Implementing Non-Volatile Memory Dump Persistence

In this lesson, you will learn how to carve out dedicated flash memory segments to preserve critical system state data following a catastrophic firmware crash.

When a system fails—due to a hard fault or a watchdog reset—the volatile context of the CPU, such as register values and the stack pointer, is lost. To debug these events effectively, we must persist this "crash context" into non-volatile memory (NVM). The challenge lies in ensuring that our crash handler doesn't overwrite active application code or essential configuration data during the write process.

Defining Reserved Flash Regions

The first step is to define a static, reserved memory section in your linker script. By creating a custom section (e.g., .crash_data), you instruct the linker to isolate this block of flash away from the .text (code) and .data (initialized variables) segments.

/* Linker Script snippet */
MEMORY {
    FLASH_APP (rx) : ORIGIN = 0x08000000, LENGTH = 512K
    FLASH_CRASH (rw) : ORIGIN = 0x08080000, LENGTH = 4K
}

SECTIONS {
    .crash_data : {
        KEEP(*(.crash_data))
    } > FLASH_CRASH
}

Implementing Safe Persistence

Once the region is defined, your crash handler must interact with this memory safely. Since standard flash controllers require an erase cycle before writing, you cannot simply perform a memory copy. Your crash handler should:

1. Disable Interrupts: Prevent any further tasks from preempting the write process.
2. Verify Alignment: Ensure the crash data structure matches the flash controller’s page/word alignment requirements.
3. Atomic Writing: Write the "Panic Header" last. By marking the block with a "valid" flag only after the payload is fully written, you ensure that incomplete dumps—resulting from power loss during the crash—can be ignored during the next boot cycle.

By placing this segment at the end of your flash memory space, you minimize the risk of fragmentation. During the system’s post-reset initialization, the bootloader should inspect FLASH_CRASH. If the "valid" flag is set, it offloads the dump to a debugger or internal log before clearing the region for the next capture.

Key Takeaway: By using linker-defined reserved memory segments and an atomic "validity" flag, you can guarantee that critical crash diagnostic data survives system resets without corrupting your active application code.