AD6636 BIST module testing, how to configure register

Dear Sir/Madam,

I could control the read and write of the register. However, when I set BIST Control to enable, and set value to timer, input and channle register, the readout value of BIST I/Q Path Signature Register  is still the default value 6636. I thought it should be a randam value, but it doesn't. I wonder  how to set corresponding register.