« Previous

Index

Next »

---

Firmware Coding Standard — Power Management (FreeRTOS)

1) Purpose

Define how firmware manages power consumption using FreeRTOS and hardware features. Ensure predictable entry/exit from low-power states, no data loss, and consistent wakeup behavior.

---

2) Principles

• Idle = sleep: The system must automatically reduce power in idle periods.
• Deterministic wakeup: Resume must be fast, bounded, and consistent.
• State retention: Critical state must survive suspend/resume cycles.
• Granularity: Match power mode depth to idle duration (tickless idle, STOP, STANDBY).

---

3) FreeRTOS Integration

• Enable tickless idle (configUSE_TICKLESS_IDLE = 1) to allow long sleep when tasks are blocked.
• Application must implement vPortSuppressTicksAndSleep() (or vendor HAL equivalent) to enter MCU sleep modes.
• All sleep entry/exit paths must be bounded and ISR-safe.

---

4) Task & Driver Rules

• Tasks must block on IPC (queue, semaphore, event group), never spin.

• Drivers must:

  • Support suspend/resume APIs (drv_suspend(), drv_resume()).
  • Ensure DMA transfers complete before entering deep sleep.
  • Disable unused peripherals and clocks when not in use.
• ISR wakeup sources must be documented (GPIO, RTC, timers).

---

5) Power Modes

• Idle/Sleep: CPU stopped, peripherals on. Entry: automatic with tickless idle.
• Stop/Standby: Most peripherals off, SRAM partially retained. Entry: via PM manager task, requires app approval.
• Shutdown: Minimal state retention, only RTC/backup. Must store reset cause.

---

6) Retention & State Saving

• All critical state (system counters, last error, watchdog marks) saved in:

  • Retention RAM, or
  • Backup registers.
• Buffers in volatile RAM must be flushed (e.g., log ring to flash).
• DMA-in-progress must be aborted or safely resumed.

---

7) Time & Tick Handling

• On wake, update tick count to account for time spent in sleep.
• Use RTC as reference clock for deep sleep modes.
• Periodic tasks using vTaskDelayUntil() remain aligned across sleep/wake cycles.

---

8) Testing & Measurement

• Measure average current for:

  • Active (CPU busy).
  • Idle (RTOS tickless sleep).
  • Stop/Standby (deep sleep).
• Validate wake sources and latency under stress (e.g., burst GPIO interrupts).
• Ensure watchdog continues running in sleep if configured.

---

9) Anti-Patterns

• Busy-wait loops in tasks (prevents sleep).
• Leaving unused peripherals powered.
• Entering deep sleep with DMA or peripheral transaction in flight.
• Failing to log wake source (debug nightmare).
• Using sleep APIs directly in random tasks (must be centralized).

---

10) Review Checklist (Power)

• Tickless idle enabled and tested.
• Tasks block properly; no spin loops.
• Drivers implement suspend/resume.
• Deep sleep only entered after confirming no active DMA/critical tasks.
• Wake sources documented and verified.
• Reset cause and wake source logged on startup.
• Average current measured in all power modes.

---

11) CI/Lint Gates

• Fail build if:

  • Calls to vendor HAL_Delay() or busy loops found.
  • Direct sleep API calls from tasks (must go through PM manager).
  • Tickless idle disabled in release config.

---

12) Example Patterns

PM manager task

void pm_task(void *arg) {
    for (;;) {
        EventBits_t sys_state = xEventGroupWaitBits(sysEvt, EVT_IDLE, pdTRUE, pdTRUE, portMAX_DELAY);

        if (sys_state & EVT_IDLE) {
            if (can_enter_stop_mode()) {
                enter_stop_mode();
            }
        }
    }
}

Tickless idle override

void vPortSuppressTicksAndSleep(TickType_t expectedIdleTime) {
    if (expectedIdleTime > MIN_SLEEP_TICKS) {
        prepare_for_sleep();
        enter_sleep_mode();   // WFI/WFE or vendor HAL
        restore_after_sleep();
    }
}

Wake source logging

uint32_t src = hw_get_wakeup_source();
LOG_INFO("PM", "Wake from source=0x%08X", src);

---

« Previous

Index

Next »