I2C Protocol Reference

I2C (Inter-Integrated Circuit, pronounced "eye-squared-see") is a two-wire serial protocol for connecting sensors, displays, and other low-speed peripherals to a microcontroller. It is the most common protocol you will encounter in ESP32 projects because it keeps wiring simple — many devices share just two lines.

🔗How I2C Works

I2C is a master-slave protocol. The ESP32 acts as the master: it generates the clock signal and initiates every transaction. Slave devices (sensors, displays, etc.) only respond when spoken to.

Communication happens over two open-drain lines:

Both lines need pull-up resistors to the supply voltage. Most breakout boards already include them — more on this below.

🔗Addressing

Every device on the bus has a unique 7-bit address (values 0x00--0x7F), giving a theoretical maximum of 128 devices. In practice, some addresses are reserved, so you can have up to 112 devices on a single bus.

When the master wants to communicate, it sends a start condition followed by the 7-bit address and a read/write bit. Only the device with the matching address responds.

The full byte sent on the wire is:

$$\text{Address byte} = (\text{7-bit address} \ll 1) ;|; \text{R/W bit}$$

You usually do not need to think about this — the Wire library handles it for you. You just pass the 7-bit address (e.g., 0x76).

🔗Clock Stretching

Some slower devices hold the SCL line LOW to pause the master while they prepare data. This is called clock stretching. The ESP32 supports it, but poorly-designed devices can occasionally cause bus lockups this way.

graph TB
    ESP32["ESP32<br/>(Master)"]

    subgraph I2C Bus
        direction LR
        SDA["SDA line"]
        SCL["SCL line"]
    end

    BME["BME280<br/>0x76"]
    OLED["SSD1306 OLED<br/>0x3C"]
    BH["BH1750<br/>0x23"]
    SHT["SHT31<br/>0x44"]

    ESP32 --- SDA
    ESP32 --- SCL

    SDA --- BME
    SDA --- OLED
    SDA --- BH
    SDA --- SHT

    SCL --- BME
    SCL --- OLED
    SCL --- BH
    SCL --- SHT

All devices share the same two wires. The master selects which device to talk to by sending its address.

🔗ESP32 I2C Pins and Controllers

The ESP32 has two independent I2C controllers: I2C0 and I2C1. Each can be assigned to almost any GPIO pin through the ESP32's GPIO matrix.

Tip: If you have two devices with the same I2C address, put them on separate controllers instead of trying to use them on the same bus.

🔗Remapping Pins

Wire.begin(16, 17);  // SDA on GPIO 16, SCL on GPIO 17

Avoid GPIOs 6--11 (flash), 34--39 (input-only), and check your board's pinout for availability.

🔗Speed Modes

Set the clock speed after Wire.begin():

Wire.setClock(400000);  // 400 kHz fast mode

The effective data rate is lower than the clock frequency due to protocol overhead. At 400 kHz, expect roughly $50\,\text{KB/s}$ of actual throughput.

🔗Pull-Up Resistors

I2C lines are open-drain, meaning devices can only pull them LOW. Pull-up resistors are required to bring the lines back HIGH. Typical values are 4.7 k$\Omega$ for 3.3 V systems.

Do you need to add them? Usually not. Most breakout boards (Adafruit, SparkFun, generic purple boards) include pull-ups on the module. If you connect multiple boards that each have pull-ups, the effective resistance drops — this is fine for two or three boards but can cause signal problems with many devices. In that case, remove the extra pull-ups or use a single pair of external resistors.

🔗I2C Address Scanner

When a device does not respond, the first debugging step is to scan the bus. This sketch checks every address and prints the ones that acknowledge:

#include <Wire.h>

void setup() {
  Serial.begin(115200);
  delay(1000);
  Wire.begin();  // Default: SDA=21, SCL=22

  Serial.println("Scanning I2C bus...\n");

  int found = 0;
  for (byte addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    if (Wire.endTransmission() == 0) {
      Serial.printf("  Device at 0x%02X\n", addr);
      found++;
    }
  }
  Serial.printf("\n%d device(s) found.\n", found);
}

void loop() {}

A longer version with device identification is in the Communication Protocols overview.

🔗Common I2C Addresses

Many devices have configurable addresses (usually via an ADDR pin or solder bridge) so you can put two of the same sensor on one bus.

If a scanner finds an address you do not recognize, check datasheets or look it up on i2cdevices.org.

🔗Common Issues

🔗Used In

These articles on this site use I2C:

• BME280 Temperature, Humidity & Pressure Sensor
• SHT31 Temperature & Humidity Sensor
• BH1750 Light Sensor
• SGP30 Air Quality Sensor
• INA219 Current & Power Sensor
• VL53L0X Time-of-Flight Distance Sensor
• SSD1306 OLED Display
• I2C LCD Display