RTC Operation with Super Cap

[JWPtn]

I would like to use the RTC in the nRF52833 as a clock-calendar.  The device will normally be powered by the USB Vbus but must have power fail protection to insure the integrity of the time.  For various reasons I cannot use a battery as for backup so I would like to use a super cap instead.  Would connecting the super cap to DEC4, perhaps with a resistor-diode network to limit initial inrush current but allow full output current, be acceptable?  

[ketiljo]

No, that won't work. The POR circuitry is sensing on the VDD pin and will put the device in reset when VDD becomes too low. 

[JWPtn]

Okay, thank you for the quick response.  

[hmolesworth]

Maybe instead of DEC4 connect your circuit directly to VCC, and add a schottky to back-drive VCCH. I have a USB/coin cell design which does this. Instead of using BAV99 schottkys I suggest you switch to Maxim ideal diode MAX40200 to reduce losses. See MAX40200