Why an STM32L476's RTC counts more than 24h and don't icnreases the day
I've designed a board that has a STM32L476.
This part is supplied using a triple system of dual battery + power ( when it is available)
We've manufactured more than 20k units of them and they are working ok
A few days ago 2 boards came back to our design lab that exhibit the following symptoms :
-> The RTC keeps counting past 23:59:59 until 39:59:59
-> Neither on 23h nor 39h rollover the day gets increased
-> When trying to set the time we get the following data:
If the time is less than 20h, 20h get added to it
Else the hour is correctly set
-> Rollover behavior don't changes
If we force a reset of the CPU, the problem dissapears.
More data:
-> The RTC access is done using HAL libs.
-> The RTC isn't reset when setting the date/time
We still have one untouched board for testing, but any try to use a debugger would trigger a CPU reset, making the probblem dissapear. (This is why we lost ont of the boards )
Does anybody have a hint about what the problem could be ?