The same clock path can pass at zero slack and then fail when one stage adds a little more delay. The arithmetic here treats each delay and the 6 second budget as exact training numbers; real timing decisions include derating, clock uncertainty, routing variation, setup characterization, and measured tolerance.
Changing one stage delay can turn zero slack into negative slack. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
One extra second breaks the same path
Changing only the last stage to 2 seconds makes the total delay 7 seconds.
2+3+2=7s
Delay rows cross from closed to rejected
Each row keeps the same clock budget and recomputes total delay and slack.
dtotal6s7s8sschain0s−1s−2sgatepassfailfail
The delay total exceeds the relative budget
The budget remains 6 seconds, so the slack becomes -1 second and the path is rejected.