How to Fix Use of Global Variables in ISRs: Step-by-Step Guide

Understand the Implications of Global Variables in ISRs

• Global variables in Interrupt Service Routines (ISRs) can lead to issues like race conditions, side effects, and increased complexity in code maintenance.
• Global variables are accessible from any part of the code, which undermines the intended modularity and encapsulation.

Identify Global Variables Used in ISRs

• Conduct a code audit to find all instances where global variables are used within ISRs.
• Use static code analysis tools to automate the detection of global variables used in ISRs.

Evaluate the Necessity of Each Global Variable

• Consider whether the global variable must be shared across different execution contexts or if its use can be isolated.
• Replace the global variable with an ISR-local variable if persistence across ISR invocations is not needed.

Employ the Use of Local Variables

• Whenever possible, declare variables within the ISR itself to limit their scope and prevent unintended side effects.

Use Volatile Keyword Wisely

• For variables shared between ISRs and the main program, use the `volatile` keyword to prevent compiler optimizations that can lead to unexpected behavior.
• Example: `volatile int sharedVar;` ensures that each access reads the variable directly from memory.

Guard Critical Sections with Interrupt Enable/Disable

• Use interrupt disabling functions to protect accesses to shared variables.
• Example in C:

        
          \__disable_irq();
          sharedVar = 50;
          \__enable_irq();
        
      

Refactor Code Using Function Parameters

• If possible, pass data to the ISR using parameters instead of global variables. This can be achieved with callback functions or context structures passed to the function setting up the interrupt.

Consider Using Message Queues or Buffers

• For communication between ISRs and other code sections, use message queues or circular buffers to hold data, minimizing the risk associated with global variables.
• Implement an appropriate synchronization mechanism when accessing these buffers.

Use Static Variables When Limited Scope is Required

• If a variable should retain its value between ISR invocations but remain non-global, declare it as `static` within the ISR.
• Example:

        
          void ISR\_Handler(void) {
            static int lastValue = 0;
            // ISR logic here
          }
        
      

Review and Test Extensively

• Conduct thorough testing to ensure that changes made to manage global variables do not introduce new issues or bugs.
• Consider using hardware debugging tools to monitor variable states and ensure ISR execution integrity.