How many memory ranks does the PPC440MC_DDR2 controller support?
It is unclear in the data sheet, as there is a C_NUM_RANKS_MEM parameter, and a section on allowing up to four external ranks of memory.
The PPC440MC_DDR2 only supports a single rank. Using more than one rank will not assert the external CS_n signal correctly, and is not currently planned to be fixed.
The documentation will be updated to support only single rank memory layouts, starting in PPC440MC_DDR2 v2.00.b.