transfer

Data bytes are then transmitted serially, with each bit followed by a clock pulse on the SCL line. The master or slave can acknowledge receipt by sending an acknowledgement bit.

Stop Condition

The master terminates communication by generating a stop condition (transition from low to high while SCL is high).

I2C Addressing and Bus Arbitration

Having grasped the core principles of I2C communication, let's delve deeper into some technical aspects-

Addressing Scheme

I2C employs a 7-bit or 10-bit addressing scheme to identify individual slaves on the bus. This allows for connection of up to 128 (7-bit) or 1024 (10-bit) slaves to a single I2C bus.

Bus Intervention

With multiple masters on the bus, a collision can occur if both attempt to initiate communication simultaneously. I2C incorporates a wired-AND arbitration mechanism to resolve such conflicts. The master transmitting the lowest address "wins" and regains control of the bus.

Data Rates

I2C operates at relatively slow speeds compared to other protocols like SPI (Serial Peripheral Interface Bus). Standard I2C supports data rates of up to 100kbps, with Fast I2C extending this to 400kbps and High-Speed I2C reaching speeds of 3.4Mbps.

Error Detection

I2C incorporates a basic error detection mechanism using Acknowledgement (ACK) and Not Acknowledgement (NACK) bits. After transmitting a data byte, the master waits for an ACK bit from the slave, signifying successful reception. If an error occurs, the slave transmits a NACK bit, alerting the master to resend the data.

Easy Implementation

Compared to some communication protocols, I2C boasts a relatively straightforward implementation. Its two-wire interface and well-defined communication protocol make it a user-friendly choice for embedded system developers.

I2C Bus- Ideal Applications

The I2C (Inter-Integrated Circuit) bus, with its unique blend of features, carves a niche in various embedded system applications. Unlike UART (Universal Asynchronous Receiver/Transmitter) which excels in point-to-point communication, I2C thrives in multi-device environments. Let's delve into the ideal applications where I2C shines and explore the technical considerations for successful implementation.

Sensor Networks

I2C's multi-master capability makes it perfect for connecting multiple sensors to a central microcontroller. This allows for efficient data acquisition from various sensors like temperature, pressure, or accelerometers, all on the same bus. The low pin count of I2C minimizes wiring complexity, a crucial factor when dealing with numerous sensors on a PCB.

Real-Time Clock (RTC) Modules

RTC modules, responsible for keeping track of time, often utilize I2C for communication. The synchronous nature of I2C ensures reliable time synchronization between the RTC and the main system controller.

Memory Expansion

I2C can be employed to connect external memory devices like EEPROMs (Electrically Erasable Programmable Read-Only Memory) or small SRAM chips (Static Random-Access Memory) to a microcontroller. This allows for data storage and retrieval beyond the on-chip memory limitations of the microcontroller.

LCD and Display Drivers

Many Liquid Crystal Display (LCD) and Organic Light-Emitting Diode (OLED) display drivers communicate with the main controller via I2C. The simplicity of I2C makes it suitable for transmitting control signals and display data without requiring a complex communication interface.

Battery Management Systems (BMS)

In battery-powered devices, I2C is a suitable choice for connecting battery monitoring and charging ICs to the main controller. The low power consumption of I2C aligns well with battery-conscious designs.

Debunking the I2C Data Packet

Data on the I2C bus is transferred in packets, each containing 8 or 10 bits depending on the addressing scheme used (7-bit or 10-bit). Each packet is structured as follows-

Data Byte

The first 8 bits of the packet hold the actual data being transferred.


The ninth bit serves as an acknowledgement mechanism. The addressed slave generates an ACK (low value) to indicate successful reception or a NACK (high value) if there's an issue.

This bit-by-bit transfer with acknowledgement ensures data integrity on the bus.

Modes of Operation

The I2C protocol offers different operating modes to cater to varying communication speed requirements-

Standard Mode (Std. Mode)

The most basic mode supports data transfer rates of up to 100 kbps. It's suitable for low-speed peripherals.

Fast Mode (Fast Mode)

Offers an increased data rate of up to 400 kbps, making it ideal for applications requiring faster communication.

High-Speed Mode (HS Mode)

This mode pushes the limits to 3.4 Mbps, catering to high-bandwidth needs.

There are even faster modes like Ultra-Fast Mode (UF Mode) reaching speeds of 5 Mbps, but their implementation is less widespread.

The specific mode supported by a device is determined by its capabilities and configuration settings.


The I2C protocol's simplicity, efficiency, and multi-device support have solidified its position as a cornerstone of embedded system design. Its ability to cater to varying communication speeds and offer features like clock stretching and error detection make it a versatile choice for connecting diverse peripherals to a microcontroller. As advancements continue in the embedded system domain, I2C is poised to remain a reliable and efficient workhorse for intra-board communication.

This deep dive has explored the intricacies of the I2C protocol, equipping you with a solid understanding of its core functionalities, communication mechanisms, and advanced features. This knowledge empowers you to effectively leverage I2C in your embedded system designs, enabling seamless communication between various integrated circuits on a single board.


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