How to disable peripherals and set PLL bypass mode with 6431?

Dear Sir,

I already studied the spec spru 342, 871, 978 about power saving on 643x.

I write some API to disable peripheral and set PLL in bypass mode(27MHz).

But when I enable the peripheral and set the PLL back to 297MHz, my system can not work anymore, all the functions are dead.

Is there any sample code to disable peripherals and set PLL to bypass mode (and set them back to original state)?

Or any other document? Really appreciate your help!!

Best Regards,

Eric Fang