Introduction

In 2024, communication protocols such as I2C, SPI and UART remain essential components of the tech world. Used for communication between various electronic devices, these protocols are the invisible heart that keeps our interconnected embedded systems and peripherals operating optimally.

In this article, we'll dissect these three protocols to provide you with a clear and practical understanding of their features, benefits, and limitations. Whether you're a developer, an electronics student, or simply curious about how devices communicate, this guide is designed for you.

We will begin by exploring the operating methods of each protocol, supported by explanatory GIFs for direct visualization. Next, we will compare I2C, SPI and UART from different angles: speed, ease of use, and typical use cases. Our objective ? Help you choose the protocol best suited to your current and future projects.

Let's dive into the practical world of bus protocols in 2024 together.

The I2C Protocol: Intuitive Communication

Basic principles and operating procedure

First, I2C (Inter-Integrated Circuit) is a synchronous and bidirectional serial communication protocol in half-duplex mode. Then, it allows the connection of multiple devices, masters or slaves, on a bus. In addition, communications are always made at the initiative of a master towards one or more slaves, without direct communication between masters or slaves. Finally, the protocol uses two lines: SDA (Serial Data Line) for data and SCL (Serial Clock Line) for the clock.

This simplified GIF illustrates the basic I2C process, showing how the master initiates communication with a slave using its address, followed by a command. For reasons of clarity, we omit some subtleties such as acknowledgment after each data transfer

How I2C works: Reading and Writing

The I2C protocol, a two-way communication system, is divided into two main operations: reading (from master to slave) and writing (from slave to master). Each operation follows distinct key steps to ensure smooth and accurate data transmission

Reading (Slave toward Master) :

1. START condition : Reading begins with a START condition initiated by the master. We achieve this by lowering the data line (SDA) low while keeping the clock line (SCL) high. This distinct signal tells all devices on the bus that the master is about to begin a new transmission.
2. Slave Address and Read Bit : The master then sends the address of the targeted slave on the bus. This address is followed by a control bit, set to 1 to indicate a read operation. Each bit is transmitted sequentially, with a change on the rising edge of each clock pulse.
3. Acknowledgment (ACK/NACK) : After receiving its address, the slave responds with an acknowledgment (ACK) bit if ready to communicate, pulling the SDA line low for one clock cycle. A non-acknowledgment (NACK) would be signaled by leaving the SDA line high.
4. Receipt of Data : The slave then begins to send the data to the master, byte by byte.
5. Acquittal by the Master : After receiving each byte, the master sends an acknowledgment (ACK) bit, pulling the SDA line low, to signal the slave to continue sending data. If the master does not wish to receive more data, it sends a non-acknowledgement (NACK) after the last byte received.
6. STOP condition : The communication ends with a STOP condition. The master generates this signal by switching the SDA line from low to high while the SCL line is high. This change indicates the end of the reading session and frees the bus for other communications

Writing (Maître toward Slave) :

1. START condition : Just like in the reading process, writing begins with a START condition initiated by the master. This signal is generated by pulling the data line (SDA) low, while the clock line (SCL) is held high, signaling the start of a transmission.
2. Slave Address and Write Bit : The master then transmits the address of the recipient slave, followed by a control bit set to 0 to indicate a write operation.
3. Acknowledgment (ACK/NACK) : The slave, after receiving and recognizing its address, sends an acknowledgment bit (ACK) by pulling the SDA line low for one clock cycle. A non-acknowledgment (NACK) would be indicated by an SDA line held high.
4. Sending Data by the Master : After receiving the ACK from the slave, the master starts sending the data, byte by byte.
5. Acknowledgment by the Slave : At the end of each byte transmitted, the slave acknowledges reception by sending an acknowledgment bit (ACK), thus confirming successful reception of the byte. If there is a problem, a NACK can be sent.
6. STOP/RESTART condition : The operation ends with a STOP condition, generated by the master by passing the SDA line from low to high while SCL is high, thus indicating the end of transmission and release of the bus. If the master wishes to continue with another write or read operation, it can generate a RESTART condition.

Thus, this distinction between read and write modes in the operation of I2C allows for efficient two-way communication between devices connected to the bus.

Strengths, weaknesses and use cases

Forces :

• Low pin consumption, using only two wires.
• Allows communication with multiple slaves.
• Relatively simple setup and use.

weaknesses :

• Limited speed compared to other protocols like SPI.
• Sensitivity to interference over long distances.
• Increasing complexity with increasing number of slaves.

Use cases : Concerning its applications, I2C shines in contexts requiring simple and economical communications in terms of connectivity. It particularly excels in integration with small sensors, LCD screens, and RTC (Real Time Clock) modules. Additionally, I2C is useful in temperature control devices, battery management systems, and LED controllers for its efficiency in compact circuits. However, in projects requiring rapid or long-distance data transfers, it would be better to opt for other protocols.”

SPI: High Speed ​​and Efficiency

SPI Operation and Key Features

SPI (Serial Peripheral Interface) stands out for its high speed, making it a preferred choice for fast communications. Unlike I2C, SPI works with four wires: MISO (Master In Slave Out), MOSI (Master Out Slave In), SCK (Serial Clock) and SS (Slave Select), allowing full-duplex communication (send and simultaneous reception). Despite its simplicity and speed, SPI requires more pins than I2C, which may be a factor to consider in circuit design.

The GIF shown here demonstrates typical SPI operation, showing how the master controls communication with the selected slave, while facilitating fast and direct data exchange.

Stages of SPI transmission :

1. Clock Signal Generation : The master starts the communication by generating a clock signal which synchronizes the data exchange.
2. Slave Selection : The master activates the desired slave by lowering the SS line to a low voltage level.
3. Data Exchange : The master begins by sending data to the slave through the MOSI line, bit by bit, often starting with the most significant bit. At the same time, the slave can also send data to the master through the MISO line, usually starting with the least significant bit.
4. Simultaneous Communication : Unlike a simple command-and-response model, SPI allows simultaneous two-way communication. As long as the SS line remains enabled, the master and slave can continue to exchange data at the same time, allowing efficient and fast data transmission in both directions.

Advantages, disadvantages and applications

Benefits :

• Fast and efficient data transfers.
• Full-duplex communication for simultaneous sending and receiving.
• Simplicity of design and implementation.

Drawbacks :

• Using multiple pins, which can be a problem in space limited designs.
• Less efficient for managing multiple slaves compared to I2C.
• Susceptible to interference at high speeds or over long distances.

Applications : SPI is ideal for situations requiring fast and reliable data transfers, such as TFT displays, SD memory cards, and wireless communication modules. However, its effectiveness decreases in complex systems with many slaves.

UART: The Universal Solution

Explanation of how the UART works

UART (Universal Asynchronous Receiver/Transmitter) is a serial communications protocol widely used for its versatility and simplicity. Unlike I2C and SPI, UART requires only two wires to operate: TX (transmit) and RX (receive). This protocol allows asynchronous communication, that is to say without a shared clock between the transmitter and the receiver. Data is organized into packets, with each packet containing a start bit, 5 to 9 data bits, an optional parity bit, and one or two trailing bits​.

Detailed operating mode of the UART

1. High State Transmission Line : Usually, the transmission line is kept at a high voltage level in the absence of data transmission​.
2. Starting the Transmission : To begin transmission, the transmitting UART pulls the transmission line up and down for one clock cycle. The UART receiver detects this voltage change and starts reading the bits at the baud rate frequency.
3. Baud Rate : The baud rate, which measures data transfer speed, should be approximately the same for both UARTs. The most common baud rates are 9600, 19200, 38400, 57600 and 115200 bits per second. The baud rate difference between the two UARTs must not exceed 10%​.
4. Data Frame : The frame contains the actual data transferred, usually sent with the least significant bit first​.
5. Parity Bit : The parity bit is used for error detection. It allows the receiving UART to determine if any changes occurred during transmission​.
6. End Bits : To signal the end of the data packet, the transmitting UART raises the transmission line high for at least two bit durations​.

Advantages, disadvantages and applications

Benefits :

• Simplicity with only two wires needed.
• Flexibility due to no need for a clock signal.
• Use of parity bit for error detection.

Drawbacks :

• Limiting data frame size to a maximum of 9 bits.
• Unsuitable for systems with multiple slaves or masters.
• Requirement of similar baud rate for reliable communication.

Applications :

UART is widely used for point-to-point communications in various fields:

• Connections between Microcontrollers and Peripherals : For simple and direct data exchange.
• GPS Modules and Serial Interfaces with Computers : For reliable, low-complexity communications.
• Industrial Machines : UART is commonly used in industrial equipment for robust communications.
• Use of RS Standards (such as RS-232, RS-485) : These standards enable UART communication over longer distances and offer the possibility of creating multi-slave networks using appropriate transceivers, thus increasing the flexibility and breadth of UART applications.

Comparative Analysis: I2C vs SPI vs UART

Comparison table: speed, complexity, utilization, and duplex

Characteristic	I2C	SPI	UART
Vitesse	Average (up to a few Mbit/s)	High (several Mbit/s to a few tens of Mbit/s)	Moderate (lower than that of SPI)
Complexity	Moderate (2 wires, management of several slaves)	Moderate to high (4-wire, full-duplex)	Moderate (2 wires, full-duplex)
Use	Ideal for short-distance communications with multiple devices	Perfect for fast data transfers	Suitable for simple and long distance serial communications
Duplex	Half-duplex (two-way communication but not simultaneous)	Full-duplex (simultaneous two-way communication)	Full-duplex (simultaneous two-way communication)

Tips for choosing the right protocol for your project

1. Communication speed : SPI for speed, UART for flexibility, and I2C for less speed-demanding configurations.
2. Circuit design : I2C for efficient space management with multiple devices, SPI for performance in larger designs, and UART for simplicity and versatility.
3. Distance and communication environment : UART is robust over long distances, while I2C is better suited to short distances.
4. Duplex Requirements : SPI and UART provide full-duplex capabilities, while I2C is limited to half-duplex.

By considering these elements, you will be better equipped to choose the protocol that will best meet your project requirements.

Conclusion

Through this article, we have explored the subtleties and specificities of the three major communication protocols: I2C, SPI and UART. Each of them features a unique balance of speed, complexity, usability and duplex capabilities, providing varied solutions for communication challenges in electronics projects.

• I2C stands out for its simplicity and ability to manage multiple slaves with a minimum of pins, making it ideal for short distance configurations.
• The SPI, with its high speed and full-duplex mode, is perfect for fast and efficient data transfers in systems where space is not a major issue.
• UART, versatile and robust, shines in long-distance communications and less speed-demanding configurations.

By understanding the strengths and limitations of each protocol, you can choose the one that best fits your project requirements. We strongly encourage you to experiment with these protocols and expand your knowledge. Practice and exploration are the keys to mastering these essential tools of electronic communication.

Remember, learning and innovation go hand in hand. In 2024, with constant technological advances, mastering these protocols opens up a world of possibilities in the field of electronics and embedded computing. So, get started, experiment and discover the infinite potential of communication between devices!

Additional Resources

In addition to the information above, here is an updated list of resources in English and French on I2C, SPI and UART protocols, offering a mix of perspectives and in-depth dives:

1. Article on element14 Community :
   • Title: “An Introduction to the UART, SPI, and I2C Communications Protocols”
   • Description: A basic article for understanding UART, SPI and I2C protocols.
   • Link : Visit article .
2. Blog on Seeed Studio (in English) :
   • Title: “UART vs I2C vs SPI – Communication Protocols and Uses”
   • Description: A blog that explores the differences between UART, I2C, and SPI, and their uses.
   • Link : Visit the blog .
3. Article on Moussasoft (in French) :
   • Title: “SPI, I2C and UART communication protocols”
   • Description: An introduction to SPI, I2C and UART protocols, in French.
   • Link : Read the article .
4. Article on Passion Électronique (in French) :
   • Title: “[Intro] Serial communication protocols: UART, I2C and SPI”
   • Description: An introductory guide to serial communication protocols.
   • Link : Discover the article .
5. Courses on OpenClassrooms (in French) :
   • Title: “Communicate serially – Develop in C for the embedded”
   • Description: A detailed online course on serial communication, including UART, SPI and I2C.
   • Link : Access the course .

In addition to external sources, explore our internal articles for a deeper understanding

1. Embedded Systems Fundamentals: A Beginner's Guide
   • Read the article
   • Learn the basics of embedded systems and their importance in modern technology.
2. Embedded Artificial Intelligence: The Evolution of Electronic Systems
   • Read the article
   • Explore how artificial intelligence is transforming embedded electronic systems.