Reading data from I2C device in Embedded C - Mini Project: Build & Apply

Reading data from I2C device

📖 Scenario: You are working on a small embedded system that reads temperature data from a sensor connected via the I2C bus. The sensor's data register holds the temperature value you want to read.

🎯 Goal: Build a simple program that reads one byte of data from the sensor's data register over I2C and prints the temperature value.

📋 What You'll Learn

Create a variable to hold the I2C device address

Create a variable to hold the register address to read from

Use a function call to read one byte from the I2C device at the given register

Print the read temperature value

💡 Why This Matters

🌍 Real World

Reading sensor data over I2C is common in embedded systems like weather stations, thermostats, and IoT devices.

💼 Career

Embedded developers often need to interface with sensors using I2C to collect data for processing and control.

1

Set up I2C device and register addresses

Create a variable called device_address and set it to 0x48. Create another variable called temp_register and set it to 0x00.

Embedded C

# Create variables for device and register addresses
# Your code here

Hint

The device address is usually given in hexadecimal. Use unsigned char for these variables.

2

Create a variable to hold the temperature data

Create a variable called temperature of type unsigned char to store the data read from the sensor.

Embedded C

unsigned char device_address = 0x48;
unsigned char temp_register = 0x00;

// Create a variable to hold the temperature data
# Your code here

Hint

Use unsigned char for the temperature variable since the sensor returns one byte.

3

Read temperature data from the I2C device

Use the function i2c_read_byte(device_address, temp_register) to read one byte from the sensor and store it in the variable temperature.

Embedded C

unsigned char device_address = 0x48;
unsigned char temp_register = 0x00;
unsigned char temperature;

// Read one byte from the I2C device
# Your code here

Hint

Call the i2c_read_byte function with the device and register addresses as arguments.

4

Print the temperature value

Use printf to display the text "Temperature: " followed by the value of temperature and a newline.

Embedded C

unsigned char device_address = 0x48;
unsigned char temp_register = 0x00;
unsigned char temperature;

temperature = i2c_read_byte(device_address, temp_register);

// Print the temperature value
# Your code here

Hint

Use printf("Temperature: %u\n", temperature); to print the value as an unsigned integer.

Practice

1. What is the main purpose of reading data from an I2C device in embedded C?

easy

A. To send commands to the device only

B. To get sensor or hardware information from the device

C. To reset the device hardware

D. To format the device memory

Solution

1. Step 1: Understand I2C device communication

   I2C devices often provide sensor or hardware data that the program needs to read.

2. Step 2: Identify the purpose of reading data

   Reading data means fetching information like sensor values, not sending commands or resetting.

3. Final Answer:

   To get sensor or hardware information from the device -> Option B

4. Quick Check:

   Reading I2C = Getting device data [OK]

Hint: Reading I2C means fetching data from device [OK]

Common Mistakes:

• Confusing reading with sending commands
• Thinking reading resets the device
• Assuming reading formats memory

2. Which of the following is the correct way to declare a buffer to read 4 bytes from an I2C device in embedded C?

easy

A. int buffer[1];

B. int buffer = 4;

C. char buffer;

D. char buffer[4];

Solution

1. Step 1: Understand buffer declaration for byte storage

   To store multiple bytes, we need an array of chars (bytes) of size 4.

2. Step 2: Check each option

   char buffer[4]; declares a char array of size 4, suitable for 4 bytes. Others are either single variables or wrong types.

3. Final Answer:

   char buffer[4]; -> Option D

4. Quick Check:

   Buffer for 4 bytes = char array size 4 [OK]

Hint: Use char array for byte buffers [OK]

Common Mistakes:

• Using single char instead of array
• Using int for byte buffer
• Declaring wrong array size

3. Given this code snippet, what will be the value of result after reading 2 bytes from the I2C device?

uint8_t buffer[2];
int result = i2c_read(device_addr, reg_addr, buffer, 2);

Assume i2c_read returns 0 on success and -1 on failure.

medium

A. 0

B. -1

C. 2

D. Undefined

Solution

1. Step 1: Understand i2c_read return values

   The function returns 0 if the read was successful, -1 if it failed.

2. Step 2: Analyze the code snippet

   The code reads 2 bytes and stores in buffer. If successful, result = 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Success return = 0 [OK]

Hint: i2c_read returns 0 on success [OK]

Common Mistakes:

• Assuming return is number of bytes read
• Confusing success with failure codes
• Ignoring return value meaning

4. Identify the error in this code snippet for reading 1 byte from an I2C device:

uint8_t data;
int status = i2c_read(device_addr, reg_addr, &data, 1);
if (status = 0) {
    process(data);
}

medium

A. Using assignment (=) instead of comparison (==) in if condition

B. Incorrect buffer type for reading data

C. Missing initialization of device_addr

D. Wrong number of bytes requested

Solution

1. Step 1: Check the if condition syntax

   The condition uses single '=' which assigns 0 instead of comparing.

2. Step 2: Understand effect of assignment in if

   Assignment returns 0 (false), so process(data) never runs even on success.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in if condition -> Option A

4. Quick Check:

   Use '==' for comparison in if [OK]

Hint: Use '==' to compare, not '=' in conditions [OK]

Common Mistakes:

• Confusing '=' and '==' in if statements
• Ignoring return status check
• Not using address-of operator for buffer

5. You want to read 3 bytes from an I2C device register and combine them into a 24-bit integer. Which code snippet correctly reads and combines the data?

uint8_t buf[3];
int status = i2c_read(device_addr, reg_addr, buf, 3);
uint32_t value = ???;

hard

A. value = buf[0] | (buf[1] << 8) | (buf[2] << 16);

B. value = (buf[2] << 16) | (buf[1] << 8) | buf[0];

C. value = (buf[0] << 16) | (buf[1] << 8) | buf[2];

D. value = buf[0] + buf[1] + buf[2];

Solution

1. Step 1: Understand byte order for 24-bit integer

   Assuming buf[0] is most significant byte, shift it left by 16 bits.

2. Step 2: Combine bytes with bitwise OR

   Shift buf[1] by 8 bits, add buf[2] as least significant byte.

3. Step 3: Check each option

   value = (buf[0] << 16) | (buf[1] << 8) | buf[2]; correctly combines bytes in big-endian order. Others mix order or sum bytes incorrectly.

4. Final Answer:

   value = (buf[0] << 16) | (buf[1] << 8) | buf[2]; -> Option C

5. Quick Check:

   Shift and OR bytes for 24-bit value [OK]

Hint: Shift bytes by 16, 8, 0 bits and OR [OK]

Common Mistakes:

• Mixing byte order
• Adding bytes instead of bitwise OR
• Not shifting bytes correctly