marinauterion opened a new pull request, #18988:
URL: https://github.com/apache/nuttx/pull/18988
## Summary
On some boards, the PCF85263 RTC does not count between reboots. After
inspecting the registers
at boot, `STOP_ENABLE` (register `0x2E`) had bit 0 set (`STOP=1`), which
freezes the RTC counter.
The exact trigger is unknown - not all boards exhibit the issue. The bit is
battery-backed and
persists across reboots, so once set (e.g. by a power glitch or undefined
hardware state) the
RTC stays frozen until explicitly cleared. The old driver never did this.
Fix: write `0x00` to `STOP_ENABLE` on init, which is the correct reset value
per the datasheet.
Fix: set time properly:
Due to datasheet the set_time should be as follow:
1. set stop_enable
2. clear prescaler
3. set time
4. clear stop_enable
## Impact
On affected boards, system time resets to the last value written via
`system_time set` after
every reboot, regardless of how much time has passed:
1. Set time to 10:00am
2. Turn off device, wait 2 minutes
3. Turn on device → time is 10:00am again
## Testing
Tested on PixHawk v6x (STM32H753IIK6)
Boot logs before and after the fix:
### Before Fix
```
Boot 1:
> pcf85263: STOP_ENABLE before init: 0xc1 <-- STOP=1 (bit 0 set)
> board: RTC time: 1970-01-01 00:00:05
wait 1 min
Boot 2:
> pcf85263: STOP_ENABLE before init: 0xc1 <-- STOP=1 (persistent)
> board: RTC time: 1970-01-01 00:00:06 <-- RTC did not count while
powered off
```
### After Fix
```
Boot 3 (first boot after fix - STOP still set from before, now cleared by
driver):
> pcf85263: STOP_ENABLE before init: 0xc1 <-- STOP=1, driver clears it
> board: RTC time: 1970-01-01 00:00:05
> system_time set 1779897071 <-- Wed May 27 2026 15:51:11 GMT+0000
wait 1 min
Boot 4 (STOP was cleared on Boot 3, RTC counted during power-off):
> pcf85263: STOP_ENABLE before init: 0x00 <-- STOP=0
> board: RTC time: 2026-05-27 15:52:11 <-- correct time, RTC counted
```
### Automated Power Cycle Test
41 iterations: power on → set time → power off → power on → compare RTC vs
PC time.
`STOP` was `0` on every boot after the fix. The remaining ~1s diff is a
measurement artifact
from integer truncation in the test script, not an RTC error.
```
run=0 diff=1s uC=1779952089 laptop=1779952090
run=1 diff=1s uC=1779952165 laptop=1779952166
run=2 diff=0s uC=1779952241 laptop=1779952241
run=3 diff=0s uC=1779952317 laptop=1779952317
run=39 diff=0s uC=1779955049 laptop=1779955049
run=40 diff=0s uC=1779955125 laptop=1779955125
run=41 diff=1s uC=1779955201 laptop=1779955202
```
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