The volatile Keyword in Embedded C Programming

Created on: 2024-10-25

When should the volatile keyword be used in embedded C programming?

In embedded C programming, the volatile keyword is essential for ensuring that the compiler does not optimize access to variables that may change unexpectedly. Some common scenarios where volatile should be used follow.

1. Hardware Registers

When accessing memory-mapped hardware registers (e.g., UART status registers, GPIO registers), you should declare them as volatile. The value of these registers can change independently of the program flow (e.g., due to hardware events).

volatile uint32_t *const UART_SR = (uint32_t *)0x40013800; // Example address

2. Interrupt Service Routines (ISRs)

Variables modified within an ISR that are also accessed in the main program should be declared as volatile. This ensures the main program always reads the most recent value.

volatile int flag = 0;

void EXTI0_IRQHandler(void) {
    flag = 1; // Set flag in ISR
}

3. Shared Variables in Multi-threading or Interrupt Context

If you have variables shared between different threads or between an ISR and the main program, declare them as volatile to prevent the compiler from optimizing reads and writes, which could lead to inconsistent behavior.

volatile int shared_data = 0;

4. External Events

Variables that may change due to external events (like sensor data) should be declared as volatile to prevent the compiler from caching their values and to ensure they are read from memory each time they are accessed.

Summary of Where to use the volatile Keyword in Embedded C

Use the volatile keyword in the following situations:

  • Accessing hardware registers.
  • Modifying or accessing variables in ISRs that are also used in the main code.
  • Shared variables in multi-threaded contexts or between ISRs and the main program.
  • Variables subject to change from external events.

Using volatile appropriately helps ensure that your embedded application behaves correctly and reliably in response to hardware and timing conditions.