Multiple I2C devices on same bus in Raspberry Pi - Mini Project: Build & Apply

Multiple I2C Devices on Same Bus

📖 Scenario: You have a Raspberry Pi connected to two I2C sensors on the same bus. You want to read data from both sensors without conflicts.

🎯 Goal: Write a Python program that communicates with two I2C devices on the same bus by specifying their addresses and reading data from each.

📋 What You'll Learn

Create a dictionary called devices with two I2C device addresses as keys and their names as values

Create a variable called bus_number and set it to 1 (the default I2C bus on Raspberry Pi)

Use a for loop with variables address and name to iterate over devices.items()

Inside the loop, create an SMBus object for bus_number and read a byte from each device address

Print the device name and the byte read in the format: Device {name} at address {address} returned {data}

💡 Why This Matters

🌍 Real World

Many projects use multiple sensors on the same I2C bus to save wiring and pins. This skill helps you manage multiple devices efficiently.

💼 Career

Understanding how to communicate with multiple I2C devices is important for embedded systems, IoT development, and hardware interfacing roles.

1

Create the I2C devices dictionary

Create a dictionary called devices with these exact entries: 0x40: 'Temperature Sensor' and 0x48: 'Light Sensor'

Raspberry Pi

# Create the devices dictionary with addresses and names
# Your code here

Hint

Use curly braces {} to create a dictionary with keys as addresses and values as names.

2

Set the I2C bus number

Create a variable called bus_number and set it to 1, which is the default I2C bus on Raspberry Pi

Raspberry Pi

devices = {0x40: 'Temperature Sensor', 0x48: 'Light Sensor'}
# Set the I2C bus number
# Your code here

Hint

Just assign the number 1 to the variable bus_number.

3

Read data from each I2C device

Use a for loop with variables address and name to iterate over devices.items(). Inside the loop, create an SMBus object for bus_number and read a byte from each device address using read_byte(address). Store the result in a variable called data

Raspberry Pi

devices = {0x40: 'Temperature Sensor', 0x48: 'Light Sensor'}
bus_number = 1
from smbus2 import SMBus
# Loop over devices and read a byte from each
# Your code here

Hint

Use for address, name in devices.items(): to loop. Use with SMBus(bus_number) as bus: to open the bus. Then read a byte with bus.read_byte(address).

4

Print the data from each device

Inside the for loop, add a print statement to display the device name, address, and data in this exact format: Device {name} at address {address} returned {data}

Raspberry Pi

devices = {0x40: 'Temperature Sensor', 0x48: 'Light Sensor'}
bus_number = 1
from smbus2 import SMBus

for address, name in devices.items():
    with SMBus(bus_number) as bus:
        data = bus.read_byte(address)
        # Print the device info and data
        # Your code here

Hint

Use an f-string in the print statement: print(f"Device {name} at address {address} returned {data}").

Practice

1.

What is the main requirement to connect multiple I2C devices on the same bus?

Choose the best answer.

easy

A. Each device must have a unique I2C address.

B. Each device must use a different bus.

C. Devices must be connected with separate wires.

D. Devices must have the same address to communicate.

Solution

1. Step 1: Understand I2C bus addressing

   I2C devices share the same two wires but must have unique addresses to avoid conflicts.

2. Step 2: Identify the correct requirement

   Since devices share the bus, unique addresses allow the master to select each device individually.

3. Final Answer:

   Each device must have a unique I2C address. -> Option A

4. Quick Check:

   Unique addresses = multiple devices on one bus [OK]

Hint: Remember: unique addresses prevent device conflicts on I2C [OK]

Common Mistakes:

• Thinking devices need separate wires
• Assuming devices can share the same address
• Believing each device needs its own bus

2.

Which Python code snippet correctly initializes communication with two I2C devices at addresses 0x20 and 0x21 on the same bus?

import smbus2
bus = smbus2.SMBus(1)

# What comes next?

easy

A. device1 = smbus2.SMBus(0x20)
device2 = smbus2.SMBus(0x21)

B. device1 = bus.read_byte(0x20)
device2 = bus.read_byte(0x21)

C. device1 = 0x20
device2 = 0x21

D. device1 = bus
device2 = bus

Solution

1. Step 1: Understand bus and device addressing

   The bus object is created once; devices are identified by their addresses.

2. Step 2: Assign device addresses as variables

   Assigning device addresses to variables allows easy communication calls later.

3. Final Answer:

   device1 = 0x20\ndevice2 = 0x21 -> Option C

4. Quick Check:

   Bus once, devices by address [OK]

Hint: Create one bus object; use addresses to identify devices [OK]

Common Mistakes:

• Trying to create multiple bus objects for each device
• Calling read_byte immediately without setup
• Assigning bus object directly to devices

3.

Given this Python code snippet on a Raspberry Pi with two I2C devices at addresses 0x20 and 0x21:

import smbus2
bus = smbus2.SMBus(1)

value1 = bus.read_byte(0x20)
value2 = bus.read_byte(0x21)
print(value1, value2)

What will be the output if device 0x20 returns 10 and device 0x21 returns 20?

medium

A. 10 20

B. 20 10

C. 0 0

D. Error: Address not found

Solution

1. Step 1: Read bytes from each device address

   The code reads one byte from address 0x20 and one from 0x21.

2. Step 2: Print the values in order

   Since value1 reads from 0x20 (returns 10) and value2 from 0x21 (returns 20), print outputs "10 20".

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   Read from 0x20 then 0x21 = 10 20 [OK]

Hint: Read bytes by address order matches output order [OK]

Common Mistakes:

• Mixing up device addresses and values
• Assuming output is reversed
• Expecting error without checking device presence

4.

Consider this Python code snippet intended to read from two I2C devices on the same bus:

import smbus2
bus = smbus2.SMBus(1)

value1 = bus.read_byte(0x20)
value2 = bus.read_byte(0x20)
print(value1, value2)

Why might this code cause a problem when trying to read two different devices?

medium

A. The code is missing pull-up resistors on the bus.

B. Both reads use the same address, so the second device is never accessed.

C. The addresses 0x20 and 0x20 are invalid on the bus.

D. The bus object is created twice, causing conflict.

Solution

1. Step 1: Check device addresses used in reads

   Both read_byte calls use address 0x20, so only one device is accessed twice.

2. Step 2: Understand impact on multiple devices

   To read two different devices, their unique addresses must be used; otherwise, the second device is ignored.

3. Final Answer:

   Both reads use the same address, so the second device is never accessed. -> Option B

4. Quick Check:

   Same address twice = one device only [OK]

Hint: Use different addresses to access multiple devices [OK]

Common Mistakes:

• Assuming bus object creation causes error
• Thinking addresses 0x20 are invalid
• Confusing hardware wiring issues with code errors

5.

You have three I2C sensors connected to the same Raspberry Pi bus with addresses 0x20, 0x21, and 0x20. You want to read data from all three devices. What is the best way to fix this setup?

hard

A. Use separate I2C buses for devices with the same address.

B. Use software to ignore address conflicts and read anyway.

C. Connect devices with the same address to different GPIO pins.

D. Change the address of one device so all three have unique addresses.

Solution

1. Step 1: Identify address conflict

   Two devices share address 0x20, causing bus conflicts and communication errors.

2. Step 2: Resolve conflict by changing address

   Most I2C devices allow changing their address via hardware pins or software configuration to ensure uniqueness.

3. Step 3: Alternative solutions

   Using separate buses or GPIO pins is complex; ignoring conflicts causes errors.

4. Final Answer:

   Change the address of one device so all three have unique addresses. -> Option D

5. Quick Check:

   Unique addresses fix conflicts [OK]

Hint: Ensure all devices have unique addresses to avoid conflicts [OK]

Common Mistakes:

• Trying to read devices with duplicate addresses
• Assuming software can fix hardware address conflicts
• Using separate GPIO pins without separate buses