In this lesson, you will learn how to turn your processor’s hardware exception vectors into an early-warning system that detects and logs critical execution faults before they propagate into system-wide failures.
The Role of the Exception Vector Table
In embedded architectures, the processor maintains an Exception Vector Table—a memory-mapped array of addresses pointing to the code that should run when a hardware error occurs. While many developers leave these as simple infinite loops (or "hangs"), in safety-critical design, these vectors serve as the "ground truth" for hardware-level integrity.
By strategically populating these vectors with custom diagnostic routines, you transition from a "silent crash" to a "reported failure." When a hardware fault occurs—such as an illegal instruction, a bus error, or a divide-by-zero—the CPU automatically jumps to the specific vector assigned to that fault.
Implementing a Diagnostic Trap
Instead of a simple while(1), your exception handlers should function as a "Black Box" logger. When the processor enters the handler, you are in a privileged state, but memory may be unstable. Your priority is to capture the processor's state and context for post-mortem analysis.
void HardFault_Handler(void) {
// 1. Capture stack frame (registers at time of fault)
uint32_t *sp = (uint32_t *)__get_MSP();
uint32_t pc = sp[6]; // Program Counter where fault occurred
// 2. Log fault context to non-volatile memory or safe buffer
FaultLog_Save(FAULT_TYPE_HARD, pc);
// 3. Trigger safe recovery (e.g., system reset or fail-safe state)
System_EnterSafeState();
NVIC_SystemReset();
}
Architecting for Detection
The power of this approach lies in granularity. By implementing specific handlers for UsageFault, BusFault, and MemManageFault, you can distinguish between a logic bug (illegal instruction) and a hardware integrity issue (memory access violation).
When designing your architecture, treat these vectors as the final line of defense. By logging the Program Counter (pc) and relevant status registers (like the Configurable Fault Status Register on ARM Cortex-M), you can perform remote diagnostics to identify which module triggered the fault. This data is critical for refining your Fault Injection Testing results and ensures that when the system does fail, it fails into a known, logged, and safe state rather than behaving unpredictably.
Key Takeaway: Transforming exception vectors into active diagnostic handlers turns catastrophic hardware failures into actionable telemetry, ensuring your system captures the "who, where, and why" of a crash.