TMS320F28075: can't write HIBBOOTMODE

I need to set a value in RAM memory after WD reset. Due to all RAM memory is cleared, I am trying to use HIBBOOTMODE register to do so.

When I try to write that register, its value doesn't chage, it keeps being 0xF. What should I do to be able to write it?

Register is not locked: CpuSysRegs.CPUSYSLOCK1.bit.HIBBOOTMODE  = 0;

Regards, 

Paloma