Cortex-M0+ hangs on return

It can be frustrating when a Cortex-M0+ processor seems to hang or freeze during operation. One common situation where this occurs is when the processor hangs on return from a function call. There are several potential reasons why a Cortex-M0+ may hang on return, along with some steps you can take to debug and resolve the issue.

Possible Causes

Here are some of the most likely reasons that a Cortex-M0+ processor could hang when trying to return from a function:

• Stack corruption: If the call stack gets corrupted somehow, the processor may not be able to reliably return to the correct address. Stack corruption could happen due to buffer overflows, stack overflows, or improper stack manipulation.
• Incorrect stack linkage: The compiler uses the stack to store information like the return address for functions. If the stack linkage is set up incorrectly, the return address could be corrupted.
• Improper exception return: If an exception occurs and the exception handler does not properly return using special exception return instructions, it could cause the main stack to be corrupted.
• Infinite recursion: If a function calls itself recursively without ever reaching a base case, it could continually use up stack space resulting in a stack overflow.
• Insufficient stack space: Similarly, if the call stack grows too large for the allocated stack size, it could cause stack overflow issues.
• Bad function pointers: If a function pointer is corrupted and leads to calling invalid code when trying to return, it could lead to a hang.
• Faulty hardware: In rare cases, faulty hardware like a defective processor or memory could exhibit issues only under certain code execution patterns.

Debugging Steps

Debugging a Cortex-M0+ hang on return issue requires methodically ruling out the possible causes. Here are some steps to take:

1. Check stack pointer: When the hang occurs, check the stack pointer address. Does it point somewhere invalid? This could indicate stack corruption.
2. Examine call stack: Use a debugger or add trace print statements to examine the call stack. Look for corruption or unexpectedly deep call stacks.
3. Verify return addresses: Check that return addresses on the stack are valid code locations. Invalid return addresses may indicate stack issues.
4. Add stack overflow checks: Add a stack overflow exception handler or monotonic stack usage checks to detect stack issues.
5. Verify stack linkage: Check that the compiler is properly linking call frames on the stack. Consult compiler documentation.
6. Check exception handling: Verify exception handlers properly return using ERET instead of BX instructions.
7. Simplify code: Try to simplify the code path leading up to the hang to isolate the problem area.
8. Reproduce on simulator: If possible, reproduce the hang on an instruction set simulator where you have more visibility.
9. Stress test hardware: Run hardware diagnostics to rule out a CPU, memory, or bus issue.

Resolving Underlying Issues

Once the source of the return hang is identified, you can take action to resolve it:

• Fix stack corruption issues by correcting buffer/stack overflows and preventing stack manipulation bugs.
• Adjust linker scripts or compiler settings to ensure correct stack frame linkage.
• Use ERET appropriately from exception handlers and avoid corrupting main stack.
• Detect and prevent stack overflows by checking stack usage and/or increasing stack size.
• Validate function pointers before calling functions.
• Eliminate infinite recursion scenarios.
• Replace faulty hardware like memory chips or even the processor itself if needed.

Prevention Tips

Here are some general tips to help prevent Cortex-M0+ hangs on return in your projects:

• Use stack overflow protection and monitoring features offered by many compilers and debuggers.
• Initialize stack areas to known values to help detect corruption.
• Validate inputs to avoid buffer overflows and memory safety bugs.
• Limit stack usage by reducing use of local variables and recursive functions.
• Profile worst-case stack usage to set stack sizes appropriately.
• Use static analysis and sanitizers to detect issues before runtime.
• Perform extensive testing to catch problems early.
• Use assertions or other checks to validate assumptions and correct behavior.

Summary

Cortex-M0+ hangs when returning from functions can stem from a variety of issues like stack corruption, stack overflows, or hardware defects. A systematic debugging approach examining stack pointers, return addresses, exception handling, stack usage, and hardware can uncover the root cause. With careful coding practices, testing, and debugging, these frustrating return issues can be prevented or resolved.