ADUcM330 LFOSC Calibration

I want to calibrate the LFO (32 K Hz) of ADuCM330 for which I have been referring to ADuCM330/ADuCM331 Hardware Reference Manual UG-716   

LOW FREQUENCY OSCILLATOR (LFOSC) CALIBRATION

 The value of the register TRMVAL (Oscillator trim value) does not get altered even though it is RW type.

I have tried to force-write a value. Still it remains the same as default..

Even after the end of the calibration cycle, the value remains default.