STM32F103RCT6 RTC init hangs in LL_RTC_Init() on custom board (LSE/Vbat related?)

[Zaeem-Ahmed]

Hello everyone,

I’m working with an STM32F103RCT6 on a custom board, and I’m trying to use the built-in RTC using ST Low-Layer (LL) drivers.

When I enter debug mode, the application gets stuck inside LL_RTC_Init(). It neither returns SUCCESS nor ERROR — it keeps looping internally, apparently waiting for a flag that never gets set.

Here is the function from my application:

void LL_RTC_Init_Custom()
{ // TODO: RTC clock needs to be set to LSE. On the dev kit, LSE isn't available
  LL_RTC_InitTypeDef rtcInit =
  { .AsynchPrescaler = RTC_CLK_PRESCALE, .OutPutSource =           LL_RTC_CALIB_OUTPUT_NONE };

  ErrorStatus status = LL_RTC_Init(RTC, &rtcInit);
  if (status != SUCCESS)
     Error_Handler();
}

Inside this function, it stucks here, in this while loop below.

ErrorStatus LL_RTC_ExitInitMode(RTC_TypeDef *RTCx)
{
  __IO uint32_t timeout = RTC_INITMODE_TIMEOUT;
  ErrorStatus status = SUCCESS;
  uint32_t tmp = 0U;

  /* Check the parameter */
  assert_param(IS_RTC_ALL_INSTANCE(RTCx));

  /* Disable initialization mode */
  LL_RTC_EnableWriteProtection(RTCx);

  /* Wait till RTC is in INIT state and if Time out is reached exit */
  tmp = LL_RTC_IsActiveFlag_RTOF(RTCx);
  while ((timeout != 0U) && (tmp != 1U))
  {
    if (LL_SYSTICK_IsActiveCounterFlag() == 1U)
    {
      timeout --;
    }

    tmp = LL_RTC_IsActiveFlag_RTOF(RTCx);
    if (timeout == 0U)
    {
      status = ERROR;
    }
  }
  return status;
}

While debugging, it looks like LL_RTC_Init() is waiting on a ready flag (likely related to RTC clock or synchronization), but in reality that flag never becomes set, so the code loops forever.

Some additional context:

• Custom board, not an ST dev kit

• Using LL drivers, not HAL

• RTC clock is intended to be LSE, but I suspect LSE may not be running or available

• Backup domain access and RTC clock configuration may be relevant

My questions:

1. Is this behavior expected if LSE is not present or not oscillating?

2. Does LL_RTC_Init() block indefinitely if the RTC clock source is invalid?

3. If I leave the Vbat pin unconnected, can RTC still work? since the Vdd is present.

4. If the 3.3 appears at the Vbat pin can I rule out all the power related issues?

Any guidance or debugging tips would be appreciated.
Thanks in advance.

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Edited to apply source code formatting - please see How to insert source code for future reference.

[Andrew Neil]

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@Zaeem-Ahmed wrote:

• Custom board, not an ST dev kit

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So you'll need to give details of your board - schematics, etc:

How to write your question to maximize your chances to find a solution

PS:

Also, have you tried this on an ST board - to rule out any hardware issues?

And Using ST Examples?