Writing commands to I2C device in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this I2C write command code?

Consider the following Python code snippet running on a Raspberry Pi to write a byte to an I2C device. What will be printed when this code runs successfully?

Raspberry Pi

import smbus2
bus = smbus2.SMBus(1)
address = 0x20
try:
    bus.write_byte(address, 0xFF)
    print("Write successful")
except Exception as e:
    print(f"Error: {e}")

A No output

B Error: [Errno 121] Remote I/O error

C Write failed

D Write successful

💡 Hint

The code tries to write a byte and prints a success message if no error occurs.

The code writes a byte 0xFF to the device at address 0x20. If the device is connected and responds, the write succeeds and prints 'Write successful'.

🧠 Conceptual

intermediate

Which I2C address format is correct for smbus2 write commands?

When using smbus2 on Raspberry Pi to write to an I2C device, which address format should you use?

A 8-bit address including the read/write bit, e.g., 0x40

B 7-bit address without the read/write bit, e.g., 0x20

C Decimal address only, e.g., 32

D Any address format works as smbus2 auto-corrects

💡 Hint

I2C addresses are usually 7 bits; the library handles the read/write bit.

smbus2 expects the 7-bit address without the read/write bit. The library adds the read/write bit automatically.

🔧 Debug

advanced

Why does this I2C write command raise an error?

This code snippet raises an error when trying to write a byte to an I2C device. What is the most likely cause?

Raspberry Pi

import smbus2
bus = smbus2.SMBus(1)
address = 0x20
bus.write_byte_data(address, 0x01)

A Address 0x20 is invalid for I2C devices

B Using wrong bus number; should be SMBus(0)

C Missing the data byte argument in write_byte_data call

D smbus2 does not support write_byte_data method

💡 Hint

Check the method signature for write_byte_data.

write_byte_data requires three arguments: address, command/register, and data byte. The code misses the data byte argument.

📝 Syntax

advanced

Which option correctly writes a block of bytes to an I2C device?

You want to write the bytes [0x01, 0x02, 0x03] starting at register 0x10 to an I2C device at address 0x20 using smbus2. Which code snippet is syntactically correct and will run without error?

A bus.write_i2c_block_data(0x20, 0x10, [0x01, 0x02, 0x03])

B bus.write_block(0x20, 0x10, [0x01, 0x02, 0x03])

C bus.write_i2c_block(0x20, 0x10, [0x01, 0x02, 0x03])

D bus.write_block_data(0x20, 0x10, [0x01, 0x02, 0x03])

💡 Hint

Check the exact method name in smbus2 for block writes.

The correct method is write_i2c_block_data. Other options are invalid method names and cause AttributeError.

🚀 Application

expert

What is the value of 'result' after this I2C write and read sequence?

Given the following code that writes a byte to register 0x05 and then reads a byte back from the same register, what is the value of 'result' if the device echoes the written byte?

Raspberry Pi

import smbus2
bus = smbus2.SMBus(1)
address = 0x20
bus.write_byte_data(address, 0x05, 0xAB)
result = bus.read_byte_data(address, 0x05)
print(result)

A 171

B 0

C Error: Remote I/O error

D None

💡 Hint

0xAB in decimal is 171. The device echoes the byte written.

The code writes 0xAB (decimal 171) to register 0x05 and reads it back. If the device echoes correctly, result is 171.

Practice

1. What is the main purpose of the write_byte_data function in the smbus2 library when working with I2C devices on Raspberry Pi?

easy

A. To open the I2C bus for communication

B. To read a byte of data from the I2C device

C. To send a command and a single byte of data to a specific register of the I2C device

D. To close the I2C bus after communication

Solution

1. Step 1: Understand the function's role

   The write_byte_data function sends a command (register address) and a byte of data to the I2C device.

2. Step 2: Differentiate from other functions

   Reading data uses read_byte_data, and opening/closing the bus uses SMBus() context or methods.

3. Final Answer:

   To send a command and a single byte of data to a specific register of the I2C device -> Option C

4. Quick Check:

   write_byte_data sends command + data [OK]

Hint: Remember: write_byte_data sends command + data [OK]

Common Mistakes:

• Confusing write_byte_data with read_byte_data
• Thinking it opens or closes the bus
• Assuming it sends multiple bytes at once

2. Which of the following is the correct syntax to write the byte value 0x1A to register 0x10 of an I2C device at address 0x20 using smbus2 in Python?

easy

A. bus.write_byte_data(0x20, 0x10, 0x1A)

B. bus.write_byte_data(0x1A, 0x20, 0x10)

C. bus.write_byte(0x20, 0x10, 0x1A)

D. bus.write_byte_data(0x10, 0x20, 0x1A)

Solution

1. Step 1: Identify the parameter order for write_byte_data

   The function signature is write_byte_data(i2c_addr, register, value).

2. Step 2: Match parameters to values

   Address is 0x20, register is 0x10, and value is 0x1A, so call is bus.write_byte_data(0x20, 0x10, 0x1A).

3. Final Answer:

   bus.write_byte_data(0x20, 0x10, 0x1A) -> Option A

4. Quick Check:

   Address, register, value order = D [OK]

Hint: Remember parameter order: address, register, value [OK]

Common Mistakes:

• Swapping address and register parameters
• Using write_byte instead of write_byte_data
• Incorrect number of arguments

3. What will be the output of the following Python code snippet on Raspberry Pi when writing to an I2C device?

from smbus2 import SMBus
bus = SMBus(1)
try:
    bus.write_byte_data(0x50, 0x01, 0xFF)
    print("Write successful")
except Exception as e:
    print(f"Error: {e}")
bus.close()

medium

A. Error: [Errno 121] Remote I/O error

B. No output

C. SyntaxError

D. Write successful

Solution

1. Step 1: Analyze the code flow

   The code opens I2C bus 1, writes byte 0xFF to register 0x01 at device 0x50, then prints success if no error.

2. Step 2: Consider normal operation

   If the device is connected and address is correct, write succeeds and prints "Write successful".

3. Final Answer:

   Write successful -> Option D

4. Quick Check:

   Successful write prints message [OK]

Hint: If no error, print shows success message [OK]

Common Mistakes:

• Assuming error without device connected
• Confusing syntax error with runtime error
• Ignoring bus.close() importance

4. Identify the error in the following code snippet that writes to an I2C device and suggest the fix:

from smbus2 import SMBus
bus = SMBus(1)
bus.write_byte_data(0x40, 0x02)
bus.close()

medium

A. Wrong device address format; use string instead of integer

B. Missing the data byte argument in write_byte_data; add the data byte

C. Incorrect bus number; change SMBus(1) to SMBus(0)

D. bus.close() should be called before write_byte_data

Solution

1. Step 1: Check write_byte_data parameters

   The function requires three arguments: device address, register, and data byte.

2. Step 2: Identify missing argument

   The code only passes two arguments, missing the data byte to write.

3. Final Answer:

   Missing the data byte argument in write_byte_data; add the data byte -> Option B

4. Quick Check:

   write_byte_data needs 3 args [OK]

Hint: write_byte_data always needs 3 arguments [OK]

Common Mistakes:

• Forgetting the data byte argument
• Assuming bus.close() order matters here
• Changing bus number without reason

5. You want to write a sequence of bytes [0x10, 0x20, 0x30] to consecutive registers starting at 0x00 on an I2C device at address 0x60 using smbus2. Which approach correctly achieves this?

hard

A. Use a loop calling write_byte_data(0x60, register, value) for each byte, incrementing register from 0x00

B. Call write_byte_data(0x60, 0x00, [0x10, 0x20, 0x30]) once to write all bytes

C. Use write_byte(0x60, 0x00, 0x10, 0x20, 0x30) to send all bytes at once

D. Write only the first byte with write_byte_data(0x60, 0x00, 0x10) and ignore the rest

Solution

1. Step 1: Understand write_byte_data limitations

   This function writes one byte to one register at a time.

2. Step 2: Write bytes to consecutive registers

   Use a loop to write each byte to registers 0x00, 0x01, 0x02 respectively.

3. Final Answer:

   Use a loop calling write_byte_data(0x60, register, value) for each byte, incrementing register from 0x00 -> Option A

4. Quick Check:

   One byte per register via loop [OK]

Hint: Write bytes one by one in a loop for consecutive registers [OK]

Common Mistakes:

• Trying to write multiple bytes in one write_byte_data call
• Using write_byte incorrectly with multiple bytes
• Ignoring the need to increment register address