I2C, SPI, and UART: Choosing the Right Communication Protocol

Every embedded system needs communication between components. The three workhorses are I2C, SPI, and UART. Understanding their trade-offs is essential for designing reliable hardware.

Quick Comparison

| Feature | UART | I2C | SPI | |---------|------|-----|-----| | Wires | 2 (TX, RX) | 2 (SDA, SCL) | 4+ (MOSI, MISO, SCK, CS) | | Speed | Up to 1 Mbps | Up to 3.4 Mbps | Up to 80 Mbps | | Devices | 2 (point-to-point) | 128 (addressed) | Unlimited (CS per device) | | Duplex | Full | Half | Full | | Complexity | Low | Medium | Medium | | Distance | Long (with RS-485) | Short (<1m) | Short (<30cm) |

When to Use UART

Best for: Debug output, GPS modules, Bluetooth modules, PC communication.

// STM32 UART transmit
HAL_UART_Transmit(&huart2, (uint8_t*)"Hello\r\n", 7, 100);

// Receive with interrupt
HAL_UART_Receive_IT(&huart2, &rx_byte, 1);

Pros: Simple, no clock line needed, long distance possible. Cons: Only point-to-point, baud rate must match on both sides.

When to Use I2C

Best for: Temperature sensors, EEPROMs, displays, accelerometers — anything on the same PCB with moderate data rates.

// Read temperature from TMP102 sensor
uint8_t data[2];
HAL_I2C_Mem_Read(&hi2c1, 0x48 << 1, 0x00, 1, data, 2, 100);
float temp = ((data[0] << 4) | (data[1] >> 4)) * 0.0625;

Pros: Only 2 wires, multi-device bus, standardized addressing. Cons: Slower than SPI, pull-up resistors needed, half-duplex.

When to Use SPI

Best for: SD cards, high-speed ADCs, displays, flash memory — anything requiring fast data transfer.

// Read accelerometer data via SPI
uint8_t tx = 0x80 | WHO_AM_I_REG;  // Read bit + register
uint8_t rx;
HAL_GPIO_WritePin(CS_GPIO_Port, CS_Pin, GPIO_PIN_RESET);
HAL_SPI_TransmitReceive(&hspi1, &tx, &rx, 1, 100);
HAL_GPIO_WritePin(CS_GPIO_Port, CS_Pin, GPIO_PIN_SET);

Pros: Fastest of the three, full-duplex, no addressing overhead. Cons: More wires (one CS per device), no built-in error checking.

Decision Flowchart

1. Need to talk to a PC or debug? → UART
2. Multiple slow sensors on one bus? → I2C
3. High-speed data transfer? → SPI
4. Long distance (>1m)? → UART over RS-485
5. Only 2 pins available? → I2C

Debugging Tips

• I2C not working? Check pull-up resistors (4.7kΩ to 3.3V), verify addresses with a bus scanner
• SPI garbled data? Check clock polarity (CPOL) and phase (CPHA) settings
• UART gibberish? Baud rate mismatch — use an oscilloscope to measure bit timing

Practice all three protocols hands-on in our Embedded Systems Engineering track.