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AutoDiscoverRTCClock adopts any device that ACKs at an RTC address, then reads it as the clock and writes it on every sync

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
embedded-iot

Research direction

Start with src/helpers/AutoDiscoverRTCClock.cpp, especially begin(), getCurrentTime() and setCurrentTime(), then review the cited RTC libraries and datasheet checks in related issue #3544. Verify that detection rules out incompatible devices at each RTC address and preserves the stated behavior for failed reads. Done means the probe no longer adopts devices based on an I2C ACK alone; #3544 describes a read-only hardware test on a RAK4631.

Written by the indexing model from the issue text.

Description

AutoDiscoverRTCClock::begin() adopts an RTC on an I2C ACK alone. Any other device that answers at one of the four RTC addresses becomes the node's clock. Its registers are then read as the time, and written on every time sync. Upstream has already hit this twice, and fixed each board by hand with DISABLE_DS3231_PROBE. The probe itself still has the defect for every other board and device.

Code references are at upstream dev 3e3150c8. The libraries are at the versions MeshCore builds against: Adafruit RTClib 2.1.4 and Melopero RV3028 1.2.0.

The defect

begin() runs i2c_probe() for each RTC address, which only checks for an ACK. It then marks the chip found:

Chip Address Adopted at Written at begin Written on every sync (setCurrentTime())
DS3231 0x68 L31-L32 (RTClib begin() only opens the device) none 00h-06h via adjust() (L87)
RV3028 0x52 L36-L41 35h, 37h, and 10h through set24HourMode() 00h-06h via setTime() (L91)
PCF8563 0x51 L44-L47 (RTClib begin() only opens the device) none time registers via adjust() (L93)
RX8130CE 0x32 L49-L53 30h, 1Ch, 1Dh, 1Eh and 1Fh (RTC_RX8130CE::begin()) time registers via adjust() (L96)

Once a device is adopted:

  • The clock is wrong. getCurrentTime() returns whatever the foreign device's registers decode to.
  • The foreign device gets written. Every sync overwrites its registers. Syncs come from the CLI clock and time commands, from app and peer timestamps, and from GPS every 30 minutes (TIME_SYNC_INTERVAL, L247).
  • The order makes it worse. The DS3231 is checked first and wins, so an IMU at 0x68 hides a real RTC elsewhere on the bus.

Address conflicts are ordinary:

  • 0x68 is the default address of the common IMUs (MPU-6050, ICM-20948, ICM-42607).
  • 0x50-0x57 is the 24-series I2C EEPROM range, which covers both 0x51 (PCF8563) and 0x52 (RV3028).

Already seen upstream

Both opt-outs are per board, so any board not yet found still has the defect, and so does any device at the other three addresses.

Proposed fix

#3544. Adopt an RTC only if its time registers read like that chip's, per its datasheet:

  • Bits documented as always 0 must read 0 (DS3231, RV3028, RX8130CE; NXP documents none for the PCF8563).
  • Not all seven registers may read 0xFF.
  • Seconds, minutes, date and month must be valid BCD in range, unless the chip's power-loss flag is set (not applied to the RX8130CE).
  • Two reads must both rule a device out, and a failed read keeps today's behaviour.

The masks and flags are cited from each vendor's datasheet in #3544, which also has a read-only hardware test on a RAK4631. The existing DISABLE_DS3231_PROBE opt-outs stay.

Related: #3545 (RV3028 EEPROM configuration), which touches the same begin() block.

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