Understanding the Problem
When you're enabling interrupts in a microcontroller and your code gets stuck in an infinite loop, it's usually indicative of an issue in how the interrupts are configured, handled, or cleared. Interrupts in embedded systems are delicate operations, and multiple factors could lead to such behavior. Let’s explore some reasons and potential solutions.
Common Causes
Incomplete ISR (Interrupt Service Routine):
If your ISR does not handle the interrupt correctly, or if there’s a logical error causing it to be retriggered without clearing the interrupt flag, the system may repeatedly enter the ISR.
Interrupt Flags Not Cleared:
Many hardware interrupts require you to clear their pending flags inside the ISR. Failing to do so will result in the ISR being called continuously.
Shared Resources without Proper Synchronization:
If interrupts and the main code (or other ISRs) share resources like variables without proper synchronization (e.g., atomic operations or mutex), it may lead to unintended behaviors or infinite looping.
Priority Inversions:
Assigning incorrect priorities to interrupts might cause lower priority interrupts to preempt higher priority ones, leading to situations where the system's control flow gets stuck.
ISR Too Slow:
An ISR that takes too long to execute can cause issues in real-time systems where other interrupts are regularly triggered. This can result in an unexpected loop if the long ISR creates conditions for re-triggering itself.
Potential Solutions
Review and Correct the ISR Code:
Ensure your ISR has minimal logic. Avoid placing blocking code or lengthy operations inside the ISR. For example:
```c
void **attribute**((interrupt)) ISR_Handler(void) {
// Handle the interrupt quickly
FlagRegister &= ~INTERRUPT_FLAG; // Clear the interrupt flag
}
```
Clear Interrupt Flags:
Always clear interrupt flags in your ISR to prevent automatic re-entry.
```c
void **attribute**((interrupt)) ISR_Handler(void) {
ProcessInterrupt();
// Clear the interrupt source
INTERRUPT_FLAG = 0;
}
```
Use Volatile Keywords:
Shared variables between ISRs and the main code should be declared as volatile. This tells the compiler not to optimize the variable access that could lead to incorrect behavior.
\`\`\`c
volatile int sharedVariable;
\`\`\`
Implement Mutual Exclusion:
Use synchronization mechanisms like atomic operations or semaphores to protect shared resources, especially when interrupts modify these resources.
Check Interrupt Priority Levels:
Ensure critical interrupts have higher priorities as required by your system’s logic. Reconfigure using priority registers if necessary.
Test with Simpler Code:
Disable other features or interrupts and test with only the problematic interrupt enabled. This approach helps isolate and identify which interrupt or combination is causing issues.
Example Encouragement of Best Practice
Implement a circular buffer mechanism to handle interrupts at a higher level, thus keeping ISRs short:
#define BUFFER_SIZE 10
volatile int buffer[BUFFER_SIZE];
volatile int head = 0;
volatile int tail = 0;
void __attribute__((interrupt)) ISR_Handler(void) {
buffer[head] = ReadSensorData();
head = (head + 1) % BUFFER_SIZE;
INTERRUPT_FLAG = 0; // Clear the interrupt
}
void MainFunction(void) {
while (1) {
if (head != tail) {
ProcessData(buffer[tail]);
tail = (tail + 1) % BUFFER_SIZE;
}
}
}
By ensuring efficient and correct ISR implementation, managing shared resources, and accurately configuring interrupts, these practices should help you avoid infinite loops associated with interrupt handling.
