Writing commands to I2C device in Raspberry Pi - Step-by-Step Execution

Concept Flow - Writing commands to I2C device

Start

↓

Open I2C bus

↓

Set device address

↓

Prepare command/data

↓

Write command to device

↓

Check for errors

↓

Close I2C bus

↓

End

This flow shows how a Raspberry Pi opens the I2C bus, sets the device address, writes a command, checks for errors, and closes the bus.

Execution Sample

Raspberry Pi

import smbus
bus = smbus.SMBus(1)
address = 0x20
bus.write_byte(address, 0xFF)
# smbus.SMBus object does not have a close() method
# so bus.close() is removed

This code opens I2C bus 1, writes the byte 0xFF to device at address 0x20.

Execution Table

Step	Action	Details	Result
1	Import smbus	Load I2C library	smbus module ready
2	Open I2C bus	bus = smbus.SMBus(1)	bus object created for bus 1
3	Set device address	address = 0x20	address stored as 32 decimal
4	Write byte	bus.write_byte(0x20, 0xFF)	0xFF sent to device at 0x20
5	End	No more commands	Program ends

💡 All commands executed successfully

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
bus	None	SMBus object for bus 1	SMBus object for bus 1	SMBus object for bus 1	SMBus object for bus 1
address	None	None	0x20 (32 decimal)	0x20 (32 decimal)	0x20 (32 decimal)

Key Moments - 3 Insights

Why do we need to open the I2C bus before writing?

The bus must be opened to create a communication channel; see Step 2 in execution_table where bus object is created before any write.

What happens if the device address is wrong?

The write command will fail or no device will respond; Step 4 shows writing to address 0x20, if incorrect, an error or no effect occurs.

Why close the bus after writing?

The smbus.SMBus object does not require explicit closing; resources are managed by the OS. Closing is not needed.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'address' after Step 3?

A None

B 0xFF

C 0x20 (32 decimal)

D SMBus object

💡 Hint

Check variable_tracker row for 'address' after Step 3

At which step is the byte 0xFF sent to the device?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at execution_table action column for 'Write byte'

If we forget to close the bus, what changes in the execution_table?

A No change at all

B Step 4 would fail

C Step 3 would change address

D Step 5 would be missing or different

💡 Hint

smbus.SMBus does not require closing, so no change

Concept Snapshot

Open I2C bus with smbus.SMBus(bus_number)
Set device address as hex (e.g., 0x20)
Write commands using write_byte(address, data)
No need to close bus explicitly
Always check for errors during write

Full Transcript

This example shows how to write commands to an I2C device using a Raspberry Pi. First, the smbus library is imported to access I2C functions. Then, the I2C bus 1 is opened by creating an SMBus object. The device address is set to 0x20. Using the write_byte method, the byte 0xFF is sent to the device at that address. The smbus.SMBus object does not require explicit closing; resources are managed by the OS. Each step is important: opening the bus creates the communication channel, setting the correct address targets the right device, and writing sends the command. If the address is wrong, the device won't respond. Forgetting to close the bus does not cause resource conflicts because closing is not needed. This step-by-step flow helps beginners see how commands are sent to I2C devices on Raspberry Pi.

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