Why Do Embedded Developers Prefer UART Over SPI, I2C, or USRT for Debugging?

UART is the dominant debugging interface for embedded systems due to its simplicity and universality. It requires only three wires—TX, RX, and GND—and operates asynchronously, so no clock line is needed, allowing a quick five‑minute setup.

Developers can instantly view logs with printf, monitor panic messages, and interact with the system using terminal programs such as minicom or putty, making debugging straightforward and effective.

SPI, by contrast, is intended for high‑speed peripheral connections. It follows a master‑slave model, needs at least four wires (MOSI, MISO, SCK, CS), and lacks a unified protocol, which makes using it for human‑readable output cumbersome and error‑prone.

I2C also falls short because it is a synchronous bus that requires a clock line, offers limited speeds (100 kbps standard, 400 kbps fast), and introduces address conflicts and bus arbitration, while also demanding additional hardware adapters for PC connectivity.

UART benefits from ubiquitous driver support: virtually every PC can communicate via a cheap USB‑to‑TTL adapter, and operating systems (Linux, Windows, macOS) provide native serial console tools, delivering a true "plug‑and‑play" experience.

The historical inertia of serial consoles reinforces this choice. Since early Unix systems, UART has been the standard for bootloader (U‑Boot) and kernel messages, with tools and documentation built around it, making a switch to other buses costly in terms of driver development and tooling.

In real‑world projects, such as automotive ECUs that host CAN, LIN, SPI, I2C, and UART, engineers still rely on UART for debugging because the other interfaces serve dedicated functional roles.

While SPI or I2C can be repurposed for debugging in special scenarios—e.g., when UART pins are occupied or ultra‑high‑speed data is needed—they are exceptions rather than the rule.

Ultimately, the principle "use the simplest solution that works" leads developers to choose UART for everyday embedded debugging.