How to Use I2C: Examples, Pinouts, and Specs

I2C (Inter-Integrated Circuit) is a serial communication protocol developed by Inter Integrated Circuit. It is designed for short-distance communication between devices on a single bus. I2C supports multiple masters and multiple slaves, making it highly versatile for embedded systems. The protocol uses only two wires: a data line (SDA) and a clock line (SCL), which simplifies circuit design and reduces pin usage.

• Communication with sensors (e.g., temperature, pressure, and accelerometers)
• Interfacing with EEPROMs and real-time clocks
• Connecting microcontrollers and peripheral devices
• Display modules like OLEDs and LCDs
• Embedded systems requiring low-speed, short-distance communication

• Manufacturer: Inter Integrated Circuit
• Part ID: i2c
• Communication Type: Serial, synchronous
• Number of Wires: 2 (SDA - Serial Data, SCL - Serial Clock)
• Data Transfer Rate:
  • Standard Mode: Up to 100 kbit/s
  • Fast Mode: Up to 400 kbit/s
  • Fast Mode Plus: Up to 1 Mbit/s
  • High-Speed Mode: Up to 3.4 Mbit/s
• Voltage Levels: Typically 3.3V or 5V (depending on the system)
• Addressing: 7-bit or 10-bit addressing modes
• Maximum Devices on Bus: Limited by bus capacitance (typically up to 127 devices with 7-bit addressing)

The I2C protocol does not have a specific physical pinout since it is implemented in microcontrollers and devices. However, the two key lines are:

1. Connect the SDA and SCL Lines:

   • Connect the SDA line of the master device to the SDA line of the slave device(s).
   • Similarly, connect the SCL line of the master to the SCL line of the slave(s).

2. Pull-Up Resistors:

   • Add pull-up resistors (typically 4.7kΩ or 10kΩ) to both the SDA and SCL lines. These resistors are necessary to ensure proper signal levels on the bus.

3. Power Supply:

   • Ensure all devices on the I2C bus operate at the same voltage level (e.g., 3.3V or 5V). Use level shifters if devices operate at different voltage levels.

4. Addressing:

   • Assign a unique address to each slave device. Most devices have configurable address pins or fixed addresses specified in their datasheets.

5. Master-Slave Communication:

   • The master initiates communication by sending a start condition, followed by the slave address and read/write bit.
   • Data is transferred in 8-bit packets, with an acknowledgment (ACK) bit after each byte.

• Bus Capacitance: Ensure the total capacitance of the bus does not exceed 400pF to maintain signal integrity.
• Clock Stretching: Some slave devices may hold the SCL line low to delay communication. Ensure the master supports clock stretching if required.
• Noise and Interference: Use short wires and proper shielding to minimize noise on the bus.

Below is an example of interfacing an I2C temperature sensor with an Arduino UNO:

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

#define SENSOR_ADDRESS 0x48 // Replace with your sensor's I2C address

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

void loop() {
  Wire.beginTransmission(SENSOR_ADDRESS); // Start communication with the sensor
  Wire.write(0x00); // Send a command to read temperature (check sensor datasheet)
  Wire.endTransmission(); // End transmission

  Wire.requestFrom(SENSOR_ADDRESS, 2); // Request 2 bytes of data from the sensor
  if (Wire.available() == 2) { // Check if 2 bytes are available
    int temp = Wire.read() << 8 | Wire.read(); // Combine the two bytes
    Serial.print("Temperature: ");
    Serial.println(temp / 256.0); // Convert and print the temperature
  }

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

1. No Communication on the Bus:

   • Check the pull-up resistors on the SDA and SCL lines.
   • Verify the wiring and ensure all devices share a common ground.
   • Confirm the slave address matches the device's datasheet.

2. Data Corruption or Noise:

   • Use shorter wires and proper shielding to reduce noise.
   • Ensure the bus capacitance is within the acceptable range.

3. Clock Stretching Issues:

   • Verify if the slave device uses clock stretching and ensure the master supports it.

4. Multiple Devices Not Responding:

   • Check for address conflicts. Each device must have a unique address.

Q: Can I connect devices with different voltage levels on the same I2C bus?
A: Yes, but you will need level shifters to safely interface devices operating at different voltage levels.

Q: How many devices can I connect to an I2C bus?
A: The number of devices is limited by the bus capacitance (typically up to 127 devices with 7-bit addressing).

Q: What happens if two masters try to communicate at the same time?
A: I2C includes an arbitration mechanism to prevent data collisions. The master with the higher priority (lower address) will take control of the bus.

Q: Do I always need pull-up resistors?
A: Yes, pull-up resistors are essential for proper operation of the I2C bus. Without them, the lines may not return to a high state.