Power Management of PowerPC in Virtex-II Pro: Is it possible to decrease the clock dynamically?
You cannot change or stop the clock to PPC405 while it is operating -- you must put the processor into the wait state first. When the processor is in the wait state, you may do anything you wish to its clock. After the clock is stable again, you can bring the processor out of the wait state by using an interrupt or by resetting the core.
To change the clock on the PLB or any other synchronous bus, you must be sure that none of the masters or slaves connected to that bus have outstanding transactions.