Cannot modify DDRSCLSTART register in PIC32MZ1025DAG169? (cbeif)

I am trying to use the internal 32MB DDR SDRAM in the PIC32MZ DA When I try to set a bit in the DDRSCLSTART register, the watch windowin the debugger shows no change to the bit. What would prevent me from writing to this register? static void DDR_PHY_Cali..