Embedded Firmware Dev

Embedded Firmware Development

FreeRTOS

• Give each task a clear ownership boundary. Shared resources need a deliberate synchronization strategy.
• Prefer task notifications for one-to-one wakeups, queues for data transfer, event groups for combined state, mutexes for mutual exclusion, semaphores for one-way signaling.
• Use a mutex (not binary semaphore) when mutual exclusion matters and priority inheritance is needed.
• Make every blocking wait explicit: use a timeout unless an infinite wait is deliberate.
• Keep timer callbacks short and non-blocking — use them to schedule work, not do it.
• Avoid holding locks across flash, storage, or long operations.
• Size task stacks from worst-case call chains. Recheck high-water marks after adding buffers or deeper call trees.
• Use ISR-safe APIs for interrupt-to-task handoff. Keep ISR state capture minimal.
• Avoid priority inversion: don't hold shared locks across blocking I/O or long processing.
• If a task can be paused/restarted/signaled from multiple places, define resume, timeout, and recovery paths explicitly.
• For cross-task pointer ownership: make lifetime and invalidation rules obvious.
• Prefer the smallest critical section that protects the state transition. Don't wrap whole operations in locks when narrower ordering suffices.
• For objects handed between threads: define whether the receiver owns, borrows, or copies before crossing the boundary.

Interrupts / ISR

• Keep the interrupt path short, deterministic, and bounded. Capture minimum state, clear the source, defer expensive work.
• Do not block, sleep, allocate heap, or call non-ISR-safe APIs from an ISR.
• Prefer top-half/bottom-half split when the handler needs more than quick state capture and wakeup.
• Make shared-state ownership explicit. Use minimum synchronization for the data being shared.
• If an ISR wakes a task, use the ISR-safe RTOS primitive and preserve yield-from-ISR behavior.
• Define clear read/clear/re-enable ordering to avoid losing edges or creating re-trigger loops.
• Avoid logging and complex branching in the hot interrupt path.
• If code runs from both task and ISR context, separate wrappers so the ISR-safe path stays obvious.

Async Lifecycle Cleanup

• Any async flag (pending, in-progress, busy, data-ready) that can be set during normal operation must be explicitly cleared in every stop, init, reset, power-off, and error-recovery path. A stale flag silently blocks the next operation.
• When adding a new async operation, audit all lifecycle entry points and ensure each path resets flags to known-safe.
• Cleanup must happen before any new operation is attempted, not after.

// CORRECT: every async flag cleared in stop path. No stale state survives restart.
uint8_t comm_stop_sample(void) {
    g_comm.data_ready         = false;
    g_comm.state              = COMM_STATE_IDLE;
    g_comm.activating         = false;
    g_comm.command_fail_count = 0;
    g_comm.protocol_locked    = false;
    g_comm.communication_lost = false;
    g_comm.warmup_start_time  = 0;
    comm_command_complete();                     // Release any pending I/O
    memset(&g_comm_data, 0, sizeof(g_comm_data));// Reset cached data
    comm_process_faults(false);                  // Clear fault detection state
    return COMM_OK;
}

// BAD: half the flags survive — next start inherits stale state
void comm_stop_bad(void) {
    g_comm.state = COMM_STATE_IDLE;              // Only state changed
    // Missing: data_ready, protocol_locked, communication_lost, fail_count...
    // Next start: data_ready==true blocks first sample; fail_count persists
}

// CALLER GUARD: clear stale flags at power-off boundaries before re-init
void comm_handler(void) {
    if (!power_get_status() && g_comm.command_pending) {
        comm_command_complete();                 // Clear before deinit
    }
    if (power_get_status()) {
        comm_state_process();
    }
}

One-Shot Event Consumption

• When a low-level driver produces a transient event that multiple higher-level consumers need, use an atomic check-and-clear (consume) API rather than shared flags each consumer clears manually.
• Manual clearing by multiple consumers creates races: consumer A clears before B reads, or B reads a flag already set again by the next cycle.
• The consume primitive returns whether the event occurred and atomically clears the latch — every interested consumer observes the event exactly once per occurrence.

// Atomic check-and-clear: every consumer sees the event exactly once
static volatile uint32_t event_latch;

uint32_t event_consume(uint32_t mask) {
    uint32_t primask = __get_PRIMASK();
    __disable_irq();
    uint32_t pending = event_latch & mask;
    event_latch &= ~mask;               // Clear consumed bits atomically
    if (!primask) __enable_irq();
    return pending;                     // Non-zero = event occurred this cycle
}

// Each consumer independently observes — no races between consumers
void ui_consume(void) {
    if (event_consume(EVT_SENSOR_READY)) update_display();
}
void log_consume(void) {
    if (event_consume(EVT_SENSOR_READY)) write_log();
}

Storage / Persistence

• Separate object corruption from schema change. Rebuild the whole store only when versioned layout rules require it.
• Prefer recoverable write paths: write primary → read back and verify → write backup → read back and verify.
• During delete, reset, or migration, preserve at least one valid recoverable copy.
• Prefer targeted repair and re-sync over destructive reinitialization.
• Treat startup repair, steady-state writes, emergency writes, and factory reset as separate paths with explicit guarantees.

Boundary Analysis

• When thresholds trip at edges, inspect debounce latency, sample timing, and ring-vs-bounded assumptions before tuning constants.
• When behavior diverges by mode, compare all branches side by side instead of debugging only the failing branch.
• When a defect appears on only one trigger path, diff which state each caller resets, preserves, or derives.
• For transient inconsistencies, inspect raw state, derived state, and cached state separately — stale data in any layer masquerades as a timing problem.
• When a system undergoes mode switch, direction reversal, or re-initialization, allow a bounded tolerance window for the first post-transition deviation. Treating it with steady-state thresholds produces false error accumulation.

Deep Reference

This skill's references/ directory contains in-depth material. Load when you need more than the core rules:

See also: Skill("state-machine-design") for state transition rules, Skill("debug-methodology") for debugging process, Skill("hardfault-triage") for stack overflow and ISR crash triage.