Reading sensor data over I2C in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this I2C sensor read code?

Consider this Python code snippet running on a Raspberry Pi to read 2 bytes from an I2C sensor at address 0x48. What will be printed?

Raspberry Pi

import smbus
bus = smbus.SMBus(1)
address = 0x48
raw = bus.read_i2c_block_data(address, 0, 2)
value = (raw[0] << 8) | raw[1]
print(value)

A An integer between 0 and 65535 representing the sensor data

B A list of two integers representing the raw bytes read

C A TypeError because read_i2c_block_data returns a string

D A runtime IOError because the sensor address is invalid

💡 Hint

The read_i2c_block_data returns a list of integers representing bytes.

The code reads two bytes from the sensor and combines them into a single 16-bit integer using bit shifting and bitwise OR. The print outputs this integer value.

🧠 Conceptual

intermediate

Which I2C bus number should you use on Raspberry Pi 4?

On a Raspberry Pi 4, which bus number corresponds to the default I2C pins (SDA, SCL) you should use in smbus.SMBus()?

A 3

B 0

C 2

D 1

💡 Hint

The default I2C bus on Raspberry Pi models after version 1 is bus 1.

The Raspberry Pi 4 uses I2C bus 1 for the default SDA and SCL pins. Bus 0 is reserved for system use.

🔧 Debug

advanced

Why does this I2C read code raise an IOError?

This code snippet raises an IOError when run on Raspberry Pi. What is the most likely cause? import smbus bus = smbus.SMBus(1) address = 0x50 data = bus.read_byte(address) print(data)

A The bus number 1 is invalid on Raspberry Pi

B The sensor at address 0x50 is not connected or powered on

C read_byte requires a command argument which is missing

D The address 0x50 is reserved and cannot be used

💡 Hint

IOError usually means no device responded at that address.

An IOError during I2C read usually means the device at the given address is not present or not responding. The bus number and method usage are correct.

📝 Syntax

advanced

Which option correctly reads a signed 16-bit value from I2C sensor?

You want to read a signed 16-bit integer from an I2C sensor at address 0x40 using smbus. Which code snippet correctly converts the two bytes to a signed integer?

Raspberry Pi

import smbus
bus = smbus.SMBus(1)
address = 0x40
raw = bus.read_i2c_block_data(address, 0, 2)

A

value = (raw[1] << 8) | raw[0]
if value > 32767:
    value -= 65536

B

value = raw[0] + (raw[1] << 8)
if value > 32767:
    value -= 65536

C

value = (raw[0] << 8) | raw[1]
if value > 32767:
    value -= 65536

D

value = (raw[0] << 8) + raw[1]
if value > 32767:
    value -= 65536

💡 Hint

The sensor sends high byte first, then low byte. Use parentheses carefully.

The sensor data is big-endian: high byte first. Option C correctly shifts raw[0] by 8 bits and ORs with raw[1]. It then converts to signed by subtracting 65536 if value > 32767.

🚀 Application

expert

How many bytes will this I2C read return?

You run this code on Raspberry Pi to read data from an I2C sensor: import smbus bus = smbus.SMBus(1) address = 0x68 count = 6 data = bus.read_i2c_block_data(address, 0x3B, count) print(len(data)) How many bytes will be printed?

A 6

B 5

C 7

D 0

💡 Hint

The count argument specifies how many bytes to read.

The read_i2c_block_data method reads exactly the number of bytes specified by the count argument. Here count=6, so 6 bytes are returned.

Practice

1. What is the main purpose of using the I2C protocol on a Raspberry Pi when working with sensors?

easy

A. To communicate with multiple sensors using unique addresses

B. To increase the Raspberry Pi's processing speed

C. To power the sensors directly from the Raspberry Pi

D. To store sensor data permanently on the Raspberry Pi

Solution

1. Step 1: Understand I2C communication basics

   I2C allows multiple devices to connect using unique addresses on the same bus.

2. Step 2: Identify the role of I2C with sensors

   The Raspberry Pi uses I2C to talk to sensors by addressing each one uniquely.

3. Final Answer:

   To communicate with multiple sensors using unique addresses -> Option A

4. Quick Check:

   I2C = Multiple devices, unique addresses [OK]

Hint: I2C uses addresses to talk to many devices [OK]

Common Mistakes:

• Thinking I2C increases processing speed
• Confusing communication with power supply
• Assuming I2C stores data permanently

2. Which Python library is commonly used on Raspberry Pi to read data from an I2C sensor?

easy

A. pygame

B. smbus

C. matplotlib

D. requests

Solution

1. Step 1: Recall common Python libraries for hardware

   For I2C communication on Raspberry Pi, the smbus library is standard.

2. Step 2: Eliminate unrelated libraries

   pygame is for games, matplotlib for plotting, requests for web requests, so they don't fit.

3. Final Answer:

   smbus -> Option B

4. Quick Check:

   Python I2C library = smbus [OK]

Hint: Use smbus for I2C sensor communication [OK]

Common Mistakes:

• Choosing pygame or matplotlib for sensor reading
• Confusing web libraries with hardware libraries

3. Given this Python code snippet reading from an I2C sensor, what will be printed?

import smbus
bus = smbus.SMBus(1)
address = 0x48
value = bus.read_byte_data(address, 0x00)
print(value)

medium

A. The string 'value' printed literally

B. An error because SMBus(1) is invalid

C. The I2C address 0x48 printed as decimal

D. The byte value stored at register 0x00 of device 0x48

Solution

1. Step 1: Understand the code's function

   The code reads one byte from register 0x00 of the device at I2C address 0x48.

2. Step 2: Identify the output of print(value)

   It prints the numeric byte value read from that register, not the address or a string.

3. Final Answer:

   The byte value stored at register 0x00 of device 0x48 -> Option D

4. Quick Check:

   read_byte_data returns register byte value [OK]

Hint: read_byte_data reads register value, not address [OK]

Common Mistakes:

• Thinking SMBus(1) is invalid (it's correct for Raspberry Pi)
• Confusing address with data value
• Expecting string output instead of number

4. What is wrong with this code snippet for reading I2C sensor data?

import smbus
bus = smbus.SMBus(0)
address = 0x40
data = bus.read_byte_data(address, 0x01)
print(data)

medium

A. read_byte_data requires three arguments, missing one

B. The address 0x40 is invalid for I2C devices

C. SMBus(0) is incorrect; Raspberry Pi usually uses bus 1

D. print(data) should be print(bus.read_byte_data)

Solution

1. Step 1: Check SMBus bus number

   On Raspberry Pi, I2C bus 1 is standard; bus 0 is usually invalid or unused.

2. Step 2: Verify other code parts

   Address 0x40 is valid, read_byte_data takes two arguments, and print(data) is correct.

3. Final Answer:

   SMBus(0) is incorrect; Raspberry Pi usually uses bus 1 -> Option C

4. Quick Check:

   Use SMBus(1) on Raspberry Pi [OK]

Hint: Use SMBus(1) for Raspberry Pi I2C bus [OK]

Common Mistakes:

• Using SMBus(0) instead of SMBus(1)
• Thinking I2C addresses like 0x40 are invalid
• Misunderstanding read_byte_data arguments

5. You want to read temperature data from a sensor at I2C address 0x5A which stores temperature in two registers: 0x07 (MSB) and 0x08 (LSB). How do you correctly combine these bytes to get the temperature value in Python?

hard

A. temp = (bus.read_byte_data(0x5A, 0x07) << 8) | bus.read_byte_data(0x5A, 0x08)

B. temp = bus.read_byte_data(0x5A, 0x07) + bus.read_byte_data(0x5A, 0x08)

C. temp = bus.read_byte_data(0x5A, 0x08) << 8 | bus.read_byte_data(0x5A, 0x07)

D. temp = bus.read_byte_data(0x5A, 0x07) * bus.read_byte_data(0x5A, 0x08)

Solution

1. Step 1: Understand byte order for temperature

   MSB is the high byte, so shift it left by 8 bits to make space for LSB.

2. Step 2: Combine MSB and LSB correctly

   Use bitwise OR to combine shifted MSB with LSB for full 16-bit value.

3. Final Answer:

   temp = (bus.read_byte_data(0x5A, 0x07) << 8) | bus.read_byte_data(0x5A, 0x08) -> Option A

4. Quick Check:

   Combine MSB << 8 with LSB using OR [OK]

Hint: Shift MSB 8 bits left, OR with LSB to combine bytes [OK]

Common Mistakes:

• Adding bytes instead of bitwise OR
• Swapping MSB and LSB order
• Multiplying bytes instead of combining bits