What are the differences between I2C and SPI protocols in Arduino?

Here’s a clear, technical comparison of I2C and SPI protocols for Arduino projects, including pros/cons and typical use cases:

---

1. Key Differences

---

2. Protocol Details

I2C

• Pins:

  • SDA (data) + SCL (clock) (+ pull-up resistors!)

  • Example: BMP280 (temperature sensor), OLED displays (SSD1306).

• Addressing:

  • 7-bit or 10-bit slave addresses (e.g., 0x68 for MPU6050).

• Data format:

  • Start bit → Slave address + R/W bit → Data → Stop bit.

SPI

• Pins:

  • MOSI (Master Out Slave In)

  • MISO (Master In Slave Out)

  • SCK (clock)

  • SS/CS (Slave Select, one pin per slave)

  • Example: SD cards, RF24L01 (radio modules).

• Data format:

  • Full-duplex (simultaneous send/receive).

  • No addressing – SS pin selects the slave.

---

3. Pros and Cons

---

4. Code Examples (Arduino)

I2C (Wire Library)

cpp

#include <Wire.h>  
void setup() {  
  Wire.begin(); // Master  
  Wire.beginTransmission(0x68); // Slave address (e.g., MPU6050)  
  Wire.write(0x6B); // Register  
  Wire.write(0);     // Data  
  Wire.endTransmission();  
}

SPI (SPI Library)

cpp

#include <SPI.h>  
void setup() {  
  SPI.begin();  
  digitalWrite(SS, LOW); // Activate slave  
  SPI.transfer(0x55);    // Send data  
  digitalWrite(SS, HIGH);  
}

---

5. When to Use Which?

• Use I2C when:

  • Pin count is limited.

  • Many sensors (e.g., temperature, pressure) are connected.

  • Speed is secondary (e.g., environmental sensors).

• Use SPI when:

  • High speed is needed (e.g., SD cards, displays).

  • Full-duplex communication is required.

  • Pin count isn’t an issue.

---

6. Typical Applications

---

Summary

• I2C: Simple, pin-efficient, but slower – ideal for sensors.

• SPI: Fast, robust, but pin-heavy – best for high-speed data.

Tip: I2C is often better for multi-sensor projects, while SPI excels in speed-critical applications (e.g., graphics).