How to Use I2C: Examples, Pinouts, and Specs

I2C (Inter-Integrated Circuit) is a multi-master, multi-slave, packet-switched, single-ended, serial communication bus. It is widely used in embedded systems to connect low-speed devices such as sensors, EEPROMs, real-time clocks, and microcontrollers. I2C is known for its simplicity and efficiency, requiring only two communication lines: a data line (SDA) and a clock line (SCL). This makes it ideal for applications where minimizing pin usage is critical.

• Communication between microcontrollers and peripheral devices (e.g., sensors, displays)
• Reading and writing data to EEPROMs or real-time clocks
• Interfacing with ADCs (Analog-to-Digital Converters) and DACs (Digital-to-Analog Converters)
• Connecting multiple devices on a shared bus in embedded systems

• Communication Type: Serial, synchronous
• Number of Wires: 2 (SDA - Serial Data, SCL - Serial Clock)
• Voltage Levels: Typically 3.3V or 5V (depending on the system)
• Speed Modes:
  • Standard Mode: Up to 100 kHz
  • Fast Mode: Up to 400 kHz
  • Fast Mode Plus: Up to 1 MHz
  • High-Speed Mode: Up to 3.4 MHz
• Addressing: 7-bit or 10-bit addressing
• Pull-Up Resistors: Required on both SDA and SCL lines (typical values: 4.7 kΩ or 10 kΩ)

I2C does not have a specific pinout since it is a protocol, but the two key lines are:

1. Connect the SDA and SCL Lines:
   • Connect the SDA and SCL pins of all devices on the bus.
   • Use pull-up resistors on both lines to ensure proper signal levels.
2. Assign Unique Addresses:
   • Each slave device must have a unique 7-bit or 10-bit address.
3. Configure the Master Device:
   • The master device (e.g., a microcontroller) initiates communication and controls the clock.
4. Send and Receive Data:
   • The master sends the slave address, followed by a read/write bit.
   • Data is transmitted in 8-bit packets, with an acknowledgment (ACK) after each byte.

• Pull-Up Resistors: Ensure the correct value of pull-up resistors is used to maintain signal integrity.
• Bus Speed: Match the speed mode (e.g., Standard, Fast) to the capabilities of all devices on the bus.
• Address Conflicts: Avoid address conflicts by verifying that all devices have unique addresses.
• Cable Length: Keep the bus length short to minimize signal degradation and noise.

Below is an example of interfacing an I2C temperature sensor (e.g., TMP102) with an Arduino UNO:

#include <Wire.h> // Include the Wire library for I2C communication

#define TMP102_ADDRESS 0x48 // I2C address of the TMP102 sensor

void setup() {
  Wire.begin(); // Initialize I2C communication
  Serial.begin(9600); // Start serial communication for debugging
}

void loop() {
  Wire.beginTransmission(TMP102_ADDRESS); // Start communication with TMP102
  Wire.write(0x00); // Point to the temperature register
  Wire.endTransmission(); // End transmission

  Wire.requestFrom(TMP102_ADDRESS, 2); // Request 2 bytes of data from TMP102
  if (Wire.available() == 2) { // Check if 2 bytes are available
    int msb = Wire.read(); // Read the most significant byte
    int lsb = Wire.read(); // Read the least significant byte
    float temperature = ((msb << 8) | lsb) >> 4; // Combine bytes and shift
    temperature *= 0.0625; // Convert to Celsius
    Serial.print("Temperature: ");
    Serial.print(temperature);
    Serial.println(" °C");
  }

  delay(1000); // Wait 1 second before the next reading
}

1. No Communication on the Bus:

   • Cause: Missing or incorrect pull-up resistors.
   • Solution: Verify that pull-up resistors (e.g., 4.7 kΩ) are connected to SDA and SCL.

2. Address Conflicts:

   • Cause: Two devices on the bus share the same address.
   • Solution: Check the datasheets of all devices and configure unique addresses.

3. Data Corruption:

   • Cause: Excessive noise or long bus length.
   • Solution: Shorten the bus length and use proper shielding.

4. Device Not Responding:

   • Cause: Incorrect wiring or wrong address.
   • Solution: Double-check the wiring and ensure the correct address is used in the code.

• Q: Can I connect multiple masters on the same I2C bus?

  • A: Yes, I2C supports multi-master configurations, but arbitration is required to avoid conflicts.

• Q: What happens if I don’t use pull-up resistors?

  • A: The SDA and SCL lines will not function correctly, as they rely on pull-up resistors to maintain high logic levels.

• Q: How do I determine the correct pull-up resistor value?

  • A: The value depends on the bus capacitance and speed. A typical value is 4.7 kΩ, but you can calculate it using the formula: R = tr / (Cbus * Vcc).

• Q: Can I use I2C with 3.3V and 5V devices on the same bus?

  • A: Yes, but you may need a level shifter to ensure proper voltage compatibility.

This documentation provides a comprehensive guide to understanding and using the I2C protocol effectively in your projects.