STM31L431 RTC calibration with LSI

[RFlod.2]

I need to calibrate STM32l431 RTC in software for using LSI clock.

When I compare STM32 RTC with GPS reference I get the following 

GPS 1759492828 1759493307 => RTC 1759500022 1759500494
Diff GPS: 479 seconds
Diff STM32: 472 seconds
=> 7 seconds = 1.48% difference and
Prefered frequency to RTC 32473

I thought smooth calibration could be used for this kind of calibration but it seems it can only calibrate small changes for crystal.

Any ideas how this can be done? I cannot modify PCB for external pulses, I need to correct in software.

Clock will be syncronised 8, 16  and 24 hours interval, so accuracy does not need to be like a crystal.

[mbarg.1]

LSI is NOT 32768 but 32000 Hz

You have to change pre-scaler in RTC. I guess async prescaler should be 124 (count 125) instead of 127 (count 128).

Print RTC actual registers and let us discuss, i do not have any L4 to make a screenshot.

Please mark as solution if it works for you.

[RFlod.2]

Yes LSI frequency is 32000 Hz. 

This is RTC init:

/* Configure RTC */
	RTCHandle.Instance = RTC;
	/* Set the RTC time base to 1s */
	/* Configure RTC prescaler and RTC data registers as follow:
	- Hour Format = Format 24
	- Asynch Prediv = Value according to source clock
	- Synch Prediv = Value according to source clock
	- OutPut = Output Disable
	- OutPutPolarity = High Polarity
	- OutPutType = Open Drain */
	RTCHandle.Init.HourFormat = RTC_HOURFORMAT_24;
	RTCHandle.Init.AsynchPrediv = 127;
	RTCHandle.Init.SynchPrediv = 255;
	RTCHandle.Init.OutPut = RTC_OUTPUT_DISABLE;
	RTCHandle.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
	RTCHandle.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;

	HAL_RTC_Init(&RTCHandle);

	LL_RTC_TIME_SetFormat(RTC, LL_RTC_HOURFORMAT_24HOUR);
	LL_RTC_TIME_Init(RTC, LL_RTC_FORMAT_BIN, &time);
	LL_RTC_DATE_Init(RTC, LL_RTC_FORMAT_BIN, &date);

	rtcSet = false;

I would like to calculate the prescaler based om measured GPS diff, since it will be different on different CPU and different temperatures

[mbarg.1 · Accepted solution]

As in previous mail:

your dividers are (127 + 1) * (255 + 1) = 32768 !

if you want to use 32kHz, move async divider to 124 (count 125) and get:

dividers (124 + 1) * (255 + 1) = 32000.

Than you can trim.

[RFlod.2]

Aha now I get it!!! 

Thanks for notifying me on dividers!!!!

I will try this!

[mbarg.1]

Don't foget to mark as an accepted solution - will help others !