Can the LSI clock drive the RTC in Vbat mode in the STM32H7x3?

[rammit]

RM0433 Rev 6 refernce manual discrepency about the RTC clock sources.

The H7 errata and clock tree indicates the LSI is in the backup domain but the top of page 351 it says it is not:

The LSE clock is in the Backup domain, whereas the other oscillators are not. As a

consequence:

• If LSE is selected as RTC clock, the RTC continues working even if the VDD supply is

switched OFF, provided the VBAT supply is maintained.

• If LSI is selected as the RTC clock, the AWU state is not guaranteed if the VDD supply

is powered off.

Which seems to copied from the F7 manual RM0385.

Please confirm.

Also, can the RTC clock source be switched to LSI from HSE during a brown out interrupt?

Or perhaps a better question is, what is the best way to clock the RTC during a VDD power loss if we want to avoid using the LSE clock?

[Tesla DeLorean]

How hard would it be to just test these hypothesis?

Most every STM32 I've encountered does NOT have LSI in the backup domain, nor any of the other internal source.

If you have specific concerns, perhaps these are discussions you could have with your local FAE

[waclawek.jan]

> The H7 [errata and] clock tree indicates the LSI is in the backup domain

I see something different:

I don't see any such indication in the errata either.

JW

[rammit]

Thank you both for your response.

We've been using the F7 which does not have LSI as clock source, but noticed the H7 does have LSI as a clock source and made an assumption which was cemented after quickly reading Errata 2.2.19. In its defense it does not mention LSI being powered by Vbat.

We are switching to the H7 and do not currently have one in house to test my hypothesis.

You are right Figure 39 RCC Block diagram shows the LSI in the VDD domain.

But Figure 17 Power supply overview shows LSI in the backup domain.

[waclawek.jan]

> But Figure 17 Power supply overview shows LSI in the backup domain.

Fair point, indeed there it is.

@Imen DAHMEN​ , @Amel NASRI​  , can you please clarify whether LSI is in VBAT/backup domain or in VDD domain, in STM32H7? One of those Fig.17 and Fig.39 must be wrong.

JW

[Imen.D]

Hello,

Thanks for bringing this typo to our attention. We confirm the contradiction, we are fixing this for next release of the reference manual.

So, we will correct Figure 17. Power supply overview: LSI needs to be moved from the backup domain to the VDD domain.

The best way to clock the RTC during a VDD power loss is to use the LSE as both LSI and HSE are off.

Best Regards,

Imen

[waclawek.jan]

Hi @Imen DAHMEN​ , thanks for the clarification.

JW