The nRF51 series reference manual v3.0 documents 3 Real Time Counters: RTC0, RTC1 and RTC2 (pp 17 and 109). However, RTC2 does not appear in the SDK documentation (e.g. no base address is listed in components/drivers_nrf/hal/nrf51.h). Furthermore, my attempts to read or write any of the RTC registers from RTC2's base address of 0x40024000 results in no effect, and all values read are 0. I am able to successfully read/write RTC0 & RTC1 as documented.
So... does RTC2 exist, or is the 3.0 reference manual incorrect? Or am I misreading it somehow? It certainly appears like RTC2 does not exist, despite what the reference manual says.
The documentation for RTC2 does seem different, in that it does not have an assigned ID according to the table on pg 17. Would that imply that (if it exists) it can't be used as a source for interrupts?
