# I2C with the Wire Library — Arduino & Embedded C

Source: https://www.geekswithgeeks.com/en/arduino-embedded-c/i2c-bus

> Two wires, many devices, addresses and pull-ups.

## Addresses and pull-ups

**I2C** uses two shared lines, **SDA** (data) and **SCL** (clock), plus ground. The controller addresses each device by a **7-bit address** (often written like `0x3C` or `0x76`), so many sensors can share the bus as long as addresses differ; some modules let you change the address with a jumper. The lines are open-drain and need **pull-up resistors** (commonly a few kΩ); many breakout boards include them. On the Uno R3, SDA is A4 and SCL is A5 (also on dedicated header pins); on other boards check the pinout. I2C is slower than SPI (100 kHz and 400 kHz are common speeds) but needs few pins.

## An I2C scanner

Lists responding addresses; a standard first step when wiring a new module.

```cpp
#include <Wire.h>

void setup() {
  Serial.begin(115200);
  Wire.begin();                       // join the bus as controller
  Serial.println(F("Scanning..."));
  for (uint8_t addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    if (Wire.endTransmission() == 0) {  // 0 = device acknowledged
      Serial.print(F("Found device at 0x"));
      if (addr < 16) Serial.print('0');
      Serial.println(addr, HEX);
    }
  }
  Serial.println(F("Done."));
}

void loop() {}
```

## Watch the voltage

A 3.3 V sensor on a 5 V Uno R3 bus may need a level shifter, depending on the module. Check both the sensor and the breakout board datasheets.

**Quiz:** How does an I2C controller select which device to talk to?

- [ ] Each device needs its own pair of wires
- [ ] By pulling that device's chip-select pin low
- [ ] By choosing a different baud rate
- [x] By sending the device's address on the shared bus

*Answer:* By sending the device's address on the shared bus. Addressing lets many devices share SDA and SCL.