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STM32F407I with LQFP176 RTC registers resets when Vbat below 3.1V

Question asked by strobl.christian on Apr 21, 2015
Latest reply on Oct 28, 2015 by joee
Hi everyone.
In a new development with STM32F407I i have some problems with backup domain especially with internally switch Vdd/Vbat to backup domain.
I use a CR3032 backup battery ( 3V lithium ) connected to Vbat pin via a BAT48C double Schottky diode and 1uF cermet cap directly on Vbat pin, second part of this diode is connected to Vdd 3,3V.
Pin PDR_ON is connected to VDD 3,3V.  ADC is running with separate 3.3V, Vref is also external 2.5V, so all voltages are generated separately from one main voltage.
VDD analog and Vref voltage needs more time to go to 0V as Vdd after power down.
After power down ( switch off main voltage) and power up all registers of  RTC are lost. If cap of Vbat pin is increased to 470uF ( and charged via BAT48C to 3.3V VDD ), i can see after power down that LSE assigned to RTC stops working when Vbat drops after approx. 60 sec. below 3.1 Volts, and after powering up all registers of RTC are lost.
The measured current in to Vbat pin with PDR_ON connected to VDD is in power off state approx 2uA ( when Vbat is above 3.1V), this is ok, when Vbat drops below 3.1V also current drops to approx. 0.1uA. When  PDR_ON  is connected to VSS, there are no problems with backup domain after switch off, Vbat can go below more then 2V without loss of datas. But there is another problem with current: in this case the Vbat current is near 2mA, so battery will be discharged rapidly.
I use PC13 as  standard input, PI8 is not connected, PA0 is used as an analog input to ADC,  PC14 PC15 is connected to a 32kHz Osc., RTC uses this as LSE. Setup of RTC shall be ok and RTC works fine, no wakeup or tamper pin is assigned to RTC, buffered RAM is not used.

I have built two prototypes and both shows the same behavior.

Other types of STM32F407 with 144pin or 100pin LQFP in other developments works fine with same hardware setup, also the software setup of RTC is the same.

I didn`t found any restrictions to battery voltage in the data sheets, 3.0V shall be good.

Is there anyone with the same problem and has found the cause or a solution?

What is about the power down reset to backup domain described in data sheets? It shall be only when BOTH Vdd and Vbat where at the SAME time low?

Greetings and thanks for every idea!