Serial Communication Explained: UART, SPI, and I2C

Microcontrollers need to talk to sensors, displays, and other devices. They do this using serial communication protocols—standardised ways of sending data one bit at a time over wires. The three most common protocols in electronics projects are UART, SPI, and I2C.

Quick Comparison

| Feature | UART | SPI | I2C | |---------|------|-----|-----| | Wires needed | 2 (TX, RX) | 4 (MOSI, MISO, SCK, CS) | 2 (SDA, SCL) | | Speed | Up to 115200 baud (typical) | Up to 10+ MHz | Up to 400 kHz (standard) | | Devices | Point-to-point (1:1) | 1 master, many slaves | Many masters, many slaves | | Addressing | None needed | Chip Select pin per device | 7-bit address per device | | Best for | GPS, Bluetooth modules, Serial Monitor | Displays, SD cards, fast sensors | Sensors, EEPROMs, RTCs |

UART (Universal Asynchronous Receiver-Transmitter)

UART is the simplest protocol. Two devices exchange data over two wires: TX (transmit) and RX (receive). Cross-connect them: Device A's TX goes to Device B's RX, and vice versa.

Key Characteristics

• Asynchronous: No clock signal. Both devices must agree on the same baud rate (e.g., 9600, 115200).
• Point-to-point: Connects exactly two devices.
• Full duplex: Can send and receive simultaneously.

Common Uses

• Arduino Serial Monitor (USB-UART)
• GPS modules (NEO-6M outputs NMEA sentences via UART)
• Bluetooth modules (HC-05, HC-06)
• GSM modules (SIM800L)

SPI (Serial Peripheral Interface)

SPI is faster than UART and I2C. It uses a clock signal (synchronous) and separate data lines for each direction.

Wiring

• MOSI (Master Out Slave In): Data from master to slave
• MISO (Master In Slave Out): Data from slave to master
• SCK (Serial Clock): Clock signal from master
• CS/SS (Chip Select): One per slave device (LOW = selected)

Key Characteristics

• Synchronous: Clock signal ensures perfect timing.
• Fast: 1–10+ MHz typical. Great for displays and SD cards.
• Multiple slaves: Each needs its own CS pin. Uses more GPIO pins.

Common Uses

• TFT/OLED displays
• SD card modules
• RFID readers (MFRC522)
• High-speed sensors (accelerometers)

I2C (Inter-Integrated Circuit)

I2C uses only two wires for communication, regardless of how many devices are connected. Each device has a unique 7-bit address.

Wiring

• SDA (Serial Data): Bidirectional data line
• SCL (Serial Clock): Clock signal from master
• Both lines need pull-up resistors (4.7kΩ to VCC). Many breakout boards include these.

Key Characteristics

• Two-wire bus: Many devices share the same two wires.
• Addressed: Each device has a unique address (e.g., OLED at 0x3C, RTC at 0x68).
• Moderate speed: 100 kHz (standard), 400 kHz (fast mode).

Common Uses

• OLED displays (SSD1306)
• RTC modules (DS3231)
• Temperature sensors (BME280, BMP280)
• EEPROM chips
• I/O expanders (PCF8574)

Which Protocol Should You Use?

• Connecting to Serial Monitor or Bluetooth? → UART
• Need fast data transfer (display, SD card)? → SPI
• Many slow sensors on few pins? → I2C
• Not sure? → Check the sensor/module datasheet. It will specify which protocol it supports.

Browse our Sensor Project Kits to practice with all three protocols.