Why does the Backup Registers not reset even if VDD and VBAT is below < 0,6V

[MAxel.2]

I'm using STM32G491 and I've a SuperCap mounted on VBAT Pin and using LSE with 32.768 KHz.

Everything works at it should but after removing voltage supply so that VDD drops to 0V, and discharge the SuperCap to 0.6V for about 15Seconds. After this I Apply power again and VDD gets 3.3V again I can see that the TAMP registers is not cleared and the the RCC_BDCR_RTCEN says that the RTC is already started. Can the powersupply during Shutdown mode be powered from somewhere else. I can also see that the RTC counter has stopped during parts of the 15sec with low voltage.

[waclawek.jan]

ST does not specify a threshold on the Backup domain reset.

Based on the brownout erratum, it can be anywhere between 100mV and 700mV, "with the exact position depending mainly on the device and on the temperature".

LSE usually stops oscillating at higher voltages than that.

JW

[MAxel.2]

thanks that explains why have the problems that I've.