TimeQuest Path Waveform Clock Pessimism

Pessimism should always be enabled.

Within a timing corner, their are two timing "sub-models", basically a fast and a slow. So when doing setup analysis on a path, it will use the slow sub-model for the source clock and data delay, and the fast sub-model for the destination clock path. For hold analysis on the exact same path, it will do the opposite. If you compare setup and hold analysis on the exact same path, you can visually see different numbers.

Part of this is rise-fall variation, but a large component is for On-Die Variation, which models the fact that different paths may not be at the exact fastest or slower allowed at the same time. This is all a good thing.

The problem is that the same clock feeds the source and destination. It's impossible to have on-die variation and rise/fall variation on the exact same signal, so it goes up to the point until the clock tree split, figures out how much pessimism was added because of these two models, and removes it. Again, it makes timing easier and is correct.