Question: I have a question regarding the RTC internal of the nRF51822. Just to be sure, could you confirm the RTC internal is never in sleep mode, it is works independently.
Answer: It depends on what power mode the chip is in. By internal RTC I assume that you mean the internal 32 kHz RC oscillator (aka RCOSC). In System OFF mode, the core functionality is off, including the RTC and RCOSC. Refer to the Power Management chapter in the Reference manual. In System ON mode, the CPU can be either active or sleeping. However the RCOSC will still not run unless you are using a peripheral that needs it (e.g. the RTC) or you start it manually.