As discovered while adding hints for #938, it seems that the RTC interface is unclear about RAM access.
All the RTCs (R3, R4+, and Groove) do have persistent RAM.
Groove seems not to list it at all, R3 and R4+ both list 64 bytes.
But according to the Datasheets it is:
The MEGA65 should provide a consistent interface to the first 16 bytes (lowest common available space) for access. The source internal documentation must be corrected.
As discovered while adding hints for #938, it seems that the RTC interface is unclear about RAM access.
All the RTCs (R3, R4+, and Groove) do have persistent RAM.
Groove seems not to list it at all, R3 and R4+ both list 64 bytes.
But according to the Datasheets it is:
The MEGA65 should provide a consistent interface to the first 16 bytes (lowest common available space) for access. The source internal documentation must be corrected.