RV3028 loses the time on a power cut: ZephCore never enables its backup switchover #33

Found while hardware-testing work/rak4631-rak12002-rtc (fork issue #4) on a RAK4631 with a RAK12002 fitted, 2026-10-03.

What happens

With the RV3028 declared, the time survives a reset or reboot: after a CLI time set and a soft reboot, the boot log shows RTC rtc-rv3028@52: restored <time>. It does not survive a power cut. After removing USB and battery for about 30 s, the boot log shows:

<wrn> zephcore_rtc: rtc-rv3028@52 present, power-loss flag set — clock will be set on the next GPS/app/CLI sync

So the chip lost power, came back through a power-on reset with PORF set, and holds no time until the next sync.

Why

ZephCore's discovery (adapters/clock/ZephyrRTCDiscover.c) only reads and writes the time block and clears the power-loss flag. It never configures the RV3028's backup-supply switchover, the EEPROM Backup register at 0x37. Other code that drives this chip does:

• Zephyr's own RV3028 driver writes 0x37 at init (drivers/mfd/mfd_rv3028.c), and its binding makes backup-switch-mode required.
• MeshCore writes 0x37 = 0xB4 on every boot (src/helpers/AutoDiscoverRTCClock.cpp), which is direct switching plus trickle charging (1 kΩ).

What a fix needs to decide

• Whether to write 0x37 at all from the generic, data-only discovery, or add an optional per-descriptor init write (for example init-reg / init-val in the zephcore,rtc-i2c binding) that only the RV3028 descriptors set.
• Trickle charging is the hazard. It is right for a supercapacitor or a rechargeable cell, and wrong for a primary cell. What backs the RAK12002, and any other board using the RV3028 descriptor, must be established from its schematic before a fix enables it. Switchover without trickle charging is the conservative choice.
• Writing the RAM copy of 0x37 every boot, as MeshCore does, avoids EEPROM wear but is reloaded from EEPROM on reset. Writing the EEPROM copy persists it but is a one-time configuration change to the module.

Hardware to verify a fix: the same RAK4631 + RAK12002, sync, power cut, boot, expect restored.