Writing commands to I2C device in Raspberry Pi - Mini Project: Build & Apply

Writing commands to I2C device

📖 Scenario: You have a Raspberry Pi connected to a small I2C device, like a sensor or a display. You want to send commands to this device to control it or request data.In this project, you will learn how to write commands to an I2C device using Python on the Raspberry Pi.

🎯 Goal: Build a simple Python program that writes specific command bytes to an I2C device at a given address.

📋 What You'll Learn

Use the smbus2 library to communicate over I2C

Set the I2C device address correctly

Write a command byte to the device

Print confirmation of the command sent

💡 Why This Matters

🌍 Real World

Many sensors and small devices use I2C to communicate. Writing commands lets you control these devices, like turning on a display or starting a sensor measurement.

💼 Career

Understanding I2C communication is important for embedded systems, hardware interfacing, and IoT jobs where you connect software to physical devices.

1

Set up the I2C bus and device address

Import the SMBus class from the smbus2 library. Create a variable called bus that opens I2C bus number 1. Create a variable called device_address and set it to 0x40 (the device's I2C address).

Raspberry Pi

from smbus2 import SMBus
# Your code here

Hint

Use SMBus(1) to open the I2C bus on Raspberry Pi.

The device address is a hexadecimal number 0x40.

2

Define the command byte to send

Create a variable called command and set it to the integer 0x10. This represents the command byte you want to send to the device.

Raspberry Pi

from smbus2 import SMBus
bus = SMBus(1)
device_address = 0x40
# Your code here

Hint

The command is a single byte, so use an integer like 0x10.

3

Write the command byte to the I2C device

Use the write_byte method of bus to send the command byte to the device_address.

Raspberry Pi

from smbus2 import SMBus
bus = SMBus(1)
device_address = 0x40
command = 0x10
# Your code here

Hint

Call bus.write_byte(device_address, command) to send the command.

4

Print confirmation of the command sent

Write a print statement that outputs: "Command 0x10 sent to device at address 0x40".

Raspberry Pi

from smbus2 import SMBus
bus = SMBus(1)
device_address = 0x40
command = 0x10
bus.write_byte(device_address, command)
# Your code here

Hint

Use print("Command 0x10 sent to device at address 0x40") exactly.

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