Understanding the Problem
To safely access shared variables between an Interrupt Service Routine (ISR) and the main code in a C environment, one must focus on both synchronization and data integrity. The primary challenge is ensuring that shared variables are not corrupted by concurrent access from both the ISR and the main program. ISRs have higher priority and can preempt other running code, which can lead to race conditions if multiple access points are not properly managed.
Atomic Operations
Use volatile keyword: Declare shared variables as volatile so the compiler will not optimize out reads or writes due to its unpredictable nature.
```c
volatile int sharedVar;
```
Prefer atomic operations for single read/write operations, as they minimize the risk of data corruption.
If you need to read or write a shared variable atomically, consider using atomic instructions or built-in functions provided by some compilers.
```c
// Example using GCC built-in functions
**atomic_store_n(&sharedVar, 1, **ATOMIC_SEQ_CST);
int val = **atomic_load_n(&sharedVar, **ATOMIC_SEQ_CST);
```
Critical Sections
Disable Interrupts: Temporarily disable interrupts when accessing shared variables to protect critical sections that involve multiple memory operations.
```c
void updateSharedVariable() {
// Disable interrupts
__disable_irq();
// Critical section
sharedVar += 1;
// Enable interrupts
__enable_irq();
}
```
Use Spin Locks or Mutex (If Available): While not commonly available on microcontrollers, if you're using a more advanced embedded system environment, spin locks or mutexes can ensure that only one piece of code accesses the critical section at a time.
```c
void updateWithLock() {
spin_lock();
// Critical Section
sharedVar++;
spin_unlock();
}
```
Double Buffering
Double Buffer: Another technique is double buffering, which involves using two sets of data: one for ISR and one for the main program. The ISR writes to one buffer, and once a condition is met, the main program safely swaps to the reading buffer.
```c
volatile int bufferA, bufferB;
volatile int* readBuffer = &bufferA;
volatile int* writeBuffer = &bufferB;
void ISR_Handler() {
*writeBuffer = // Input data
}
void swapBuffers() {
__disable_irq();
volatile int* temp = readBuffer;
readBuffer = writeBuffer;
writeBuffer = temp;
__enable_irq();
}
```
Memory Barriers
Memory Barriers: Ensure that operations execute in the exact order you intend. Memory barriers, also known as fences, help prevent code optimization from reordering operations, which is crucial in embedded systems for predictable ISR interaction.
```c
// Example barrier in GCC
**asm** **volatile**("": : :"memory");
```
Proper Testing
Testing with Edge Cases: Thoroughly test your implementation under realistic and stress scenarios. Try to simulate conditions that would cause an ISR to preempt the main code to ensure the integrity of your solution.
Static Analysis Tools: Utilize static analysis tools to identify potential data access issues and race conditions in your codebase.
In conclusion, maintaining shared variable integrity between ISR and main code in C requires a combination of programming techniques and careful application of hardware features. Choose the appropriate technique based on your specific requirements and hardware constraints for optimal results.
