Multiple I2C devices on same bus in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when reading from two I2C devices on the same bus?

Consider two I2C devices connected to the same Raspberry Pi I2C bus with addresses 0x20 and 0x21. The code reads one byte from each device. What will be printed?

Raspberry Pi

import smbus
bus = smbus.SMBus(1)

addr1 = 0x20
addr2 = 0x21

byte1 = bus.read_byte(addr1)
byte2 = bus.read_byte(addr2)

print(f"Device 1 returned: {byte1}")
print(f"Device 2 returned: {byte2}")

A Device 1 returned: 20\nDevice 2 returned: 10

B RuntimeError: Remote I/O error

C Device 1 returned: 10\nDevice 2 returned: 20

D Device 1 returned: 0\nDevice 2 returned: 0

💡 Hint

Each device has a unique address, so reading from each returns its own data.

When multiple I2C devices share the same bus, each device is accessed by its unique address. Reading from address 0x20 returns data from device 1, and reading from 0x21 returns data from device 2. The output shows the values read from each device.

🧠 Conceptual

intermediate

Why must each I2C device have a unique address on the same bus?

On a Raspberry Pi I2C bus, why is it important that each connected device has a unique address?

A To prevent data collisions and ensure the master communicates with the correct device

B To allow devices to share the same memory space

C To enable devices to send data simultaneously without interference

D To reduce the power consumption of the bus

💡 Hint

Think about how the Raspberry Pi knows which device to talk to.

Each I2C device must have a unique address so the master (Raspberry Pi) can select and communicate with the correct device. If two devices share the same address, the bus cannot distinguish between them, causing data collisions or communication errors.

🔧 Debug

advanced

Identify the error when two devices share the same I2C address

The following code tries to read from two devices on the same I2C bus, but both devices have the address 0x20. What error will most likely occur?

Raspberry Pi

import smbus
bus = smbus.SMBus(1)

addr1 = 0x20
addr2 = 0x20

byte1 = bus.read_byte(addr1)
byte2 = bus.read_byte(addr2)

print(byte1, byte2)

A No error, prints two bytes

B ValueError: Invalid address

C TypeError: unsupported operand type(s)

D RuntimeError: Remote I/O error

💡 Hint

Think about what happens when two devices respond to the same address.

If two devices share the same I2C address on the bus, the master cannot distinguish between them. This usually causes a communication failure, resulting in a 'Remote I/O error' when trying to read data.

📝 Syntax

advanced

Which code snippet correctly scans all devices on the I2C bus?

Choose the code that correctly scans the I2C bus for connected devices and prints their addresses.

A

for addr in range(0x03, 0x78):
    bus.read_byte(addr)
    print(f"Device at {addr}")

B

for addr in range(0x03, 0x78):
    try:
        bus.read_byte(addr)
        print(f"Device found at 0x{addr:02X}")
    except OSError:
        pass

C

for addr in range(0x03, 0x78):
    try:
        bus.write_byte(addr, 0)
        print(f"Found device at {addr}")
    except ValueError:
        continue

D

for addr in range(0x00, 0x7F):
    if bus.read_byte(addr):
        print(f"Device at {addr}")

💡 Hint

Scanning requires catching errors when no device responds.

Option B tries to read a byte from each possible address and catches OSError if no device responds. This is the correct way to scan the I2C bus. Other options either do not handle exceptions or use incorrect methods.

🚀 Application

expert

How to handle two I2C devices with the same fixed address on one bus?

You have two I2C sensors that both use the fixed address 0x40 and cannot be changed. You want to connect both to the same Raspberry Pi I2C bus. What is the best approach to read data from both devices?

A Use an I2C multiplexer to switch between devices, selecting one at a time

B Connect both devices directly and read data simultaneously from address 0x40

C Change the Raspberry Pi I2C bus speed to avoid address conflicts

D Use software to merge data from both devices at the same address

💡 Hint

Think about hardware solutions to address conflicts.

When two devices share the same fixed I2C address, the bus cannot distinguish them. Using an I2C multiplexer allows the Raspberry Pi to select which device is connected to the bus at a time, avoiding address conflicts and enabling communication with both devices.