Question About RTC Interface and Back-Feeding Protection for RV-8263-C7 with nRF52832

[Avesta]

Hello,

I’m designing a wearable device based on the nRF52832. To maintain time and date when the MCU and other sensors are completely disconnected from the 3.3 V supply, I plan to include an external RTC — the RV-8263-C7.

Please see the attached document. Instead of using the Backup Supercap or Battery LMR (Note 4), I intend to use a standard rechargeable battery that may have a voltage of up to 4.2 V.

My question is: do I need to include an interface component — for example, an analog bus switch such as the TMUX1511 — to prevent possible back-feeding from the SDA/SCL lines to the MCU?

Best regards,
Hossein

[khelmutlord]

If power is entirely disconnected, the protection diodes of the nRF52832 can conduct and draw "phantom power". In system off, without RAM retention, the nRF52832 draws about 300nA. 

https://docs.nordicsemi.com/bundle/ps_nrf52832/page/pmu.html

If this consumption is acceptable, then no external component is necessary. If you do intend to completely disconnect the power supply, you will need some sort of protection.

Additionally, VDD on the nRF52832 is 3.6V at maximum, apply 4.2V could damage the part.

Take a look at our nPM1300 or nPM1304, which are intended for rechargeable applications.

[Avesta]

Thank you so much.

[hmolesworth]

Allowing any phantom power (aka back-drive or back-feed) into a port pin on the nRF52 is a design flaw. The external source raises the internal VDD in the absence of normal VDD power and often prevents correct operation of the internal reset circuitry; this leads to random and unpredictable cases when disconnecting and reconnecting the 3.3V which can only be recovered by removing all power sources including the source of the back-drive. Simply repeating 3.3V disconnect-connect in this scenario does not fix the hangup.

The correct fix is usually to use a single level-shifter which is powered from the nRF52 3.3V; this allows higher battery voltages as well as removing back-drive. Without this design flaw fix, at some future date sensors in the field will require an expensive site visit to recover. Search for phantom power on the dev zone; I responded to multiple posts with more details. Finally if your bench test doesn’t exhibit this behavior, rest assured that some sensors in the field will :-)