Using RTC_WAKEUPCLOCK_CK_SPRE_16BITS as wakeuptimer

Question

Good Day.I want to put my stm32wb device to sleep standby mode. and wakeup using RTC after specific time.I am following the PWR_STANDBY_RTC example which works.I would like to sleep for up to 12 hours and would like to use RTC_WAKEUPCLOCK_CK_SPRE_16BITS and not RTC_WAKEUPCLOCK_RTCCLK_DIV16 since the latter only allows up to 33 seconds.I am sucessfuly using RTC_WAKEUPCLOCK_RTCCLK_DIV16 but I am puzzled by value.for example if I set up counter to: HAL_RTCEx_SetWakeUpTimer_IT(&hrtc, 60, RTC_WAKEUPCLOCK_CK_SPRE_16BITS);it wakes up roughly 16 minutes later. I have tried various values but it seems a rough value is i must divide my second by 16 to get more or less correct time.For exampleHAL_RTCEx_SetWakeUpTimer_IT(&hrtc, 60/16, RTC_WAKEUPCLOCK_CK_SPRE_16BITS);Gives me roughtly 60 second wakeup time.Help would be greatly appreciated.I m using external 32khz clock and bluetooth functionality works great.

Scott Löhr · Accepted Answer

It would seem that you have (mis-) configured the RTC Asynchronous and/or Synchronous Prescaler(s) such that the ck_spre is only 1 pulse every 16 seconds.

For a 12 hours sleep, why wouldn't you be using STOP2 instead of Standby?

HSpre.1 · Answer

Thank you very much for your help.The device can be changed updating a characteristic in GATT profile. The device would vary between 5 minutes and 12 hours depending on specific use in field.I have opted for Standby and not STOP2 because I want to preserve GPIO on certain pins for external hardware.I have found the problem with prescalaers. IT is a setting i did not understand/ set under STM32_WPAN which sets up these prescalers in app_conf.hIn the CUBE setup under Middleware STM32_WPANConfigurationThere is a application parameter called CFG_RTCCLK_DIVIDER_CONFThis was set to 0 which which caused CFG_RTC_ASYNCH_PRESCALER = 16 CFG_RTC_SYNCH_PRESCALER = 32767I know changed this value to 2Which changed the setup toCFG_RTC_ASYNCH_PRESCALER = 2 CFG_RTC_SYNCH_PRESCALER = 16383below new code for reference if anyone else possibly comes upon this. CFG_RTCCLK_DIVIDER_CONF is set in CUBE#define CFG_RTCCLK_DIVIDER_CONF 2#if (CFG_RTCCLK_DIVIDER_CONF == 0)/** * Custom configuration * It does not support 1Hz calendar * It divides the RTC CLK by 16 */#define CFG_RTCCLK_DIV (16)#define CFG_RTC_WUCKSEL_DIVIDER (0)#define CFG_RTC_ASYNCH_PRESCALER (CFG_RTCCLK_DIV - 1)#define CFG_RTC_SYNCH_PRESCALER (0x7FFF)#else#if (CFG_RTCCLK_DIVIDER_CONF == 2)/** * It divides the RTC CLK by 2 */#define CFG_RTC_WUCKSEL_DIVIDER (3)#endif#if (CFG_RTCCLK_DIVIDER_CONF == 4)/** * It divides the RTC CLK by 4 */#define CFG_RTC_WUCKSEL_DIVIDER (2)#endif#if (CFG_RTCCLK_DIVIDER_CONF == 8)/** * It divides the RTC CLK by 8 */#define CFG_RTC_WUCKSEL_DIVIDER (1)#endif#if (CFG_RTCCLK_DIVIDER_CONF == 16)/** * It divides the RTC CLK by 16 */#define CFG_RTC_WUCKSEL_DIVIDER (0)#endif#define CFG_RTCCLK_DIV       CFG_RTCCLK_DIVIDER_CONF#define CFG_RTC_ASYNCH_PRESCALER  (CFG_RTCCLK_DIV - 1)#define CFG_RTC_SYNCH_PRESCALER   (DIVR( LSE_VALUE, (CFG_RTC_ASYNCH_PRESCALER+1) ) - 1 )#endif

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