在锁相环时钟频率切换过程中，只有当锁相环稳定后CPU才会切换到新的PLL设置。因此在设置完PLLCR后需要等待PLL稳定。PLL的切换时间大约等于131072个输入时钟周期。 -The PLL clock frequency switching process only when the phase-locked loop stable CPU will switch to the new PLL settings. After setting PLLCR need to wait for the PLL to stabilize. The switching time of the PLL is approximately equal to 131 072 input clock cycles.