Hard Fault Handler Problem – Cortex-M0+

The Cortex-M0+ processor from ARM is an extremely popular 32-bit microcontroller that is used in a wide range of embedded systems. As with any microcontroller, robust fault handling is essential for stable operation. One common issue that can occur with Cortex-M0+ devices is problems with the hard fault handler.

The hard fault handler in Cortex-M0+ is software code that executes when a hard fault exception occurs. A hard fault indicates an error has happened that the processor cannot recover from on its own. Potential causes include an invalid memory access, divide by zero, or programming errors. Without a proper hard fault handler, the system will likely crash or hang when these errors occur. Implementing an effective handler is therefore critical.

Causes of Hard Fault Handler Issues

There are several possible reasons why the hard fault handler code may not work properly on a Cortex-M0+ processor:

• Buggy or incomplete handler code – The handler code itself contains bugs or is missing key functionality.
• Improper handler registration – The handler function is not correctly registered in the vector table.
• Stack corruption – The stack is corrupted at the time the handler is called, leading to crashes.
• Invalid stack pointer – The MSP stack pointer is not set up correctly for processing the exception.
• Priority issues – Higher priority interrupts disrupt handler execution.
• Fault during handler – A fault occurs within the handler itself, causing nesting issues.

These types of problems can manifest in different ways during operation. Typical symptoms include the processor entering a crash state, rebooting continuously, or locking up indefinitely.

Debugging Hard Fault Handler Issues

Debugging hard fault handler problems requires a methodical approach to isolate the root cause. Here are some effective techniques for Cortex-M0+ devices:

• Add logging – Log messages in the handler can reveal issues like double faults.
• Examine return stacks – The stacked PC values provide insight on the fault origin.
• Inspect register contents – Register values indicate the processor state at the time of fault.
• Check vector table – Confirm the handler address was correctly specified.
• Step through code – Use a debugger to step through handler execution.
• Stack overflow checks – Detect stack corruption issues.

Logging is particularly useful because it allows capturing runtime information when debugging may not be possible. The stacked PC values provide a snapshot of the code flow leading up to the fault, pinpointing where issues occurred. Register contents show the processor status like the fault status register and stacking process. Overall, leveraging a combination of techniques helps narrow down the root cause.

Resolving Hard Fault Handler Problems

Once the cause of the handler issue is identified, steps can be taken for resolution:

• Fix handler bugs – Problems in the handler code itself need to be corrected.
• Update vector table – An incorrect handler address needs to be fixed.
• Recover corrupted stack – If stack damage is causing crashes, the stack needs to be repaired.
• Adjust stack pointer – An improperly initialized MSP address must be corrected.
• Raise handler priority – The handler needs elevated priority to prevent interruptions.
• Eliminate nested faults – The code needs protection against cascading faults.

Updating the buggy handler code or vector table problems is relatively straightforward. For stack corruption or nested faults, the root cause that produces the issue needs to be addressed. This may entail fixing errant memory accesses in the application code. Prioritizing the handler via the NVIC helps minimize disruption. Added stack overflow checks can also detect corruption. With coding defects repaired and the proper handler operation verified, the overall system stability will be improved.

Best Practices for Hard Fault Handlers

Some design techniques help avoid issues with the hard fault handler on Cortex-M0+ devices:

• Minimize handler code – Simple compact code reduces bugs.
• Validate handler operation – Thoroughly test the handler with fault injection.
• Use stack checking – Detect corruption as early as possible.
• Enable fault diagnostics – Generate logs and register snapshots on faults.
• Trap handler errors – Catch faults in the handler via a wrapper.
• Handle critical tasks first – Prioritize key system state saving actions.
• Drop lower priority tasks – Suspend or abandon lower priority activity during handling.

Proper validation is essential to verify correct operation prior to deployment. Fault injection testing can validate recovery from different fault conditions. Defensively coding practices like stack checking and error trapping improves robustness and fault containment. Dedicated debugging and diagnostics capabilities provide visibility. With careful design and testing, problematic hard fault handlers can be avoided in Cortex-M0+ based systems.

Example Hard Fault Handler Code

Here is an example hard fault handler for Cortex-M0+ written in C: /* HardFault handler */ void HardFault_Handler(void) { __asm(“TST LR, #4”); __asm(“ITE EQ”); __asm(“MRSEQ R0, MSP”); __asm(“MRSNE R0, PSP”); __asm(“B hard_fault_handler_c”); } void hard_fault_handler_c(uint32_t *hardfault_args) { volatile uint32_t stacked_r0; volatile uint32_t stacked_r1; volatile uint32_t stacked_r2; volatile uint32_t stacked_r3; volatile uint32_t stacked_r12; volatile uint32_t stacked_lr; volatile uint32_t stacked_pc; volatile uint32_t stacked_psr; stacked_r0 = ((uint32_t)hardfault_args[0]); stacked_r1 = ((uint32_t)hardfault_args[1]); stacked_r2 = ((uint32_t)hardfault_args[2]); stacked_r3 = ((uint32_t)hardfault_args[3]); stacked_r12 = ((uint32_t)hardfault_args[4]); stacked_lr = ((uint32_t)hardfault_args[5]); stacked_pc = ((uint32_t)hardfault_args[6]); stacked_psr = ((uint32_t)hardfault_args[7]); /* Log handler entry */ log_hardfault_start(); /* Log stacked register contents */ log_registers(stacked_r0, stacked_r1, stacked_r2, stacked_r3, stacked_r12, stacked_lr, stacked_pc, stacked_psr); /* Perform platform specific fault handling */ platform_handle_hardfault(hardfault_args); /* Log handler exit */ log_hardfault_end(); /* Clear fault condition */ clear_fault_status(); }

This demonstrates some key implementation principles:

• Minimal assembly wraps C handler function
• Stacked register contents logged for diagnostics
• Platform specific handling delegated to separate function
• Fault status clearing required before return

The assembly wrapper preserves the stacked registers and selects the correct stack pointer before passing execution to the C code. Logging captures details like the program counter and fault status bits for post-mortem analysis. Platform portability is facilitated by isolating the platform specific handling. Clearing the status bits indicates recovery is complete. With these fundamentals covered, the handler can be customized further for each application.

Conclusion

The hard fault handler is a critical software component for achieving robust exception handling on Cortex-M0+ microcontrollers. Design defects, coding errors, and electrical issues can all produce hard faults requiring the handler to intervene. By leveraging debugging techniques like register inspection and stack checking, the root causes of handler problems can be isolated. Addressing issues in the handler code itself, correcting configuration errors, and enabling diagnostics all help resolve these problems. Using validation, prioritization, and defensive coding ultimately helps avoid hard fault handler faults. With an optimized handler, Cortex-M0+ systems gain the fault tolerance needed for stable operation in embedded applications.