Critical path

Critical path. A combinational cone connects FF_A to FF_Z through gates, each with its own delay. Several paths run through it, all with the same number of gates - so you cannot spot the worst one by counting. You have to add up the gate delays.

• Click FF_A, then follow one path's gates to FF_Z; the delay sums as you go (click the last gate again to back up). Trace each path to compare their sums.
• Submit the path with the largest total delay - arrival = clock-to-Q plus the gate-delay sum, and the biggest arrival has the least slack. That is the critical path, the one you would fix first.