Interrupts are a critical part of embedded systems programming on ARM Cortex M0 chips. However, developers frequently encounter issues getting interrupts to trigger correctly. This comprehensive guide examines the common causes of interrupt problems on Cortex M0 and provides troubleshooting tips to resolve them.
1. Interrupt Handler Not Being Called
The most common symptom of interrupt issues is that the interrupt handler function is simply not being executed when the interrupt occurs. There are several potential reasons for this:
1.1 Interrupt is Disabled
Before using an interrupt on Cortex M0, you must enable it by setting the appropriate enable bit in the Nested Vectored Interrupt Controller (NVIC). For example, to enable EXTI0 interrupt:
NVIC_EnableIRQ(EXTI0_IRQn);
Forgetting to enable the interrupt is an easy mistake that will prevent the handler from running. Double check that the right NVIC enable function is called.
1.2 Faulty Interrupt Trigger Source
The source causing the interrupt needs to be working correctly. For hardware interrupts like EXTI, verify the trigger source (e.g. pin, port, ADC channel) is properly configured to generate the interrupt signal. For software interrupts like SVC, ensure the instruction is being executed.
1.3 Issues with Interrupt Priority
Cortex M0 allows setting priorities for interrupts. Higher priority interrupts can interrupt lower priority ones. If a high priority interrupt is continually triggering, it can prevent lower priority handlers from running. Check that priorities are configured as intended.
1.4 Stack Overflow
Interrupt handlers require stack space to store context during execution. If the stack overflows, the processor will fail to execute the handler. Allocate sufficient stack for all interrupt needs.
2. Multiple Interrupt Handlers Not Working
Sometimes only a single interrupt works while others are unresponsive. There are a few possible explanations for this:
2.1 Wrong Interrupt Handler Defined
The Cortex M0 vector table associates a specific handler function with each interrupt. If the wrong function is defined for a particular interrupt, that handler will never be called. Verify the correct handler is defined for each interrupt.
2.2 Interrupt Vectors Overwritten
The interrupt vector table resides in flash and defines the handlers. If code accidentally overwrites this data, the wrong handlers will execute. Put this table in protected flash to avoid issues.
2.3 Shared Interrupt Lines
Some microcontroller pins or peripherals share a single interrupt line. If so, the ISR must check the peripheral register that triggered the interrupt. Otherwise, it won’t be able to service the correct peripheral.
3. Interrupt Running Endlessly
An interrupt that executes continually can make the system unresponsive. Several flaws can cause this:
3.1 Failing to Clear Interrupt
Interrupt flags must be cleared at the end of the handler to reset the trigger source. Otherwise the interrupt will execute repeatedly after returning.
EXTI_ClearITPendingBit(EXTI_Line0);
3.2 Interrupt Triggered Continuously
If the source of the interrupt continues to trigger it, like an unchecked ADC result or uncontrolled GPIO pin, it prevents normal system operation. Modify the trigger source to stop endlessly generating interrupts.
3.3 Hardware Issue
Faulty or miswired hardware can cause an interrupt line to be continually triggered. Check for electrical issues on the board causing unexpected interrupts.
4. Interrupt Handler Code Not Working Correctly
In some cases the interrupt runs but the handler code does not behave properly. This is not a core interrupt issue but a problem with the code itself:
4.1 Race Conditions
The interrupt handler should only modify data guarded by disable interrupts/enable interrupts calls. Otherwise race conditions may occur with main code.
4.2 Resource Conflicts
Take care that interrupt and main code do not both access the same hardware resources like serial, ADC, GPIO etc. This can cause conflicts and errors.
4.3 Improperly Functioning Code
Like any other C code, the handler itself could have bugs. Thoroughly test the handler code to identify errors in its logic and execution.
5. Interrupt Only Working Sometimes
Intermittent problems with interrupts only sometimes working suggests a timing-related bug:
5.1 Race Condition with Main Loop
If the main code accidentally disables interrupts for long periods, it can intermittently prevent handlers from running and look like flaky behavior.
5.2 Hardware Limitations
Some interrupt triggers like external GPIO transitions have hardware debouncing and filtering. This puts a limit on how frequently the interrupt can reliably occur. Understand the hardware limits.
5.3 Defective Hardware
Faulty components or electrical issues could cause an interrupt line to only be triggered some of the time. Consider hardware problems as a potential cause of inconsistency.
6. Troubleshooting Interrupt Issues
Debugging interrupt problems can be challenging. Here are some techniques to help identify the root cause:
6.1 Verify Execution of Handler
Use LEDs, log statements, oscilloscope triggers, etc. to prove the handler function is actually running when expected. This can identify handler execution issues.
6.2 Inspect Stack Usage
Stack overflows are common interrupt bugs. Inspect stack usage in the disassembly or use ITM to validate there is adequate room for context save/restore.
6.3 Review Interrupt Registers
The NVIC and interrupt control registers contain useful debug data. Review them during testing to spot disabled interrupts, priority misconfigurations, etc.
6.4 Use Interrupt Debugging Tools
Many IDEs provide interrupt-aware debugging capabilities like breakpoints triggered on ISR entry/exit. Use these advanced tools to isolate problems.
6.5 Toggle Peripherals and Interrupts
Selectively enabling/disabling peripherals generating interrupts can help narrow down any conflicts and identify the problem source.
In summary, Cortex M0 interrupts are powerful but must be used carefully. Following best practices during configuration, properly structuring handler code, and leveraging debugging tools can help identify and resolve any issues you encounter.
7. Common Interrupt Problems on Cortex M0
Here is a quick summary of some of the most frequent interrupt issues seen on Cortex M0 and how to solve them:
- Interrupt Not Enabled – Use NVIC functions to enable interrupts.
- Wrong Handler – Define the correct handler function in the vector table.
- Stack Overflow – Allocate sufficient stack space for context saving.
- Forgot to Clear Flag – Clear pending interrupt bits at ISR end.
- Shared Interrupt Line – Handle multiple sources correctly in handler.
- Continuous Interrupt Triggers – Stop peripherals continuously interrupting.
- Race Conditions – Use critical sections to protect shared resources.
- Hardware Issues – Check for electrical defects causing problems.
Staying aware of these common pitfalls will help in debugging interrupt issues on Cortex M0 projects.
8. Best Practices for Interrupts on Cortex M0
Following these interrupt guidelines and recommendations will help avoid many problems:
- Enable required interrupts using the NVIC at startup.
- Define interrupt handlers correctly in the vector table.
- Allocate sufficient stack space for handler context saving.
- Clear interrupt flags at end of handlers to reset the interrupt.
- Use critical sections around shared data accessed in main code and handlers.
- Minimize handler code length and execution time.
- Avoid accessing the same hardware from both interrupt and main code.
- Verify that handler stack usage fits within allocated space.
- Handle shared interrupt lines carefully in handlers.
- Use interrupt-aware debugging techniques.
Planning ahead and following best practices when working with Cortex M0 interrupts will help avoid many debugging headaches down the road.
9. Debugging Stalled Interrupts on Cortex M0
When an interrupt handler on Cortex M0 seems to stall or fail to complete, there are a few steps you can take to help identify what is wrong:
- Use oscilloscope to check if handler is executing at all. This can identify if the processor is even entering the handler.
- single step handler code in debugger and monitor registers/memory. Look for corruption indicating stack overflow etc.
- Log timestamp on handler entry/exit. Long execution time may indicate problem.
- Toggle GPIOs on entry/exit to identify any hangs.
- Verify no infinite loops in handler code keeping it from exiting.
- Check for proper handler return code to avoid stack issues.
- Inspect disassembly for proper exception return instructions.
- Rule out electrical issues like noise disrupting processor in handler.
By methodically stepping through the handler execution using these techniques, you can identify what is preventing the handler from properly running to completion.
10. Prioritizing Interrupts on Cortex M0
Cortex M0 allows assigning priorities to interrupts to control the order they are handled:
- Higher priority interrupts will interrupt lower priority handlers already running.
- Set priority when enabling interrupt using NVIC functions.
- 0 is highest priority, 255 is lowest on Cortex M0.
- Tailor priorities to your application needs.
- Example: Timer interrupts high priority to keep accurate time.
- Example: UART RX low priority to avoid data loss.
- Get priorities wrong and high priority handlers can block others.
- Watch for priority inversion causing subtle problems.
- Mix priorities and polling for optimal real-time performance.
With planning, Cortex M0 interrupt priorities can significantly boost responsiveness of critical system events.
11. Common Interrupt Handling Mistakes
It’s easy to make mistakes when handling interrupts in Cortex M0 code. Be on the lookout for these common blunders:
- Forgetting to actually enable the interrupt.
- Failing to clear pending interrupt at ISR end.
- Accesses to global variables without protection.
- Calling non-reentrant functions like
printf. - Accessing same hardware from main code and ISR.
- Returning improperly from handler leading to stack corruption.
- Failing to account for interrupt latency when reading hardware.
- Enabling higher priority interrupts inside a handler.
- Calling slow library functions in interrupt handlers.
Stay vigilant against these patterns to create robust interrupt handlers in your Cortex M0 projects.
