Common Variables and Data Types in Embedded C Programming

1. Common Variables in C Language

8-bit Unsigned Long Integer Range: [0-255]

16-bit Unsigned Long Integer Range: [0-65535]

32-bit Unsigned Long Integer Range: [0-4294967295]

Refer to the following:

Note:

1. Variable definitions should be at the beginning of the function, and no other statements should precede the variable definition statements.

2. In microcontroller development, only unsigned data types are typically used, unless complex floating-point calculations are involved.

3. When using statements like for loops to continuously accumulate a variable’s value, consider the overflow issue related to the data type range.

4. In STM32 development, there is no bit-type definition.

2. Enumeration

1. Enumeration Definition

It is a user-defined data type used to define a set ofnamed integer constants.

1) Definition Reference:

// Define enumeration type
enum Weekday {
    MONDAY,   // Default value is 0
    TUESDAY,  // Default value is 1
    WEDNESDAY, // Default value is 2
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
};
// Declare enumeration variable
enum Weekday today = WEDNESDAY;

2) Enumeration constants can explicitly specify any integer value; unspecified constants will increment from the previous value.

enum HttpStatus {
    OK = 200,
    CREATED = 201,
    BAD_REQUEST = 400,
    NOT_FOUND = 404,
    SERVER_ERROR = 500
};

enum Color {
    RED,    // 0
    GREEN = 5,
    BLUE,   // 6
    YELLOW = 10
};

3) Use typedef to create an alias

typedef enum {
    JAN, FEB, MAR, APR, MAY, JUN,
    JUL, AUG, SEP, OCT, NOV, DEC
} Month;

Month currentMonth = SEP; // Declare variable using alias

2. Essence and Advantages of Enumeration

• Essence: Enumeration constants are essentially integers of type int (can be negative)

Data is stored in flash, and enumeration variables can only be assigned values of identifiers within the enumeration.

• Advantages:

1. Improved Readability: if (today == WEDNESDAY) is much clearer than if (today == 2).

2. Easy Maintenance: If the value of a constant needs to be modified, it can be changed in one place in the enumeration definition.

3. Compiler Checks: Some compilers can provide better type checking than #define.

4. Debugging Friendly: Debuggers can display the names of enumeration constants instead of a meaningless number.

3. Structures

1. Definition

Definition: Different types of variables are packaged into a whole, forming a new “custom type”.

Example:

struct Tag {
    MemberList;
};          // ← Semicolon cannot be omitted

2. Principles to Follow

1. The address of the first member is 0.
2. The address of each member is a multiple of its own size.
3. The total size of the structure is a multiple of the largest type contained in its members.

Example:

struct test {
    char a;  //1
    short b;  //2
    char c;  //1
}; // The size of this structure is 6 bytes

4. Unions

1. Definition

The definition syntax of a union is similar to that of a structure, but uses the union keyword instead of struct. Below is the definition syntax for a union.

union Tag {
    MemberList;
};          // Semicolon cannot be omitted

2. Principles to Follow

All members share the same memory, and only one value can be stored at a time; size = size of the largest member.

Comparison with Structures

Dimension	struct	union
Memory	Each member has an independent address	All start from the same address
Size	Sum of all members + alignment	Size of the largest member + alignment
Usage	Package different data	Multiple “views” of the same data
Simultaneous Access	Possible	Illegal (undefined behavior)

Course reference links are as follows:

C Structures | Runoob Tutorial

“Basic Knowledge of C Language” on Variables – Bilibili