What is the difference between UART, I2C, and SPI?

UART is asynchronous and point-to-point using TX/RX; I2C is a synchronous two-wire bus (SDA/SCL) that addresses many devices; SPI is a synchronous full-duplex bus (SCLK/MOSI/MISO/SS) that is the fastest but needs one chip-select line per device.

Which serial protocol is fastest?

SPI is the fastest, commonly running at tens of MHz, because it is full-duplex and uses dedicated lines with no addressing overhead. I2C is typically 100 kHz to 3.4 MHz, and UART usually runs at 9600 to 115200 baud.

Why does I2C need pull-up resistors?

I2C lines (SDA and SCL) are open-drain: devices can only pull the line low, so external pull-up resistors (typically 4.7 kΩ) return the line to a high logic level. Without them the bus cannot signal a logic 1.

How many devices can each protocol support?

UART connects exactly two devices. I2C addresses up to 127 devices on a shared two-wire bus using 7-bit addresses. SPI supports many devices but each needs its own slave-select (SS/CS) line from the master.

What are common UART baud rates?

Standard UART baud rates include 9600, 19200, 38400, 57600, and 115200 bits per second. 9600 and 115200 are the most widely used defaults; faster rates such as 230400 and 921600 are also supported by many devices.

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Reference

UART / I2C / SPI Serial Protocols

Side-by-side comparison of the three most common embedded serial protocols — signal lines, speed, topology, duplex mode, pros and cons, wiring, and UART baud rates.

Comparison

Wiring & connections

Common UART baud rates