Debugging a Cortex-M0 Hard Fault

Experiencing a hard fault on a Cortex-M0 can be frustrating for developers. However, with some guidance, hard faults can often be debugged and resolved. This article provides an overview of techniques and strategies developers can use when debugging hard faults on the Cortex-M0 architecture.

What is a Hard Fault?

A hard fault on the Cortex-M0 occurs when an exception happens that cannot be handled by the processor itself. Hard faults are usually triggered by events like:

• An attempt to execute an undefined instruction
• A data abort, such as referencing invalid memory
• A prefetch abort, like an invalid instruction fetch
• An assertion failure in the application code

When a hard fault happens, code execution stops and the core enters an exception handler. Hard faults are disruptive, so identifying the root cause is important.

Preparing to Debug

Before you can start debugging a Cortex-M0 hard fault, make sure you have the right tools configured:

• Use a debugger that supports the Cortex-M0, like Keil MDK, IAR EWARM, or OpenOCD and GDB.
• Enable debug mode and connect your debugger.
• Build your code with debug symbols enabled.
• Have the memory map, register definitions, and datasheets available.

Debugging works best when you have insight into the running application. Consider initialize the core, enable interrupts, and get the system into a semi-working state before testing complex code.

Locating the Crash Site

When a hard fault occurs, key processor registers contain clues about the root cause. Here are the important registers to check:

• HFSR – HardFault Status Register. Indicates the source of the fault.
• CFSR – Configurable Fault Status Register. More fault details.
• BFAR – BusFault Address Register. Fault address for bus errors.
• PC – Program Counter. Points to fault location.

Analyze these registers first to identify the type of fault. Remember that the PC may point to a location after the actual fault site since the pipeline needs to drain. You’ll need to work backwards to find the root cause.

Hardware Issues

Many hard faults stem from hardware problems versus software bugs. Watch for these common hardware issues:

• Stack overflow – Check SP and MSP to see if the stack bounds were exceeded.
• Heap corruption – Use BFAR to identify the offending memory access.
• Invalid peripheral access – Enable peripherals properly and verify register maps.
• Faulty external memory – Validate external memories and interfaces.

Hardware issues manifest as bus faults or undefined instruction exceptions. The BFAR, CFSR, and HFSR registers will highlight the general area to investigate.

Software Defects

Software bugs can also lead to hard faults if the code makes invalid assumptions or accesses. Common software defects include:

• Null pointer access – Check for NULL pointers before dereferencing.
• Uninitialized variables – Double check static and global variables.
• Array overflows – Audit buffer bounds and enforce limits.
• Stack overflows – Be wary of large stack allocations.

Use BFAR to locate faulty memory addresses. Then inspect your code to identify flaws around pointer use, memory allocation, and array manipulation.

Debugging Tactics

With potential fault domains identified, you can start narrowing down root causes. Useful tactics include:

• Code inspection – Check for bugs in the identified fault area.
• Logging – Add trace messages to reconstruct the crash.
• Memory profiling – See how functions use the stack and heap.
• Assertions – Validate assumptions and inputs.

Iteratively apply these techniques to shrink the suspect code regions. Assertions aid significantly by exposing flawed logic closer to the source.

Recovery Strategies

Once the root cause is found, you can employ recovery strategies like:

• Fix defects – Correct bugs, unsafe code, and hardware issues.
• Fault handlers – Add an exception handler for specific faults.
• Watchdogs – Use a watchdog timer to escape locks.
• Reset – Safely reset the system after logging faults.

Focus first on eliminating the underlying hardware and software flaws. For unrecoverable faults, logging details helps during post-mortem debugging. Watchdogs and resets should be last resorts.

Tips for Effective Debugging

Here are some key tips to help streamline debugging Cortex-M0 hard faults:

• Review your toolchain and debugger setup first.
• Initialize hardware properly and incrementally test subsystems.
• Understand fault registers and architecture handling.
• Inspect code around identified fault regions.
• Add logging and assertions to quickly expose issues.
• Fix root causes instead of just masking faults.

Patience and a methodical approach is required to resolve elusive hard faults. Following fundamental embedded practices will help avoid defects and support debug efforts.

Conclusion

Debugging hard faults on Cortex-M0 devices certainly can be challenging. But the core itself provides valuable error information to help track down root causes. By combining this architecture insight with proven debugging techniques, developers can systematically isolate and correct hard faults.