请教：Quartus 编译警告问题

yzltax

Quartus 编译产生如下警告：
Warning: Found pins functioning as undefined clocks and/or memory enables
Info: Assuming node "key2" is an undefined clock
Info: Assuming node "key1" is an undefined clock

其中，key1,key2是INPUT引脚，实际是外接两个按键，不知上面警告是为什么，请高手指点，谢谢！