I found a strange behavior which seems to be bug on FRDM-K20D50M board,
When Kinetis MCU wake up from the low power mode by RTC overflow interrupt, bit 28 of NVIC interrupt pending register (i.e. NVIC_ISPR0 and NVIC_ICPR0) will be set instead of bit 29. Although the interrupt enable (NVIC_ICER0) bit of the overflow interrupt is set, it will be ignored.
Therefore LLWU_ME should be assigned by the value of LLWU_RTCA_ME (i.e. 0x20) instead of LLWU_RTCS_ME (i.e. 0x80).
I think right way to set RTC overflow interrupt for wake up would be as following.
However I actually set as following to work successfully.
I would like to get comments from the Kinetis MCU (Kinetis K20 only?) designer.
For your information, even on FRDM-KL25Z, the situation was the same.