Write and read the RX8130CE WEEK register as one-hot (iteration branch)

[ptr727]

Iteration branch for #15. Keep it open; never merge it. A clean single-commit branch for upstream is cut from it once it has baked. Upstream takes PRs on dev.

References:

• The manual: RX8130CE Application Manual, Seiko Epson, ETM50E-10, from the product page: https://www5.epsondevice.com/en/products/rtc/rx8130ce.html. Section, table and printed page numbers below are that document's.
• The code:
  • before: upstream dev 3e3150c8
  • after: 4cdd5d71

The defect

The RX8130CE's WEEK register (13h) is one-hot: Sunday = 01h through Saturday = 40h (§14.1.2 "Week counter", Table 15, p. 27). The manual says "Do not set '1' to more than one day at the same time".

• setTime() L98 wrote bin2bcd(t->tm_wday) & 0x07, so no day was stored correctly:
  • Sunday wrote 00h, no day at all.
  • Wednesday, Friday and Saturday each wrote two days.
  • Monday, Tuesday and Thursday each wrote another day's bit.
• getTime() L144 decoded it the same wrong way.

The fix

• L98 writes 1 << t->tm_wday. Its only caller, adjust(), passes gmtime() output, which is 0-6.
• L144-L150 read back the position of the lowest set bit among bits 0-6. A chip still holding the old zero value reads as Sunday until the next adjust() rewrites the register.

Impact and notes

• Low impact. now() and unixtime() go through mktime(), which ignores tm_wday, so the date and time MeshCore uses never depended on this. Only the chip's own week counter was wrong. That affects a weekday alarm, which MeshCore doesn't use, or any other firmware reading the chip.
• No other users. tm_wday and register 13h are used nowhere else in src/, examples/ or variants/.
• Out of scope, pre-existing in this driver, found by review:
  • begin() neither checks VLF nor initialises the time registers after power-up, which the manual requires.
  • stop() rewrites all of 1Eh, not just the STOP bit.
  • bin2bcd(tm_year - 100) is wrong outside 2000-2099.

Testing

• Build: R1Neo_repeater builds, the only board using this driver (Muzi R1 Neo), as do RAK_4631_repeater, heltec_v4_repeater and RAK_11310_repeater.
• Hardware: not tested. No RX8130CE is available here.

The same defect was found independently in ZephCore: ptr727/liquidraver-ZephCore#39.

Refs #15

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review comments remain, and the focused change is fully reviewed.

Review effort: Lite
Findings: None

What changed in this PR

Corrects RX8130CE WEEK register handling to use one-hot weekday encoding.

Changes:

• Writes weekdays as 1 << tm_wday.
• Decodes the lowest set weekday bit when reading.

File	Description
src/​helpers/​RTC_RX8130CE.cpp	Corrects WEEK register encoding and decoding.

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[ptr727]

Upstream: PR meshcore-dev#3550 (from fix/rx8130ce-week-onehot at 8c9c9772, byte-identical to this branch's 4cdd5d71) and issue meshcore-dev#3549.