negative propagation delay为什么会是负数？

The SAGE-X Standard Cell Library may contain negative propagation delays. Although most third-party verification tools can handle negative propagation delays, some tools will turn negative delays into a zero value.
传输延时应该是个正数啊，怎么还有负数这种情况?

这个跟定义的百分比还有输入信号的slew、输出的负载有关系如果输入信号的rise time很小，而负载很大，那么delay肯定是正的
如果输入信号的rise time很大，而负载很小，那delay就不好讲了