ARM Cortex-M7 UsageFaultHandler Infinite Loop Due to Unclearable Fault Status Register

ARM Cortex-M7 UsageFault Triggered by Division-by-Zero with DIV_0_TRP Enabled

The ARM Cortex-M7 processor, like other Cortex-M series processors, includes a UsageFault exception mechanism to handle various types of programming errors. One such error is a division-by-zero operation, which can be trapped by setting the DIV_0_TRP bit in the Configuration Control Register (CCR, address 0xE000ED14). When this bit is set, any attempt to perform a division by zero will trigger a UsageFault exception. In the provided scenario, the Cortex-M7 is configured to trap division-by-zero errors by setting the DIV_0_TRP bit and enabling the UsageFault exception via the USGFAULTENA bit in the System Handler Control and State Register (SHCSR, address 0xE000ED24).

The assembly code provided intentionally performs a division-by-zero operation using the udiv instruction, which causes the processor to enter the UsageFaultHandler. The UsageFaultHandler is expected to read the Fault Status Register (FSR, address 0xE000ED28) to determine the cause of the fault and clear the fault status to allow the system to recover. However, the fault status appears to be non-clearable, causing the UsageFaultHandler to execute repeatedly in an infinite loop.

The core issue lies in the inability to clear the fault status in the FSR, which prevents the system from recovering from the UsageFault. This behavior is atypical, as the FSR is generally designed to be clearable by software. The repeated execution of the UsageFaultHandler suggests that the fault condition persists even after attempting to clear the FSR, leading to a system lockup.

Fault Status Register (FSR) Write Behavior and Persistent Fault Conditions

The Fault Status Register (FSR) in the ARM Cortex-M7 is a key register for diagnosing and recovering from fault conditions. The FSR contains bits that indicate the cause of a fault, such as division-by-zero, unaligned memory access, or invalid instruction execution. These bits are typically clearable by writing to the FSR, allowing the system to recover from the fault and resume normal operation.

In the provided scenario, the FSR is read and written back in an attempt to clear the fault status. However, the fault status remains set, indicating that the write operation to the FSR is either ineffective or the fault condition is being re-triggered immediately after the FSR is cleared. This behavior can be attributed to several possible causes:

1. Incorrect FSR Write Operation: The FSR may have specific write requirements, such as needing to write a specific value or sequence to clear the fault status. Writing back the same value read from the FSR may not be sufficient to clear the fault bits.

2. Persistent Fault Condition: The fault condition may be persistent, meaning that the underlying cause of the fault (e.g., division-by-zero) is still present or being re-triggered immediately after the FSR is cleared. This could occur if the faulting instruction is re-executed before the system has a chance to recover.

3. Hardware or Configuration Issue: There may be a hardware issue or configuration setting that prevents the FSR from being cleared. This could include errata in the processor, incorrect memory mappings, or misconfigured system control registers.

4. Interrupt or Exception Priority: The priority of the UsageFault exception may be set too high, causing it to preempt other system operations and preventing the system from recovering properly. This could result in the fault condition being re-triggered before the system can handle it.

Resolving Persistent UsageFault Conditions and Ensuring Proper FSR Clearance

To resolve the issue of the UsageFaultHandler executing repeatedly due to an unclearable FSR, the following troubleshooting steps and solutions can be implemented:

1. Verify FSR Write Operation: Ensure that the FSR is being cleared correctly by writing the appropriate value. The FSR may require writing a specific bit pattern to clear the fault status. Consult the ARM Cortex-M7 Technical Reference Manual (TRM) for the exact write requirements for the FSR. For example, some fault bits may require writing a ‘1’ to clear them, while others may require writing a ‘0’.

2. Check for Persistent Fault Conditions: Investigate whether the fault condition is being re-triggered immediately after the FSR is cleared. This can be done by adding debug prints or breakpoints in the UsageFaultHandler to monitor the state of the system before and after clearing the FSR. If the fault condition is being re-triggered, identify the source of the fault and ensure that it is properly handled before exiting the UsageFaultHandler.

3. Review System Configuration: Check the system configuration to ensure that there are no hardware or configuration issues preventing the FSR from being cleared. This includes verifying the memory mappings, system control registers, and any errata related to the Cortex-M7 processor. Ensure that the DIV_0_TRP and USGFAULTENA bits are set correctly and that there are no conflicting settings that could affect the behavior of the FSR.

4. Adjust Exception Priorities: Ensure that the priority of the UsageFault exception is set appropriately to allow the system to recover from the fault. If the UsageFault exception has a higher priority than other system operations, it may preempt those operations and prevent the system from recovering properly. Adjust the priority of the UsageFault exception to a level that allows the system to handle the fault condition without causing a lockup.

5. Implement Fault Recovery Mechanism: Implement a fault recovery mechanism in the UsageFaultHandler to ensure that the system can recover from the fault condition. This may include resetting the system, restarting the faulting task, or taking other corrective actions to prevent the fault condition from persisting. Ensure that the fault recovery mechanism is robust and can handle all possible fault conditions.

6. Debugging and Testing: Use debugging tools and techniques to further investigate the issue. This may include using a debugger to step through the code, inspecting the state of the processor registers, and analyzing the system behavior during the fault condition. Perform thorough testing to ensure that the fault condition is properly handled and that the system can recover without entering an infinite loop.

By following these troubleshooting steps and solutions, the issue of the UsageFaultHandler executing repeatedly due to an unclearable FSR can be resolved. The key is to ensure that the FSR is cleared correctly, that the fault condition is not being re-triggered, and that the system is configured to handle the fault condition properly. With the appropriate measures in place, the system can recover from the UsageFault and resume normal operation.