Minimum Vbat level for TAMPER detection (STM32G491)

[unterberg_rtb]

Hi, 

We have an application on STM32G491 where need to detect a button press after the device was switched off for some minutes. To implement this, we connect a standard capacitor (100 µF) to Vbat pin, Activate battery charging to load the capacitor, start LSE clock and activate TAMPER detection on one of the tamper input pins. This works very well.

However, in the data sheet we do not find the minimum Vbat voltage for RTC or TAMP operation. The only voltage we find is that Vbat shall be between 1.55 and 3.6 Volts. But our tamper detection still works when Vbat is far below 1 Volt. 

Is there any (ST-official) document that gives us Vbat voltage range where LSE and RTC/TAMP keep working?

Regards, Norbert

[Andrew Neil · Accepted solution]

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@unterberg_rtb wrote:

The only voltage we find is that Vbat shall be between 1.55 and 3.6 Volts. But our tamper detection still works when Vbat is far below 1 Volt. 

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The specifications tell you over what range it is guaranteed to work as specified - over the entire working range of temperature, for all production, etc.

So those are the values that you need to use in your design.

If you happen to have observed it still working outside that range on a certain device under certain conditions - that's just a bonus!

[TDK]

> Is there any (ST-official) document that gives us Vbat voltage range where LSE and RTC/TAMP keep working?

The minimum VBAT voltage is listed in the datasheet.

This is the voltage needed for VBAT operation of the RTC/TAMP. Below this, it is not guaranteed to work, even though portions of it might work.

[unterberg_rtb]

Thanks for the feedback.

I know these marked items from the data sheets. I was hoping I missed a separate data sheet section or an application note which indicated there might something like a separate Vbat_tamp voltage defined.

[Andrew Neil]

No, the electrical specs are all in the Datasheet.