上电时，什么时候让6678的时钟输入有效？

SPRS691E—March 2014文档中，p122中，有如下说明：Once CVDD is valid it is acceptable that the P and N legs of these CLKs may be held in a static state (either
high and low or low and high) until a valid clock frequency is needed at that input.

请问：我怎么知道6678什么时候需要输入时钟？