write usart register "US_RTOR" but no change

[leo.ni]

Hi,all! i want to set register "US_RTOR" of usart1 as this "usart->US_RTOR = US_RTOR_TO( 8 * 4);" but it's value always zero. The register is W/R. How to do this ?Thanks!

[blue_z]

it's [sic] value always zero. The register is W/R.

Peripheral registers that do not respond properly are usually an indication of a disabled peripheral, i.e. its peripheral clock has not been enabled in the Power Management Controller.

Regards