The data sheet of the STM32F427 mentions two important details:
[3.21] When PDR_ON pin is not connected to VDD (Internal Reset OFF), the VBAT functionality is no more available and VBAT pin should be connected to VDD.
[3.17.2] PDR_ON controls the internal power reset circuitry, i.e. for PDR_ON = GND this section applies:
Internal reset OFF
This feature is available only on packages featuring the PDR_ON pin. The internal power-on reset (POR) / power-down reset (PDR) circuitry is disabled through the PDR_ON pin. An external power supply supervisor should monitor VDD and should maintain the device in reset mode [...]
As a consequence, you can work with PDR_ON=GND, but then you no longer have backup functionality via VBAT, so that if VDD fails, the backup domain is not buffered and the data in the backup RAM and in the RTC are lost.
You should also carefully check the function of your external reset module to see if it meets the requirements stated in the above data sheet, section 3.17.2.
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Good luck!
/Peter
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