What Are UART, SPI and I2C? (Serial Communication Protocols)

What Is Serial Communication?

In serial communication, data is sent one bit at a time, in sequence, over a single line (serial vs. parallel). These protocols set the rules for how and how fast the bits are sent.

UART

UART is asynchronous: there's no shared clock line; the two sides talk at a pre-agreed speed (baud rate). It uses two lines: TX (transmit) and RX (receive). It's ideal for simple point-to-point communication between two devices.

SPI

SPI is synchronous and fast; it runs on a clock line (SCK). It usually uses 4 lines: MOSI, MISO, SCK and chip select (CS). It's preferred for high-speed devices like displays and memory.

I2C

I2C uses only 2 lines (SDA data, SCL clock) and gives each device an address. That way many devices can share the same two lines, saving pins.

Comparison

Which One to Choose?

UART for a simple two-device link; SPI for high speed; I2C to connect many devices with few pins. In all three, both sides must run at the same logic level (e.g. 3.3 V).

Common Mistakes

• Forgetting the common ground: if GND isn't shared between two devices, no protocol works.
• Baud mismatch in UART: if the two sides aren't at the same speed, the data comes through garbled.
• Missing pull-ups in I2C: the SDA and SCL lines need pull-up resistors, or communication can't start.