We have a custom board with LPC1788, where we use the RTC, backed up with supercap (appr. 1 week backup time)
We have a small amount of the boards at customers (25), and normally, the RTC works fine.
But one customer has observed that the RTC changes out of nowhere:
After some days where RTC was OK, the DOM (day of month) changed to 00. But the RTC worked fine after this, and after 1 day, it changed to 1, and next days, it also increments the day.
Month, year, hour, minute and seconds was never changed, but incremented as expected.
The issue is never seen on other boards.
What can make this happen?
The voltage on the backup pin is 99,9% OK, but it has not possible to measure it.