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.