Quick Answer
Choose SPI for high-speed data transfer and multiple devices, I2C for simple two-wire interface with many devices sharing the bus, and UART for point-to-point communication or legacy sensors. SPI offers highest speed (up to 50 MHz on STM32), I2C provides easiest wiring (2 wires + ground), and UART is best for asynchronous communication.
Introduction
Interfacing external sensors with STM32 microcontrollers requires selecting the right communication protocol. Each protocol—SPI, I2C, and UART—has distinct advantages, limitations, and ideal use cases.
Core Content
1. Protocol Comparison
| Feature | SPI | I2C | UART |
|---|---|---|---|
| Speed | Up to 50 MHz | Up to 3.4 MHz | Up to 4 Mbps |
| Wires | 4+ (MOSI, MISO, SCK, CS) | 2 (SDA, SCL) | 2 (TX, RX) |
| Topology | Master-slave, multi-slave | Multi-master, multi-slave | Point-to-point |
| Addressing | Chip select per device | 7-bit/10-bit address | None |
| Full Duplex | Yes | No (half duplex) | Yes |
2. When to Use SPI
- High-speed data transfer: SD cards, displays, flash memory
- Full-duplex communication: Simultaneous send and receive
- Multiple devices: Each device has its own chip select line
- Low overhead: No addressing overhead per transfer
3. When to Use I2C
- Limited pins: Only 2 wires needed regardless of device count
- Multiple sensors: Easy to add devices to the bus
- Low-speed sensors: Temperature, humidity, accelerometers
- Board space constraints: Minimal wiring required
4. When to Use UART
- Point-to-point communication: GPS modules, Bluetooth modules
- Legacy sensors: Older devices with UART interface
- Debug output: Simple serial console for debugging
- Long-distance communication: RS232, RS485 interfaces
FAQ
Which protocol is best for battery-powered sensors?
I2C is often preferred for battery-powered sensors because it requires fewer pins (saving power on GPIO) and supports low-speed communication which consumes less power. However, SPI can be more power-efficient at high speeds due to faster data transfer.
Can I use multiple protocols simultaneously?
Yes! STM32 has multiple SPI, I2C, and UART peripherals. You can use all three protocols simultaneously, connecting different sensors via the most appropriate interface for each.
Conclusion
Selecting the right protocol for sensor interfacing:
- SPI: High speed, full duplex, multiple devices with separate CS
- I2C: Simple wiring, multiple devices, low-speed sensors
- UART: Point-to-point, legacy devices, debug interface
Need Help with Sensor Interfacing?
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