When the Cortex-M1 processor encounters a fatal error early in the boot process, it will enter hard fault mode. This indicates there is something seriously wrong with the software or hardware setup. Troubleshooting hard faults can be challenging, but there are a few key things to check that may reveal the underlying problem.
Confirm the Hardware is Functioning
First, validate the basic hardware is operating correctly. Check the core power and clock signals with an oscilloscope to ensure they are stable. Examine the supply voltages with a multimeter to confirm they are within specifications. Inspect for any issues with circuit damage or shorts. Probe test points to verify expected voltages. Faulty hardware like a defective part or PCB issue can lead to unpredictable hard faults.
Review Clock Configurations
Many hard faults are caused by improper clocking setup. Double check the CMSIS core configuration to make sure the expected MHz value is configured. Validate any external oscillators or crystals are operating at the intended frequencies. Probe clock signals and measure with a frequency counter or oscilloscope. Incorrect core or bus clocks will lead to synchronization issues and crashes.
Check Memory Interfaces
Faulty memory interfaces are a prime suspect for early hard faults. Review the memory controller configuration and ensure the expected RAM and flash chips are defined properly. Verify the memory interface pins are connected to the right external memories. Check memory access waveforms for any abnormalities. Run memory tests to confirm read/write operations. Faulty memory access will result in crashes.
Examine Stack Pointer Initialization
An invalid stack pointer is a common cause of early faults. Check the vector table to ensure the stack pointer is initialized properly on reset. Verify the stack memory region is mapped correctly in the linker script. Inspect the disassembly to confirm the stack pointer (SP) is set up as expected. Use debugger stack walk to validate the SP location. An incorrect SP will lead to immediate hard faults.
Review Interrupt Vectors
Faulty interrupt vectors can trigger early crashes. Examine the vector table configuration and validate the interrupt handlers are defined and pointed to correct ISR functions. Verify the NVIC priority levels are appropriate. Check for any extra interrupts enabled during initialization. Use debugger breakpoints to step through ISR entries. Invalid vectors or spurious interrupts can cause unexpected faults.
Validate System Initialization
Improper system startup can also lead to early hard faults. Review the SystemInit() and startup code execution flow. Check for any enabled peripherals missing configuration code. Make sure clocking for external devices is activated properly. Confirm SystemCoreClock variable matches expected core clock rate. Use debugger to walk through reset sequence. Incorrect system initialization will cause instability.
Check Compiler Settings
Compiler configuration issues can result in hard faults too. Double check the compiler include paths, defines, and flags. Verify the target architecture and core settings match the Cortex-M1. Examine the compiled disassembly for any anomalies. Zero initialize all global variables. Inspect code alignment for illegal accesses. Invalid compiler options can introduce bugs or unintended instructions.
Examine Exception Handlers
Faulty exception handlers may be triggering the hard fault. Verify the HardFault_Handler() code attempts to recover or report failure before locking up. Inspect other exception handlers for proper stack frame linkage and return handling. Check for uninitialized handler variables. Use debugger to walk through exception entry and exit flows. Bad exception handling can lead to crashes.
Monitor for Memory Corruption
Memory corruption from stray pointer access or overflow errors could be the culprit. Setup MPU regions and utilize memory watchpoints to monitor for out-of-bounds accesses. Enable the MPU early on to catch invalid memory access. Examine stack usage and watch for overflow. Check all pointers for inadvertent modifications. Memory corruption will wreak havoc once code execution goes awry.
Review Peripheral Access
Invalid access to internal peripherals could be triggering the fault too. Check for aligned 32-bit peripheral register accesses. Verify bus wait states are configured properly. Use debugger to step through any peripheral initialization code. Watch for inadvertent writes to control registers. Attempting to access a peripheral incorrectly will crash the processor.
Check Reset Sources
Determine where the reset originated from before the hard fault occurred. Was it a power-on reset, external reset, or watchdog reset? Check the reset source register to understand the cause. Some reset sources like watchdog or brown-out indicate the processor is crashing repeatedly. Recurring resets suggest a more serious systemic issue is present.
Inspect Register Contents
Debug register contents provide clues to the fault source. Inspect stack pointer to validate location. Check program counter for bad code region. Did R0-R12 get initialized properly? Examine exception link registers for handler failures. Check fault status registers for flags. Processor state registers can reveal problems leading to crashes.
Enable Debugging Early
Hard faults are easier to diagnose if debugging is enabled earlier. Initialize debug module and setup debugger connection ASAP. Configure debug breakpoint(s) to pause execution at strategic points. Single-step through code using debugger to isolate failure. Debugging provides critical visibility so utilize tools as much as possible.
Minimize Code Before Main()
Reduce the amount of code executed prior to main() to simplify debugging. Avoid complex platform initialization code before this point. Only setup the minimal hardware needed for debugging and C environment support. Postpone unnecessary peripheral code until later. The less code run upfront, the easier it is to pinpoint the fault location.
Add Assertion Checks
Strategically add assertion checks to validate proper system state during initialization. This makes it easier to detect faults closer to the root cause. For example, assert that the stack pointer is set correctly after initialization. Assert key register values post-reset to ensure they are valid. More assertions provide more insight into a crash.
Establish Failure Recovery
Consider recovery options should the system fail and end up in hard fault. Implement a soft-reset mechanism to restart the system cleanly. Or utilize watchdog reset to reset the core automatically after a fault. Providing failure recovery options helps when debugging difficult faults.
Use Logging Not Debugging
For faults that happen intermittently, logging may be more effective than debugging. Initialize the logging peripheral early. Log key register contents, messages, and timestamps into a buffer. Dump logs after crashes to get history of events leading up to failure. Debugging can only catch issues that happen consistently.
Reproduce on Test Board First
Faults exacerbated by target board design issues can waste lots of debugging time. When possible, reproduce the hard fault on a test board or dev kit first. Simplify debugging by isolating hardware factors from software faults. Fix fundamental issues on the test platform before moving to the target board.
In summary, hard faults on Cortex-M1 can have many causes. But methodically checking the hardware, clocks, memory interfaces, interrupts, initialization and peripherals will usually reveal the fault trigger. Enabling debugging and logging as early as possible provides the visibility needed to diagnose these crashes quickly.
With rigorous validation and sound debugging techniques, hard faults can be resolved efficiently even in complex embedded systems utilizing Cortex-M1 processors.
