Bit field structures in Embedded C - Mini Project: Build & Apply

Bit Field Structures

📖 Scenario: You are working on a small embedded system that controls a device with several settings packed into a single byte. To save memory, each setting uses only a few bits.

🎯 Goal: You will create a struct with bit fields to represent device settings, set configuration values, and print the packed byte value.

📋 What You'll Learn

Create a struct called DeviceSettings with bit fields

Use exact bit sizes for each field as specified

Create a variable of type DeviceSettings and assign values

Print the packed byte value as an unsigned integer

💡 Why This Matters

🌍 Real World

Bit fields are used in embedded systems to save memory by packing multiple small settings into a single byte or word.

💼 Career

Understanding bit fields helps in low-level programming, device driver development, and working with hardware registers.

1

Create the bit field structure

Create a struct called DeviceSettings with these bit fields: mode (3 bits), speed (2 bits), enabled (1 bit), and reserved (2 bits).

Embedded C

// Define the struct DeviceSettings with bit fields
// Your code here

Need a hint?

Use unsigned int for each field and specify the number of bits after a colon.

2

Create a variable and assign values

Create a variable called settings of type DeviceSettings. Set mode to 5, speed to 2, enabled to 1, and reserved to 0.

Embedded C

typedef struct {
    unsigned int mode : 3;
    unsigned int speed : 2;
    unsigned int enabled : 1;
    unsigned int reserved : 2;
} DeviceSettings;

// Create a variable settings and assign values
// Your code here

Need a hint?

Use the initializer list with values in the order of the fields.

3

Create a pointer to access the packed byte

Create an unsigned char* pointer called bytePtr that points to &settings. This will let you access the packed bits as a single byte.

Embedded C

typedef struct {
    unsigned int mode : 3;
    unsigned int speed : 2;
    unsigned int enabled : 1;
    unsigned int reserved : 2;
} DeviceSettings;

DeviceSettings settings = {5, 2, 1, 0};

// Create a pointer bytePtr to point to settings
// Your code here

Need a hint?

Cast the address of settings to unsigned char*.

4

Print the packed byte value

Use printf to print the value pointed to by bytePtr as an unsigned integer. Use printf("%u\n", *bytePtr);.

Embedded C

typedef struct {
    unsigned int mode : 3;
    unsigned int speed : 2;
    unsigned int enabled : 1;
    unsigned int reserved : 2;
} DeviceSettings;

DeviceSettings settings = {5, 2, 1, 0};

unsigned char* bytePtr = (unsigned char*)&settings;

// Print the packed byte value
// Your code here

Need a hint?

The packed byte value should print as 45 because the bits represent the combined settings.