rtc: write the RTC time only for years 2000-2099

[ptr727]

Replaces #103, cut down to the year-range check as asked in #103 (comment). #103 stays as the record of the version coordinated with Zephyr and MeshCore.

The defect. ZephCore writes the RTC year as two BCD digits with no century, as y % 100. A time before 2000, accepted by a node whose clock had no time yet, was stored as a year near the end of the century and came back at the next boot above the 2025 floor: time 900000000 (1998) was restored as 2098.

The fix.

• rtc_time_block() returns false outside 2000-2099.
• zephcore_rtc_save() then logs time outside 2000-2099 not written and writes no time, for every chip, rv3028-eeprom-config included. The chip keeps the time it has.
• rv3028_save_retry_fn() stops if its run-on time leaves the range.
• One-line updates to the Kconfig help, the header, the zephcore,rtc-i2c binding and docs/ARCHITECTURE.md.

The RV3028 write sequence is unchanged: year 00h, then 00h-05h, then the year.

Other projects. Zephyr (zephyrproject-rtos/zephyr#121389) and MeshCore (meshcore-dev/MeshCore#3421) take the same 2000-2099 write range. They also write the RV3028 with an A0h year mark and one burst, which ZephCore does not take: its 2025 restore floor already rejects the year 2000 that a cut write leaves (the last test below). A mark left by either reads here as a non-BCD year, so it gives no time.

Testing. RAK4631 + RAK12002, repeater debug builds.

• This commit, exact build: the boot restored 2026-10-07 14:17:31 UTC, and a CLI time then reboot restored the set time.
• A test-only harness on this commit, which calls zephcore_rtc_save()/zephcore_rtc_restore() and writes the chip's registers, 14/14 PASS:
  • 500 restores, none failed or went backwards. 100 random save and restore pairs, exact to the second.
  • 2000-01-01 is written and gives no time (the floor). 2099-12-31 23:59:59 is written and restored.
  • 2100-01-01 and 1999-12-31 23:59:59 are not written: the chip kept 2030-01-01 00:00:00, and that was restored.
  • All FFh, year FFh, seconds 5Ah, month 13h, year A0h: no time.
  • The current write, its burst cut after three bytes by the chip's 950 ms bus timeout: year 00, no time at the next restore. A normal save recovers.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Cancel pending RV3028 retries for out-of-range epochs to prevent stale time writes.

Review effort: Lite
Findings: 1

Open (1)

• Cancel pending retry when rejecting out-of-range epoch · New

What changed in this PR

Restricts RTC writes to years 2000–2099 to prevent century aliasing.

Changes:

• Adds year-range validation for RTC writes and retries.
• Updates RTC configuration, API, binding, and architecture documentation.

File	Summary
zephcore/​Kconfig	Documents the supported RTC write range.
zephcore/​dts/​bindings/​rtc/​zephcore,rtc-i2c.yaml	Updates RTC behavior documentation.
zephcore/​adapters/​clock/​ZephyrRTCDiscover.h	Documents the range-limited RTC persistence contract.
zephcore/​adapters/​clock/​ZephyrRTCDiscover.c	Validates years and manages RTC writes and retries.
docs/​ARCHITECTURE.md	Documents the RTC write restriction.

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