Usage fault exception in ARM Cortex M

A usage fault exception in ARM Cortex M processors indicates an invalid memory access or illegal attempt to execute an instruction. It occurs when the processor tries to access restricted memory regions, executes an undefined instruction, uses a coprocessor instruction when no coprocessor is present, accesses a disabled memory region, or performs an unaligned memory access. The usage fault handler is invoked to handle this exception.

What causes a usage fault exception?

The common causes of a usage fault exception in ARM Cortex M are:

• Accessing a disabled memory region – The Memory Protection Unit (MPU) can be used to disable access to certain memory regions. Any access to a disabled region will cause a usage fault.
• Executing an undefined instruction – This happens when the processor encounters an instruction that it cannot decode. This could be due to a bug in code.
• Using a coprocessor instruction when no coprocessor is present – Coprocessor instructions should only be executed when the corresponding coprocessor hardware is present. Otherwise, it leads to a usage fault.
• Performing an unaligned memory access – The Cortex M processors require accesses to be aligned. Any unaligned access will generate a usage fault.
• Accessing a memory location outside the allowed range – This is caused by corrupted pointers or out-of-bounds array accesses in code.
• Accessing restricted core registers – Attempts to access system control registers without appropriate privileges will result in a usage fault.

Usage fault handler

When a usage fault occurs, the fault handler is called. This is configured in the vector table, which contains the address of the usage fault handler function. The default handler in the Cortex M vector table points to a weak definition, which contains an infinite loop. Developers can override this with their own usage fault handler by defining the function and updating the vector table entry.

The usage fault handler would typically do the following:

1. Store processor context – This includes saving the stack pointer, program counter, and other registers that may be useful for debugging.
2. Log fault information – The UsageFault_IRQn status register provides details on the cause of the usage fault. This can be logged for troubleshooting.
3. Take appropriate recovery action – For recoverable errors, the handler may attempt corrective measures like terminating the faulty task. Otherwise it may reboot the system.

Debugging usage faults

To debug the cause of a usage fault exception, we need to examine the processor state when the fault occurred. Useful techniques include:

• Examining the stack – The stack contains function call history which can reveal the sequence of events leading to the fault.
• Checking UsageFault_IRQn register – This identifies the source of the fault e.g. unaligned access, invalid instruction etc.
• Enabling fault reporting – Tools like SEGGER SystemView can be used to get runtime reporting of faults.
• Attaching a debugger – A debugger snapshot taken when the usage fault occurs is very useful for analyzing register contents, stack traces and variable values.
• Tracing function calls – Logging function entry and exit can help recreate the program flow.

Based on the debugging data, we can identity flaws like:

• Buggy code accessing restricted memory regions
• Stack/heap corruption due to buffer overflows
• Improper handler for undefined instructions
• Race conditions due to missing synchronization

Fixing these root causes will help resolve the usage fault exception.

Best practices to avoid usage faults

Some good programming practices can help avoid common usage fault errors:

• Use memory protection features like MPU to restrict access to memory regions and peripherals. Define a memory map and protect each section appropriately.
• Enable alignment checks to trap unaligned accesses early during development. Enforcing aligned accesses is highly recommended.
• Validate user input and buffer lengths to prevent buffer overflows.
• Use stack overflow protection and monitoring.
• Define handlers for undefined instructions to detect errant behavior.
• Use a device driver model for cleaner interaction with peripherals.
• Employ defensive programming techniques like safe casting, bounds checking, assertions etc.
• Perform static analysis to detect issues early in the development cycle.
• Test code extensively and run fault injectors to uncover corner case flaws.

With good development practices, usage fault exceptions can be minimized. But exceptions may still occur in complex systems due to unanticipated conditions. So the usage fault handler plays an important role as the last line of defense.

Usage faults vs Hard faults

Usage faults and hard faults are two types of exceptions in Cortex M devices. The key differences are:

• Causes – Usage faults occur due to invalid memory access or illegal instructions. Hard faults represent more catastrophic errors like bus faults.
• Recoverability – Usage faults may be recoverable by terminating the offending task. Hard faults are usually unrecoverable.
• Debugging data – Usage faults provide debugging details like fault status registers. This data is not available with hard faults.
• Performance – Usage fault handlers can have some performance overhead if they are triggered frequently. Hard faults directly escalate to resets.
• System impact – Usage faults only impact the task where they occur. Hard faults bring down the entire system.

So in summary, usage faults are caused by software errors and may be recoverable via the usage fault handler. Hard faults imply serious hardware failures that require system restarts.

Conclusion

Usage fault exceptions are an important mechanism to trap illegal memory accesses and invalid instruction execution on ARM Cortex M devices. When handled properly, usage faults provide the developer valuable insights into flaws in code. This allows issues to be fixed during development rather than failing in production. With good programming practices, usage fault occurrences can be reduced. But having a robust usage fault handler is still important to handle unforeseen situations and exceptions arising from bugs.