Wire library for I2C in Arduino

Introduction

The Wire library helps your Arduino talk to other devices using I2C, a simple way to send and receive data between chips.

When you want to connect sensors like temperature or light sensors to your Arduino.

When you need to control small displays that use I2C communication.

When you want to connect multiple devices using only two wires to save pins.

When you want to read data from or send data to another microcontroller.

When you want to build projects that need easy and fast communication between parts.

Syntax

Arduino

Wire.begin();
Wire.beginTransmission(address);
Wire.write(data);
Wire.endTransmission();
Wire.requestFrom(address, quantity);
while (Wire.available()) {
  byte c = Wire.read();
}

Wire.begin() starts the I2C bus on your Arduino.

address is the 7-bit I2C address of the device you want to talk to.

Examples

This starts the Arduino as the master device on the I2C bus.

Arduino

Wire.begin();
// Start I2C as master

This sends a byte (0x00) to the device at address 0x3C.

Arduino

Wire.beginTransmission(0x3C);
Wire.write(0x00);
Wire.endTransmission();

This asks the device at 0x3C for 2 bytes and reads them one by one.

Arduino

Wire.requestFrom(0x3C, 2);
while (Wire.available()) {
  char c = Wire.read();
  // process c
}

Sample Program

This program starts I2C, sends a byte to a device at address 0x3C, then reads one byte back and prints it to the Serial Monitor every second.

Arduino

#include <Wire.h>

void setup() {
  Serial.begin(9600);
  Wire.begin(); // Start I2C as master
}

void loop() {
  Wire.beginTransmission(0x3C); // Device address
  Wire.write(0x00); // Send a command or data
  Wire.endTransmission();

  Wire.requestFrom(0x3C, 1); // Request 1 byte
  if (Wire.available()) {
    byte data = Wire.read();
    Serial.print("Received: ");
    Serial.println(data);
  }
  delay(1000); // Wait 1 second
}

Important Notes

Always check the device's 7-bit I2C address before communicating.

Use pull-up resistors on the SDA and SCL lines if your board or device does not have them built-in.

Wire library uses 7-bit addresses; some datasheets show 8-bit addresses, so be careful.

Summary

The Wire library makes I2C communication easy on Arduino.

Use Wire.begin() to start, Wire.beginTransmission() and Wire.endTransmission() to send data, and Wire.requestFrom() to receive data.

I2C uses only two wires to connect many devices, saving pins and simplifying wiring.

Practice

1. What is the main purpose of the Wire.begin() function in Arduino's Wire library?

easy

A. To request data from a device on the I2C bus

B. To send data to a device on the I2C bus

C. To end the I2C communication

D. To initialize the I2C bus and join the I2C network

Solution

1. Step 1: Understand the role of Wire.begin()

   The Wire.begin() function sets up the Arduino as a master or slave on the I2C bus and starts the communication.

2. Step 2: Differentiate from other Wire functions

   Functions like Wire.beginTransmission() and Wire.requestFrom() handle sending and receiving data, not initialization.

3. Final Answer:

   To initialize the I2C bus and join the I2C network -> Option D

4. Quick Check:

   Wire.begin() starts I2C communication [OK]

Hint: Wire.begin() always starts I2C communication [OK]

Common Mistakes:

• Confusing Wire.begin() with data sending functions
• Using Wire.beginTransmission() to start communication
• Thinking Wire.begin() sends or receives data

2. Which of the following is the correct syntax to start sending data to an I2C device with address 0x3C using the Wire library?

easy

A. Wire.start(0x3C);

B. Wire.beginTransmission(0x3C);

C. Wire.send(0x3C);

D. Wire.requestFrom(0x3C);

Solution

1. Step 1: Identify the function to start transmission

   Wire.beginTransmission(address) is used to begin sending data to the device at the given address.

2. Step 2: Check other options for correctness

   Wire.requestFrom() is for receiving data, Wire.send() is deprecated, and Wire.start() does not exist.

3. Final Answer:

   Wire.beginTransmission(0x3C); -> Option B

4. Quick Check:

   Use Wire.beginTransmission(address) to send data [OK]

Hint: Use Wire.beginTransmission(address) to start sending [OK]

Common Mistakes:

• Using Wire.requestFrom() to send data
• Using deprecated Wire.send() instead of Wire.write()
• Trying to use non-existent Wire.start() function

3. What will be the output of the following Arduino code snippet using the Wire library?

Wire.begin();
Wire.beginTransmission(0x50);
Wire.write(0x01);
Wire.endTransmission();
Wire.requestFrom(0x50, 1);
int data = Wire.read();
Serial.println(data);

medium

A. Prints the byte received from device at 0x50

B. Prints the byte 0x01

C. Prints 0 because Wire.read() returns 0 by default

D. Compilation error due to missing Wire.read()

Solution

1. Step 1: Analyze the code flow

   The code initializes I2C, sends one byte (0x01) to device 0x50, then requests 1 byte back from the same device.

2. Step 2: Understand Wire.read() behavior

   Wire.read() reads the byte received from the device. It does not return the sent byte but the device's response.

3. Final Answer:

   Prints the byte received from device at 0x50 -> Option A

4. Quick Check:

   Wire.read() returns received data, not sent data [OK]

Hint: Wire.read() reads incoming data, not what was sent [OK]

Common Mistakes:

• Assuming Wire.read() returns the sent byte
• Thinking Wire.read() returns 0 by default
• Confusing Wire.write() and Wire.read() roles

4. Identify the error in this Arduino I2C code snippet:

Wire.begin();
Wire.beginTransmission(0x40);
Wire.write(0x10);
Wire.requestFrom(0x40, 1);
Wire.endTransmission();
int val = Wire.read();

medium

A. Calling Wire.requestFrom() before Wire.endTransmission()

B. Missing Wire.beginTransmission() before Wire.write()

C. Using Wire.read() without requesting data

D. Not calling Wire.begin() to start I2C

Solution

1. Step 1: Check the order of I2C functions

   Wire.endTransmission() must be called to end sending data before requesting data with Wire.requestFrom().

2. Step 2: Identify the incorrect sequence

   The code calls Wire.requestFrom() before Wire.endTransmission(), which is incorrect and causes communication errors.

3. Final Answer:

   Calling Wire.requestFrom() before Wire.endTransmission() -> Option A

4. Quick Check:

   End transmission before requesting data [OK]

Hint: Always call Wire.endTransmission() before Wire.requestFrom() [OK]

Common Mistakes:

• Swapping the order of endTransmission and requestFrom
• Forgetting to call endTransmission() after beginTransmission()
• Calling Wire.read() without data available

5. You want to read 3 bytes from an I2C sensor at address 0x68 and store them in an array. Which code snippet correctly does this using the Wire library?

hard

A. Wire.beginTransmission(0x68);
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}
Wire.endTransmission();

B. Wire.beginTransmission(0x68);
Wire.requestFrom(0x68, 3);
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}
Wire.endTransmission();

C. Wire.requestFrom(0x68, 3);
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}

D. Wire.requestFrom(0x68, 3);
Wire.endTransmission();
for (int i = 0; i < 3; i++) {
data[i] = Wire.read();
}

Solution

1. Step 1: Understand how to receive data with Wire.requestFrom()

   Wire.requestFrom(address, quantity) requests bytes from the device. Then Wire.read() reads each byte.

2. Step 2: Check the correct sequence for reading data

   Only Wire.requestFrom() is needed to receive data. Wire.beginTransmission() and Wire.endTransmission() are for sending data, not receiving.

3. Final Answer:

   Wire.requestFrom(0x68, 3); for (int i = 0; i < 3; i++) { data[i] = Wire.read(); } -> Option C

4. Quick Check:

   Use requestFrom() then read() in loop to get bytes [OK]

Hint: Use requestFrom() then read() in loop to get bytes [OK]

Common Mistakes:

• Using beginTransmission() when only reading data
• Calling endTransmission() after requestFrom()
• Reading data before requesting it