power-mgmt-patterns

power-mgmt-patterns

Knowledge Base

Power management patterns for battery-operated embedded systems.

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Pattern 1: Sleep Entry Checklist

Before entering Stop/Standby mode, complete these steps in order:

void prepare_for_stop2(void)
{
    /* 1. Complete all pending DMA transfers */
    HAL_DMA_Abort(&hdma_spi1_tx);

    /* 2. Flush UART TX buffer */
    while (!LL_USART_IsActiveFlag_TC(USART2)) {}

    /* 3. Disable peripheral clocks */
    __HAL_RCC_SPI1_CLK_DISABLE();
    __HAL_RCC_USART2_CLK_DISABLE();
    __HAL_RCC_ADC1_CLK_DISABLE();

    /* 4. Configure unused GPIOs as analog (lowest leakage) */
    GPIO_InitTypeDef gpio = {
        .Mode = GPIO_MODE_ANALOG, .Pull = GPIO_NOPULL,
        .Pin  = GPIO_PIN_2 | GPIO_PIN_3   /* Unused pins on GPIOB */
    };
    HAL_GPIO_Init(GPIOB, &gpio);

    /* 5. Configure wake-up: EXTI0 (PA0) falling edge */
    HAL_EXTI_GetHandle(&hexti0, EXTI_LINE_0);
    /* EXTI already configured; just ensure it's enabled */

    /* 6. Enter Stop 2 */
    HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);

    /* 7. After wake: restore clocks */
    SystemClock_Config();
    MX_USART2_UART_Init();
    MX_SPI1_Init();
}

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Pattern 2: RTC Alarm Wake-Up

Wake MCU at a specific time or after an interval using the RTC alarm.

void rtc_set_wakeup_interval(uint32_t seconds)
{
    HAL_RTCEx_DeactivateWakeUpTimer(&hrtc);

    /* RTCCLK = LSE 32.768kHz, prescaler = 16 → 2048Hz */
    /* WUTR reload value = seconds * 2048 */
    HAL_RTCEx_SetWakeUpTimer_IT(&hrtc,
        seconds * 2048U - 1U,
        RTC_WAKEUPCLOCK_RTCCLK_DIV16);
}

void HAL_RTCEx_WakeUpTimerEventCallback(RTC_HandleTypeDef *hrtc)
{
    /* Woke from RTC: take sensor reading */
    schedule_sensor_read();
}

/* Usage: wake every 60 seconds */
void setup_periodic_wakeup(void)
{
    rtc_set_wakeup_interval(60U);
    prepare_for_stop2();
}

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Pattern 3: GPIO Leakage Prevention

Floating GPIO inputs near the logic threshold (VCC/2) draw up to 1mA per pin.