NUCLEO-WBA55CG - RTC WUT Fails to Wake MCU from Standby Mode

[maborbaa]

Hi everyone,

I'm new to STM32 solutions. My goal is to use the RTC Wake-Up Timer (WUT) to wake the MCU from Standby mode. I'm starting with a simple test:

• Cold Boot - Program the WUT to wake the MCU in 5 seconds and enter Standby.
• Warm Boot - Blink an LED, increment a counter (in BKPSRAM), and go back to sleep for 5 more seconds.

The Problem: The MCU enters Standby, but it never wakes up.

I am calculating the WUT for ck_spre (256 Hz) as follows: wakeup_counter = 5 (seconds) * 256 (Hz) = 1280.

HAL_Init();
SystemClock_Config();
MX_GPIO_Init();
MX_RTC_Init();

  if (__HAL_RCC_GET_FLAG(RCC_FLAG_LSERDY) == RESET)
    {
      // LSE FAILED!
      while (1)
	{
	  //(error)
	  HAL_GPIO_WritePin (LED_RED_GPIO_Port, LED_RED_Pin, GPIO_PIN_SET);
	  HAL_Delay (100);
	  HAL_GPIO_WritePin (LED_RED_GPIO_Port, LED_RED_Pin, GPIO_PIN_RESET);
	  HAL_Delay (100);
	}
    }

  uint32_t time_wkp_seg = 5;
  uint32_t wakeup_counter = time_wkp_seg * 256;

  if (__HAL_PWR_GET_FLAG(PWR_FLAG_SBF) != RESET)
    {
      // 1. Woke up from Standby

      // 2. Get the state (e.g., a counter)
      uint32_t wake_count = HAL_RTCEx_BKUPRead (&hrtc, RTC_BKP_DR0);
      wake_count++;

      // 3. Execute the wake-up task
      HAL_GPIO_WritePin (LED_GREEN_GPIO_Port, LED_GREEN_Pin, GPIO_PIN_SET); // Turn on LED
      HAL_Delay (50); // Keep it on for 50ms (to be visible)
      HAL_GPIO_WritePin (LED_GREEN_GPIO_Port, LED_GREEN_Pin, GPIO_PIN_RESET); // Turn off LED

      // 4. Save the new state
      HAL_RTCEx_BKUPWrite (&hrtc, RTC_BKP_DR0, wake_count);
      HAL_RTCEx_DeactivateWakeUpTimer(&hrtc);
      HAL_RTCEx_SetWakeUpTimer_IT (&hrtc, wakeup_counter, RTC_WAKEUPCLOCK_CK_SPRE_16BITS, 0);
    }
  else
    {
      // 5. It's a "cold" boot

      // 6. Save the initial state
      HAL_RTCEx_BKUPWrite (&hrtc, RTC_BKP_DR0, 0); // Start counter
      HAL_RTCEx_DeactivateWakeUpTimer(&hrtc);
      HAL_RTCEx_SetWakeUpTimer_IT (&hrtc, wakeup_counter, RTC_WAKEUPCLOCK_CK_SPRE_16BITS, 0);
    }

  // --- Safety Clear BEFORE sleeping ---
  __HAL_RTC_WAKEUPTIMER_CLEAR_FLAG(&hrtc, RTC_FLAG_WUTF);// Clear the RTC flag
  __HAL_PWR_CLEAR_FLAG(PWR_FLAG_SBF);      // Clear PWR flags

  HAL_PWR_EnterSTANDBYMode ();