Connecting multiple I2C devices in Arduino - Mini Project: Build & Apply

Connecting multiple I2C devices

📖 Scenario: You have two I2C sensors connected to your Arduino board. You want to read data from both sensors using their unique I2C addresses.

🎯 Goal: Write an Arduino program that initializes the I2C bus, communicates with two different I2C devices at addresses 0x3C and 0x68, and reads one byte of data from each device.

📋 What You'll Learn

Use the Wire library to communicate over I2C

Initialize the I2C bus in the setup() function

Read one byte from the device at address 0x3C

Read one byte from the device at address 0x68

Print the read bytes with labels to the Serial Monitor

💡 Why This Matters

🌍 Real World

Many electronics projects use multiple I2C sensors or modules connected to the same bus. Knowing how to communicate with each device by its address is essential.

💼 Career

Embedded systems engineers and IoT developers often work with multiple I2C devices to gather sensor data or control hardware. This skill is fundamental for hardware interfacing.

1

Setup I2C communication and Serial Monitor

Write void setup() function that starts the I2C bus with Wire.begin() and starts the Serial Monitor with Serial.begin(9600).

Arduino

#include 

void setup() {
  // Initialize I2C bus and Serial Monitor
  // Your code here
}

void loop() {
  // Empty loop
}

Hint

Use Wire.begin() to start I2C and Serial.begin(9600) to start serial communication.

2

Create variables for I2C device addresses

Declare two const byte variables named device1Address and device2Address and set them to 0x3C and 0x68 respectively.

Arduino

#include 

const byte device1Address = 0x00;  // Set device 1 address
const byte device2Address = 0x00;  // Set device 2 address

void setup() {
  Wire.begin();
  Serial.begin(9600);
}

void loop() {
  // Empty loop
}

Hint

Use const byte to define fixed I2C addresses.

3

Read one byte from each I2C device

In the loop() function, use Wire.beginTransmission() with device1Address, then Wire.requestFrom() to read one byte. Store it in byte data1. Repeat for device2Address storing in byte data2.

Arduino

#include 

const byte device1Address = 0x3C;
const byte device2Address = 0x68;

void setup() {
  Wire.begin();
  Serial.begin(9600);
}

void loop() {
  // Read one byte from device 1
  // Your code here

  // Read one byte from device 2
  // Your code here
}

Hint

Use Wire.beginTransmission(address) and Wire.requestFrom(address, 1) to read one byte from each device.

4

Print the read bytes to Serial Monitor

Add Serial.print() statements to print "Device 1 data: " followed by data1, and "Device 2 data: " followed by data2. Add Serial.println() to print each on a new line.

Arduino

#include 

const byte device1Address = 0x3C;
const byte device2Address = 0x68;

void setup() {
  Wire.begin();
  Serial.begin(9600);
}

void loop() {
  Wire.beginTransmission(device1Address);
  Wire.endTransmission();
  Wire.requestFrom(device1Address, 1);
  byte data1 = 0;
  if (Wire.available()) {
    data1 = Wire.read();
  }

  Wire.beginTransmission(device2Address);
  Wire.endTransmission();
  Wire.requestFrom(device2Address, 1);
  byte data2 = 0;
  if (Wire.available()) {
    data2 = Wire.read();
  }

  // Print the data
  // Your code here
}

Hint

Use Serial.print() and Serial.println() to show the data with labels.

Practice

1. What is the main reason multiple I2C devices can share the same two wires on an Arduino?

easy

A. They use different clock speeds to avoid conflicts

B. Each device uses a separate data wire

C. Each device has a unique address to identify it on the bus

D. They communicate one at a time using different power pins

Solution

1. Step 1: Understand I2C wiring basics

   I2C devices share two wires: SDA (data) and SCL (clock).

2. Step 2: Identify how devices are distinguished

   Devices use unique addresses so the master knows which device to talk to.

3. Final Answer:

   Each device has a unique address to identify it on the bus -> Option C

4. Quick Check:

   Unique addresses allow multiple devices on same wires [OK]

Hint: Remember: unique addresses let devices share wires [OK]

Common Mistakes:

• Thinking devices use separate wires
• Assuming different clock speeds separate devices
• Believing devices use different power pins to communicate

2. Which Arduino code snippet correctly initializes the I2C bus for multiple devices?

easy

A. Wire.init();

B. Wire.begin();

C. Wire.begin(0x3C);

D. Wire.start();

Solution

1. Step 1: Recall the correct function to start I2C

   The Arduino Wire library uses Wire.begin() to start the I2C bus as master.

2. Step 2: Check other options for correctness

   Wire.begin(0x3C) sets device as slave with address 0x3C, not for master init. Wire.init() and Wire.start() do not exist in Wire library.

3. Final Answer:

   Wire.begin(); -> Option B

4. Quick Check:

   Wire.begin() starts I2C master bus [OK]

Hint: Use Wire.begin() once to start I2C bus [OK]

Common Mistakes:

• Using Wire.begin(address) to start master
• Calling non-existent Wire.init() or Wire.start()
• Not calling Wire.begin() at all

3. Given this Arduino code snippet, what will be the output on the Serial Monitor if device at address 0x40 returns 25 and device at 0x41 returns 30?

Wire.begin();
Wire.beginTransmission(0x40);
Wire.write(0x00);
Wire.endTransmission();
Wire.requestFrom(0x40, 1);
int temp1 = Wire.read();
Wire.beginTransmission(0x41);
Wire.write(0x00);
Wire.endTransmission();
Wire.requestFrom(0x41, 1);
int temp2 = Wire.read();
Serial.print(temp1);
Serial.print(", ");
Serial.println(temp2);

medium

A. Compilation error

B. 30, 25

C. 0, 0

D. 25, 30

Solution

1. Step 1: Understand the communication sequence

   The code requests one byte from device 0x40, reads it as temp1 (25), then requests one byte from 0x41, reads it as temp2 (30).

2. Step 2: Check the Serial output commands

   Serial.print(temp1) prints 25, then ", ", then Serial.println(temp2) prints 30 with newline.

3. Final Answer:

   25, 30 -> Option D

4. Quick Check:

   Read values match device addresses in order [OK]

Hint: Match device address to read value order [OK]

Common Mistakes:

• Mixing up device addresses and values
• Assuming output is reversed
• Thinking code causes compile error

4. This Arduino code tries to read from two I2C devices but always reads zero. What is the likely error?

Wire.begin();
Wire.beginTransmission(0x50);
Wire.write(0x01);
Wire.endTransmission();
Wire.requestFrom(0x50, 1);
int val1 = Wire.read();
Wire.beginTransmission(0x50);
Wire.write(0x02);
Wire.endTransmission();
Wire.requestFrom(0x51, 1);
int val2 = Wire.read();
Serial.println(val1 + val2);

medium

A. Requesting from 0x51 instead of 0x50 causes no data

B. Missing Wire.begin() call

C. Using Wire.write() before Wire.requestFrom() is wrong

D. Serial.println() cannot print integers

Solution

1. Step 1: Analyze device addresses used

   The code writes to 0x50 twice but requests data from 0x50 first, then 0x51 second. If 0x51 device is not connected, no data is returned.

2. Step 2: Understand impact on read values

   Reading from 0x51 returns no data, so Wire.read() returns 0, causing sum to be zero or incorrect.

3. Final Answer:

   Requesting from 0x51 instead of 0x50 causes no data -> Option A

4. Quick Check:

   Address mismatch causes no data read [OK]

Hint: Check device addresses match in all calls [OK]

Common Mistakes:

• Confusing write and requestFrom addresses
• Forgetting to call Wire.begin() once
• Thinking Wire.write() before requestFrom() is invalid

5. You want to connect three I2C temperature sensors with addresses 0x48, 0x49, and 0x4A to an Arduino. Which of the following is the best approach to read all sensors reliably?

hard

A. Call Wire.begin() once, then read each sensor by its unique address with Wire.requestFrom()

B. Call Wire.begin() before reading each sensor separately

C. Use separate SDA and SCL wires for each sensor to avoid conflicts

D. Assign the same address to all sensors and read them one by one

Solution

1. Step 1: Understand I2C bus initialization

   Wire.begin() must be called once to initialize the bus as master before any communication.

2. Step 2: Read sensors by unique addresses on shared bus

   Each sensor has a unique address; read each by calling Wire.requestFrom() with that address on the same SDA and SCL lines.

3. Step 3: Evaluate other options

   Calling Wire.begin() multiple times is unnecessary. Using separate wires defeats I2C design. Assigning same address causes conflicts.

4. Final Answer:

   Call Wire.begin() once, then read each sensor by its unique address with Wire.requestFrom() -> Option A

5. Quick Check:

   One Wire.begin(), unique addresses, shared wires [OK]

Hint: Initialize once, read each device by unique address [OK]

Common Mistakes:

• Calling Wire.begin() multiple times
• Trying separate wires for each I2C device
• Using same address for multiple devices