How to Use const to Save RAM in Embedded C

In embedded C, using const tells the compiler to store variables in flash memory (program memory) instead of RAM, saving RAM space. This is especially useful for fixed data like strings or lookup tables that do not change during program execution.

Syntax

The const keyword is placed before a variable declaration to indicate that its value will not change. This allows the compiler to place the variable in read-only memory (flash) instead of RAM.

Example parts:

• const: keyword to mark data as constant
• type: data type like int, char
• name: variable name
• = value: optional initialization

c

const int fixedValue = 100;
const char message[] = "Hello";

Example

This example shows how using const stores data in flash memory, saving RAM. The message string is constant and placed in flash, while counter uses RAM.

c

#include <stdio.h>

const char message[] = "Embedded C"; // Stored in flash memory
int counter = 0; // Stored in RAM

int main() {
    printf("Message: %s\n", message);
    counter++;
    printf("Counter: %d\n", counter);
    return 0;
}

Output

Message: Embedded C
Counter: 1

Common Pitfalls

Common mistakes include:

• Not using const for fixed data, wasting RAM.
• Trying to modify const data, which causes compiler errors.
• Forgetting to initialize const variables, which is required.

Example of wrong and right usage:

c

// Wrong: Modifying const data causes error
const int value = 10;
// value = 20; // Error: assignment of read-only variable

// Right: Use non-const if modification needed
int variable = 10;
variable = 20;

Quick Reference

Tips to save RAM using const in embedded C:

• Use const for fixed strings and lookup tables.
• Initialize const variables at declaration.
• Do not modify const data after initialization.
• Check your compiler/linker map to confirm data placement.

Key Takeaways

Use const to store fixed data in flash memory, saving RAM.

Always initialize const variables when declaring them.

Do not try to modify const data; it is read-only.

Use const for strings and lookup tables to reduce RAM usage.

Verify memory placement with your compiler or linker tools.