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Understanding the Differences Between UART and I2C
Jessica Hopkins

Communication protocols are some of the essential building blocks of an embedded system. They are what governs how data is sent over the bus, including whether data is sent serially, in parallel, asynchronously, or through a master/slave paradigm.

There are multiple forms of communication that can be used in an embedded system, including a communication protocol, such as I2C, or through a physical circuit like UART. Both are often used for similar objectives, but do have some notable similarities and differences. In this article, we’ll cover the basics of I2C and UART communication, and the differences between them.

Understanding the I2C Protocol

I2C, or Inter-Integrated Circuit, is a communication protocol often used in embedded systems as a way to transfer data between a master (or multiple masters) and a single slave (or multiple slaves) device. It is a bidirectional two-wire serial bus that uses serial clock (SCL) and serial data (SDA) wires to send and manage data between devices connected to the bus.

Because I2C operates using a serial clock, it considered to be synchronous, which allows the output of bits to be synchronized to the sampling of bits by a clock signal shared between the master and the slave.

I2C two wire bus layout.

I2C is used to connect devices like microcontrollers, EEPROMs, I/O interfaces, and other peripheral devices in an embedded system. A microcontroller is often used as the master device, and other peripheral devices are used as slave devices. Because all communication takes place on only two wires, all devices must have a unique address to identify it on the bus. By using the unique address, the master device is able to signal its read/write command to exchange communication between the two over the bus.

Where is I2C Used?

What applications use I2C? I2C is a suitable protocol for connecting short-distanced, low-speed peripherals on printed circuit boards (PCBs). Common I2C applications can include reading memory, reading hardware sensors, and accessing DACs and ADCs.

Understanding UART Communication

UART, or Universal Asynchronous Receiver/ Transmitter, is a physical circuit in a microcontroller or single integrated circuit (IC) that is used to implement serial communication between devices in an embedded system. It supports bidirectional data transmission, including half-duplex and full-duplex operations. Because it doesn’t include a clock signal to synchronize the output bits from the transmitting UART to the sampling bits on the receiving UART, UART is considered to be asynchronous. Without a clock, the receiving and transmitting UART need to be on the same baud rate, or bit rate. This allows the system to know where and when the bits have been clocked. Additionally, the transmitting UART adds a start and stop bit to the data being transmitted.

UART two-wire diagram UART device communication diagram

In UART communication, UART devices communicate directly with one another, where one device is the transmitter and the other is the receiver. UART uses a two-wire system where the Rx and Tx pins are used to transfer and receive data. A controlling device, such as a microcontroller, sends parallel data to the transmitting UART via a data bus. The transmitting UART then transmits the data into serial data using a shift register and sends it to the receiving device using the Tx pin. The receiving UART uses the Rx pin to receive the data and then converts the serial data back into parallel data using its shift register, which is then sent back on the bus to the receiving end.

Where is UART Used?

Primarily, UART was widely used for PC-device communication, allowing devices like a mouse and keyboard to communicate with the PC. While USB has replaced UART and standardized these connections, UART is still used in some applications today. Today, we see UART being used often in microcontroller-based devices including GPS receivers or Bluetooth modules.

What are the Similarities and Differences Between I2C and UART?

Both I2C and UART are similar in that their purpose is to transfer data serial data at low data rates between devices, and both use a two-wire interface to achieve this. Both are also commonly used for short/medium distanced data transmission. I2C, however, uses a master/slave configuration that uses clock signals to help synchronize the data being read or transmitted by the devices. UART, on the other hand, is hardware that is responsible for implementing asynchronous serial data streams for point to point connection and includes no clock signal.

So, what would be the advantage or disadvantage to using either?

Advantages and Disadvantages to Using I2C vs UART

While both I2C and UART offer similar objectives in data transmission, there are reasons why one might be best over the other. I2C is flexible and useful for connecting multiple devices. It allows users to integrate multiple master and slave devices – up to 128 devices on a single bus. I2C also has flow control and performs data validation, so the integrity of the data and how it was transferred is reliable. I2C is also generally faster than UART, and can reach speed of up to 3.4 MHz. Some of the disadvantages of I2C include its increasing circuit complexity with additional master/slave setups, and is only able to operate in half-duplex, meaning data can only be transmitted in one direction at a time.

UART is often used for its simplicity in hardware implementations. It doesn’t require a clock signal, and is ideal for systems that need to send data between devices without waiting for a master poll request and generating unwanted traffic on the bus. Some drawbacks are that UART doesn’t offer multiple master/slave support, which can limit how many devices are used on the bus. Additionally, each UART baud rate should be in 10% of each other or else data can be corrupted.

Debugging and Developing I2C Systems

Communication protocols, including the widely used I2C protocol, are an essential component to a system’s performance, and having bugs and transmission errors can cause erratic or erroneous system behavior. Without the right tools, finding bus and data errors can be challenging. That’s why using debugging and development tools are essential for an embedded system engineer’s toolbox.

Total Phase offers numerous tools to develop and debug I2C systems, including host adapters and protocol analyzers, including the Aardvark I2C/SPI Host Adapter, the Promira Serial Platform, and the Beagle I2C/SPI Protocol Analyzer.

The Aardvark I2C/SPI Host Adapter is a general-purpose host adapter supporting I2C and SPI protocols and can operate up to 800 kHz as an I2C master and slave. It is often used to test I2C systems by emulating master/slave configurations.

The Promira Serial Platform is Total Phase’s most advanced serial device. With its field upgradeable platform, users can configure the device with applications that allow users access to I2C and SPI protocols, various speeds, and other capabilities. Depending on the application and level, this device can support up to 3.4 MHz as an I2C master and slave.

The Beagle I2C/SPI Protocol Analyzer can non-intrusively monitor I2C and SPI traffic in real time. It can monitor I2C up to 4 MHz and offers flexible search and filtering capabilities.

Are you currently developing I2C systems or are considering a future I2C project? For any questions on how our I2C tools can resolve your project bottlenecks, we’re happy to help. Please email us at sales@totalphase.com to learn more.

Want to learn about the differences between UART and SPI? Check out our article, “Understanding the Difference Between UART vs SPI”.