7.01 Syntax, Runtime, & Logic Bugs vs Microcontroller Hardware Faults (HardFault & MemManage)

1. Bug Classification Taxonomy

• Syntax Errors: Violations of the C++ grammar detected at compile-time (e.g. missing semicolons, type mismatches).
• Runtime Errors: Program halts unexpectedly during execution (e.g. division by zero, null pointer dereferencing, uncaught exceptions).
• Logic Errors: The program compiles and runs without crashing, but computes incorrect results (e.g. off-by-one loops, inverted boolean conditions).

2. Compiler Warning Diagnostics as First Line of Defense

Modern compilers can catch 90%+ of common logic bugs when configured with strict flags: -Wall -Wextra -Wpedantic -Wshadow -Wconversion -Werror. In embedded safety-critical systems, treating warnings as errors is mandatory.

1. How Bugs Manifest as Hardware Faults on ARM Cortex-M

In microcontrollers without an operating system, runtime bugs trigger hardware interrupt exceptions:

• HardFault: Triggered by unaligned memory access (if configured), executing undefined instructions, or bus errors during vector fetch.
• MemManage Fault: Triggered by Memory Protection Unit (MPU) rule violations (e.g. writing to read-only Flash or executing code from SRAM).
• BusFault: Triggered by attempting to access non-existent memory addresses or unpowered peripheral registers over the AHB/APB bus.
• UsageFault: Triggered by division by zero (when DIV_0_TRP is set in NVIC) or unaligned memory access.

2. Watchdog Timers (WDT) and Fail-Safe Recovery

Logic bugs like infinite loops or deadlock cause watchdog timer expiration. The hardware watchdog forcibly resets the microcontroller into a known safe state (e.g., de-energizing motor PWMs and logging the Program Counter register to non-volatile backup registers).

Production firmware replaces generic runtime crashes with compile-time static_assert and custom hardware fault capture handlers:

#include <cstdint>
#include <type_traits>

// 1. Compile-Time Invariant Verification
template <typename T, size_t BufferSize>
struct DmaRingBuffer {
    static_assert(BufferSize > 0, "Buffer size must be non-zero");
    static_assert((BufferSize & (BufferSize - 1)) == 0, "Buffer size must be a power of 2 for fast bitmask indexing");
    static_assert(std::is_trivially_copyable<T>::value, "DMA elements must be trivially copyable");
    
    T buffer[BufferSize];
    uint32_t head{0};
    uint32_t tail{0};
};

// 2. Hardware Fault Capture Handler (ARM Cortex-M Example)
struct FaultFrame {
    uint32_t r0, r1, r2, r3, r12, lr, pc, psr;
};

extern "C" void HardFault_Handler_C(FaultFrame* frame) {
    // Read NVIC Configurable Fault Status Register (CFSR)
    volatile uint32_t* const CFSR = reinterpret_cast<volatile uint32_t*>(0xE000ED28);
    uint32_t fault_reason = *CFSR;
    
    // Log crash telemetry (PC and LR) to persistent battery-backed backup SRAM
    // Safe shutdown: disable all actuators
    while (1) {
        // Halt or trigger controlled watchdog reset
    }
}