Problem with RTC when switching off (VDD) on H7

[AScha.3]

Hi,

We have now a problem with some boards, H7A3ZI or H743ZI , when switching off :   RTC stops working !

RTC is powered on VBAT with a standard CR2032 and was working as it should on about 500 boards since more than a year. Now (some) batteries loosing voltage and we get errors, because RTC no more working.

This happend at first at about 2,7V (cell voltage), now even on new boards with 3,0V cells.

The problem starts at the moment of power off, when VDD falling (to 2V or so) , the internal switch going to use the VBAT as supply for RTC and backup ram. On affected boards is about 5 ms (!) long the LSE oscillation away, then starting (on VBAT supplied now) again - or no more, showing the error on next startup.

Is there anything we can do, to avoid this ? Or is there some "known problem" with the H7 series, when switching to VBAT ?   (In errata i found nothing about this problem.)

[AScha.3]

Update...with GOOD solder joint, PDR_ON is at VDD , switching to VBAT as it should be.

Works even with a bad, old CR2032 , at 2,4V .