Hello.
I work on a project (the microcontroller is STM32H733VGT6) where VBAT mode is used. When the main power supply is switched off a supercapacitor supplies power to the backup domain (RTC, backup memory). This works correctly but the behavior of the system at low VBAT voltages is not what I want.
The datasheet specifies that 1.2 V is the lower limit of operation in VBAT mode. However, when the voltage drops below this value there are no guarantees what will happen. In my case the reset of backup domain occurs far below the mentioned limit (0.3 V).
According to the reference manual the VBAT voltage can be monitored and when its not between the upper and lower limit a tamper event can be generated that will reset the backup domain. For some reason I am not able to configure this event correctly. The screens below are 2 pages from the reference manual that seem to prove that what I want to achieve is possible.
Here is my attempt (one of many) to configure the tamper event correctly.
Here is the circuit. I excluded the part that actually charge the supercap as it is not important here.
Here is how I modify the circuit to verify whether the tamper event resets the backup domain. I add a resistor parallel to the supercap/resistor branch and add a voltmeter to measure the voltage.
I use different resistors to drop the voltage.
What I would expect is that after the voltage drops below 1.2 V the tamper event will occur and the backup domain will be reset (after the uC starts the RTC will start from "zero"). I drop the voltage even to 1.0 V but RTC is not reset nevertheless.
What am I missing here? Do I understand the reference manual correctly? Is my tamper event configuration correct?
