We are using Kinetis K66 controller(MK66FN2M0VLQ18) for our custom board. Actually this board is an upgraded version,old one is having K60 controller. Now we are facing an issue in RTC. Out of 10 boards 4 boards having an issue in RTC. The issue is time is not increment. We are using a crystal 32.768KHz (CFS-20632768HZFB) for RTC clock and a 3V rechargeable battery for VBAT.
In our design we are using load capacitors externally and it's value is 20pF. Our crystal having a load capacitance of 12.5pF.
We know that load capacitors can be internally provided. But for our old design we are using external load capacitors so we just followed same design. For debugging this RTC issue, we removed the external load capacitors of boards which have RTC issue.The results are some board's RTC issue resloved without giving any internal load capacitors and some baords worked with an internal capacitance of 2PF . I'm not sure why the behavior is like this.
My questions are,
1. Is it required to provide load capacitors externally? what is the good design practice ?
2. Why the behavior of RTC is varying in each boards?
3. For a 12.5pF load capacitance in crystal, what value we programmed for internal load capacitors?
Thanks in advance.