I sovled this question later,it's because that you must write a code of writing data and address before write the register FCMD.
But I need to do with another new question,I have not add watchdog in my main(),and I used 0x25 (constant writing).If I excute the program step by step ,it will be OK.But when I excute it wholy in once,the program will ran away.it will run to 0xbd ,0xc0.
I want to know ,where is the problem. anyone could support a help?thank you.