Demystifying UART, SPI, and I2C: Communication Between Chips

In both VLSI design and embedded systems, no chip operates in isolation. Whether it’s a microcontroller interfacing with sensors, a processor communicating with memory modules, or multiple peripherals synchronizing data, inter-chip communication is fundamental to building reliable and scalable hardware systems.

As systems grow in complexity, so does the demand for structured, efficient, and purpose-fit communication protocols. Direct parallel connections are rarely feasible due to board space, pin limitations, and synchronization challenges. Instead, designers turn to well-established serial communication standards that enable devices to exchange data with minimal wiring and clear electrical and timing definitions.

Three of the most widely used protocols for such tasks are:

• UART (Universal Asynchronous Receiver/Transmitter)
• SPI (Serial Peripheral Interface)
• I²C (Inter-Integrated Circuit)

Each of these protocols is optimized for different trade-offs—speed, simplicity, scalability, and wiring complexity—making them suited for specific use cases in VLSI blocks, SoCs, microcontroller boards, and embedded devices.

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Brief Overview of the Protocols

UART – Universal Asynchronous Receiver/Transmitter

UART is a point-to-point, asynchronous serial protocol used to transmit data without needing a clock line. Instead, both devices agree on a predefined baud rate. Communication occurs over two lines: TX (transmit) and RX (receive).

It’s widely used for debugging, console communication, and bootloaders, especially when simplicity and human-readable interfaces (e.g., serial terminals) are priorities. Since it doesn’t support addressing or multiple devices, it’s typically used for direct, one-to-one communication.

SPI – Serial Peripheral Interface

SPI is a synchronous, full-duplex protocol that uses four wires in its standard configuration: MOSI, MISO, SCLK, and CS (Chip Select). Data is clocked simultaneously in both directions, allowing high-speed transfers.

SPI is ideal when speed and performance matter—common in display drivers, flash memory, ADCs/DACs, and high-throughput sensors. It supports multiple slaves but requires a separate chip-select line for each, which can limit scalability.

I²C – Inter-Integrated Circuit

I²C is a synchronous, half-duplex protocol that operates on just two wires: SDA (data) and SCL (clock). It allows for multi-master and multi-slave communication, where each device has a unique address.

I²C is favored for connecting multiple low-speed peripherals, such as sensors, EEPROMs, and configuration ICs, particularly when board space and wiring simplicity are critical. Its slower speeds and shared bus make it less ideal for high-volume data transfer but excellent for control applications.

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Protocol Comparison at a Glance

Feature	UART	SPI	I²C
Communication Type	Asynchronous, full-duplex	Synchronous, full-duplex	Synchronous, half-duplex
Wires Required	2 (TX, RX)	4 (MOSI, MISO, SCLK, CS)	2 (SDA, SCL)
Multi-Device Support	No	Yes (via multiple CS lines)	Yes (via addressing)
Speed	Moderate (depends on baud rate)	Very High (10s of MHz)	Low to Moderate (up to ~3.4 MHz)
Complexity	Low	Medium	Medium-High (due to addressing and arbitration)
Best For	Debugging, serial logging	High-speed sensors, memory	Low-speed sensors, config ICs
Flow Control	Optional (hardware/software)	Master controlled	Clock stretching by slave
Hardware Requirement	UART peripheral	SPI controller	I²C controller with open-drain pins

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Which One Should You Use?

The choice of protocol depends on system constraints:

• Use UART when you need simple, direct communication, especially for console output or single peripheral interfaces.
• Use SPI for high-speed communication with a small number of peripherals that demand fast response or large data transfer volumes.
• Use I²C when you need to connect multiple peripherals with minimal wiring and address-based access, especially in sensor arrays and configuration-heavy devices.

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In the sections that follow, we’ll explore each protocol in technical depth—from electrical characteristics and signal timing to addressing, framing, and typical use cases. The goal is to not just describe them, but to understand why each behaves the way it does, and how to integrate them correctly into real-world systems.

Stay tuned for the deep dives on UART, SPI, and I²C.

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