Verification that prescaler values are configured should be added

[jan019]

RTC should contain an assertion checking if the prescaler values are configured or not. Or check if configured and if not, call the function to compute the values.

This portion of code

STM32RTC/src/rtc.c

Lines 382 to 398 in 95b7514

	if (!LL_RTC_IsActiveFlag_INITS(RtcHandle.Instance) || reset) {
	RtcHandle.Init.HourFormat = format == HOUR_FORMAT_12 ? RTC_HOURFORMAT_12 : RTC_HOURFORMAT_24;
	RtcHandle.Init.OutPut = RTC_OUTPUT_DISABLE;
	RtcHandle.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
	RtcHandle.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
	#if defined(RTC_OUTPUT_REMAP_NONE)
	RtcHandle.Init.OutPutRemap = RTC_OUTPUT_REMAP_NONE;
	#endif /* RTC_OUTPUT_REMAP_NONE */
	RTC_getPrediv((int8_t *) & (RtcHandle.Init.AsynchPrediv), (int16_t *) & (RtcHandle.Init.SynchPrediv));
	HAL_RTC_Init(&RtcHandle);
	// Default: saturday 1st of January 2001
	// Note: year 2000 is invalid as it is the hardware reset value and doesn't raise INITS flag
	RTC_SetDate(1, 1, 1, 6);
	reinit = true;
	}

is executed only when the device was previously unpowered -> RTC is not initialized. For the case when RTC is not initialized the function

STM32RTC/src/rtc.c

Line 391 in 95b7514

RTC_getPrediv((int8_t *) & (RtcHandle.Init.AsynchPrediv), (int16_t *) & (RtcHandle.Init.SynchPrediv));

is called and prescaler values are computed.

In case only a soft reset is performed, the RTC remains initialized, the above mentioned code is not executed and prescaler values are not calculated. Therefore they remain set to -1. Then when setting and RTC alarm with subseconds enabled, the following issue occurs:

• these variables remain set to the initial default value

  STM32RTC/src/rtc.c

  Lines 69 to 71 in 95b7514

  	static uint8_t predivSync_bits = 0xFF;
  	static int8_t predivAsync = -1;
  	static int16_t predivSync = -1;

• RTC_AlaramStrucutre subsecond parameters are set here

  STM32RTC/src/rtc.c

  Lines 611 to 612 in 95b7514

  	RTC_AlarmStructure.AlarmSubSecondMask = predivSync_bits << RTC_ALRMASSR_MASKSS_Pos;
  	RTC_AlarmStructure.AlarmTime.SubSeconds = predivSync - (subSeconds * (predivSync + 1)) / 1000;

• the strucutre is passed to

  STM32RTC/src/rtc.c

  Line 654 in 95b7514

  HAL_RTC_SetAlarm_IT(&RtcHandle, &RTC_AlarmStructure, RTC_FORMAT_BIN);

• where it is used to compute the value to be written to register https://github.com/stm32duino/Arduino_Core_STM32/blob/491ddc80df212c417de88d8039ff43f800cf0c28/system/Drivers/STM32G0xx_HAL_Driver/Src/stm32g0xx_hal_rtc.c#L1298 (the specific file depends on used platform, for me it was this file), https://github.com/stm32duino/Arduino_Core_STM32/blob/main/system/Drivers/STM32G0xx_HAL_Driver/Src/stm32g0xx_hal_rtc.c#L1332

This leads to reserved bits in the register being written to 1 and generally incorrect behaviour. This is not easy to debug and just a note in the readme that the function setPrediv() must be called before begin() can easly be missed.

The examples also don't call this function, which makes the issue even more hidden. Maybe some boards ensure that invalid values are not written to registers and this issue is not observed on them.

[ABOSTM]

@jan019 thanks for reporting this issue, and for your detailed analysis.
I proposed a fix in PR #72,
feel free to review it and tests it

@rozrabiak this PR should also fix your issue #70