Does STM32WL55JC1 VBAT ensures LSI Crystall maintain the RTC

[HarshaHegde]

Hi,

I'm using Nucleo-STM32WL55JC1. I used a coin cell to give power to Vbat and remove the sb21 link.
Now I tried the LSE as an RTC source and time works as expected even after I remove power from Vdd, I have used HAL_PWR_EnableBkUpAccess();
But when I tried to USE LSI crystal time is resuming from the last power removed but not the actual time.
for example, if I removed the power at 15:51:21 and resumed power after 5 minutes it shows 15:51:22 instead of 15:56:21.
I want to use LSI only because I don't want to have an external RTC with my board when I'm designing the customer board.
my doubt is if LSE works, even LSI also should work. as we can see in the diagram both LSE and LSI are in the same Backup domain.

 

Please let me know if anything missing. 
Thank you in advance!!!

[STTwo-32 · Accepted solution]

Hello @HarshaHegde 

As you can see on the paragraph 32.2 of the RM0453. The RTC is functional in VBAT mode and in all low-power modes when it is clocked by the LSE. When clocked by the LSI, the RTC is not functional in VBAT mode, but is functional in all low-power modes except Shutdown mode.

So, it will keep working in Vbat mode only will using the LSE.

Best Regards.

STTwo-32

[TDK]

LSI stops when power is removed, so you can't use it like you want.

Note that even if you could use it, LSI accuracy is awful.