What is a fault exception in the ARM Cortex-M3?

A fault exception in the ARM Cortex-M3 is an unexpected event that occurs during program execution which transfers control to the fault handler. This allows the system to gracefully handle issues and continue normal operation. Fault exceptions are synchronous events generated by the processor in response to an instruction execution error.

Overview of Exceptions in ARM Cortex-M3

The ARM Cortex-M3 processor has a variety of exceptions that handle events during program execution. These include:

• Interrupts – Asynchronous external events such as peripheral inputs.
• Exceptions – Synchronous events occurring during instruction execution.

Exceptions can be further divided into:

• Traps – Intentional exceptions used for things like system calls.
• Faults – Unintentional exceptions such as illegal instructions.

Fault exceptions are triggered by the processor itself in response to an instruction execution error. This could be an attempt to perform an invalid memory access, execute an undefined instruction, or other illegal conditions. When a fault occurs, the processor immediately stops normal program flow and transfers control to a fault handler routine.

Types of Fault Exceptions

There are several types of faults that can generate exceptions in the Cortex-M3 processor:

• Usage fault – Generated by illegal use of the processor including invalid instructions, illegal unaligned access, or entering a state where stack pointers are invalid.
• Bus fault – Caused by invalid memory access errors including invalid address alignments, non-existing addresses, or faulty access rights.
• MemManage fault – Results from MPU or MMU violations during memory access operations.
• Debug monitor fault – Triggered when debug events occur such as breakpoint match.
• SVCall – Supervisor call instruction to switch to privileged execution mode.

The Cortex-M3 fault exceptions provide a structured way for the system to handle execution errors. When a fault occurs, important processor state information is automatically saved to registers and the fault handler can then analyze the cause and recover gracefully.

Fault Exception Handling Process

Here are the key steps when a fault exception occurs in the Cortex-M3:

1. The processor detects a fault condition and immediately halts execution of the current instruction.
2. Processor state information including PC, LR, and PS registers are automatically stacked onto the fault stack frame.
3. The fault status registers are updated with information about the specific fault.
4. The processor loads the fault handler address and jumps to that routine.
5. The fault handler software analyzes the status registers to identify the source and type of fault.
6. The system handles the fault which may include correcting invalid conditions and restarting interrupted tasks.
7. The handler executes a return instruction to resume normal program execution.

This structured exception handling allows the Cortex-M3 system to recover from faults in an organized way. The processor deals with the immediate halting of operations while the handler software performs necessary analysis and recovery handling before returning to normal code.

Cortex-M3 Fault Status Registers

The Cortex-M3 processor contains a number of fault status registers that provide information to identify the source and type of fault exception:

• Fault Status Register (HFSR) – Indicates the presence and source of a hardware fault.
• Debug Fault Status Register (DFSR) – Shows debug event faults from breakpoints, watchpoints, etc.
• MemManage Fault Status Register (MMFSR) – Provides details on MPU fault causes.
• Bus Fault Status Register (BFSR) – Identifies bus fault reasons such as invalid access.
• Auxiliary Fault Status Register (AFSR) – Additional fault status details for some processor implementations.

These registers contain status bits and fields that the fault handler software uses to identify issues that triggered the exception. This allows the hander to take suitable actions based on the specific fault situation.

Handling Different Fault Conditions

Since there are various types of faults, the exception handler needs to analyze the status registers and handle each case appropriately. Some examples include:

• For a bus error fault, the handler may clear the invalid address and retry the access.
• An undefined instruction fault could skip past illegal opcode and resume execution.
• A debug monitor fault can handle breakpoint match and continue debugging.
• Stacking errors may require resetting stack pointers before resuming tasks.

The handler can also recover from some faults by reversing improper register changes and status updates before restarting instructions. Fault handling mechanisms are highly application-specific based on the system requirements.

Configuring Fault Exceptions

Several options are available for configuring fault exception handling in the Cortex-M3 system:

• Set the priority levels of different fault exceptions using nested vectored interrupt controller (NVIC) priorities.
• Define the address ranges for the main stack pointer and process stack pointer.
• Setup memory protection regions using the optional MPU to generate fault exceptions in response to access violations.
• Enable fault exceptions and set whether faults activate as hard faults using the System Handler Control and State (SHCSR) register.

Properly configuring fault handling ensures that the most critical exceptions will be prioritized by the processor when multiple fault conditions occur simultaneously.

Benefits of Cortex-M3 Fault Exceptions

Here are some of the major benefits provided by the use of fault exceptions in the Cortex-M3 architecture:

• Allow processor problems to be handled in a structured way using dedicated exception mechanisms.
• Gracefully recover from faults to improve system resilience without crashes.
• Analyze fault causes using status register details to develop correct recovery procedures.
• Isolate and handle faults locally before they propagate to create bigger failures.
• Configure fault handling priority levels and behavior as needed for the application.
• Provide detailed diagnostic information through fault status registers.

Overall, the Cortex-M3 fault exceptions and mechanisms enable robust exception handling customized for the application requirements.

Conclusion

Fault exceptions in the ARM Cortex-M3 allow synchronous processor faults during instruction execution to be handled in an organized way. The CPU immediately suspends normal program flow to execute a fault handler, which can analyze fault register details to identify and resolve the issue before resuming code execution. This provides an essential set of mechanisms for recovering from instruction faults and improving system resilience against crashes in embedded applications.