FPGA的全局时钟应该是从晶振分出来的，最原始的频率。其他需要的各种频率都是在这个基础上利用PLL或者其他分频手段得到的；因为全局时钟需要驱动很多模块，所以全局时钟引脚需要有很大的驱动能力，FPGA一般都有一些专门的引脚用于作为全局时钟用，他们的驱动能力比较强-FPGA' s global clock should be divided out from the crystal, the frequency of the most original. Other needs of the various frequencies are based on the use of this frequency PLL or other means to get　because many need to drive the global clock module, so there is great need for global clock pins drive capability, FPGA usually have some special pin is used as a global clock with their strong ability to drive