Writing commands to I2C device in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is I2C communication?

I2C (Inter-Integrated Circuit) is a simple way for devices to talk to each other using just two wires: one for data and one for clock. It lets many devices share the same wires.

beginner

Which Python library is commonly used on Raspberry Pi to write commands to an I2C device?

The 'smbus2' library is commonly used. It helps send and receive data over I2C easily.

intermediate

What does the 'write_byte_data' function do in I2C communication?

It sends a single byte of data to a specific register (or command) on the I2C device.

beginner

Why do you need the device address when writing to an I2C device?

The device address tells the Raspberry Pi which device on the I2C bus should receive the command, like sending mail to the right house.

beginner

What is the purpose of the 'bus' number in I2C communication on Raspberry Pi?

The 'bus' number selects which I2C channel on the Raspberry Pi you want to use, because some models have more than one I2C bus.

Which two wires are used in I2C communication?

A Data and Clock

B Power and Ground

C Transmit and Receive

D Reset and Enable

I2C uses two wires: one for data (SDA) and one for clock (SCL).

In Python's smbus2 library, which function writes a byte to a device register?

A read_byte_data

B write_byte_data

C write_word_data

D read_word_data

write_byte_data sends a single byte to a specific register on the I2C device.

What does the device address represent in I2C communication?

A The size of data packets

B The speed of communication

C The voltage level

D The unique ID of the device on the bus

The device address identifies which device on the I2C bus will receive the data.

Why might you need to specify the bus number on a Raspberry Pi?

A To choose the power source

B To set the baud rate

C Because the Pi can have multiple I2C buses

D To select the GPIO pins for SPI

Some Raspberry Pi models have more than one I2C bus, so you specify which one to use.

Which Python code snippet correctly writes the value 0x10 to register 0x01 of an I2C device at address 0x20 on bus 1?

A bus.write_byte_data(0x20, 0x01, 0x10)

B bus.read_byte_data(0x20, 0x01)

C bus.write_word_data(0x01, 0x20, 0x10)

D bus.write_byte(0x01, 0x10)

write_byte_data(address, register, value) sends a byte to the device register.

Explain the steps to write a command to an I2C device using Python on a Raspberry Pi.

Think about setting up communication and sending data.

Why is it important to know the device address and bus number when writing commands to an I2C device?

Compare it to sending a letter to the right house on the right street.

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