Timing analysis and CDC

A path has a clock period of 1.0 ns, a clock-to-output delay of 0.1 ns, a combinational logic delay of 0.85 ns and a setup time of 0.08 ns. If the clock arrives at the capturing flip-flop 0.05 ns later than at the launching flip-flop, what is the setup slack?

A−0.03 ns
B+0.02 ns
C−0.08 ns

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Answer: B. +0.02 ns

When the capture clock arrives late (positive skew), the capturing edge shifts later, giving more time for setup. The required time is 1.0 + 0.05 − 0.08 = 0.97 ns and the arrival time is 0.1 + 0.85 = 0.95 ns, so the slack is +0.02 ns. −0.03 ns ignores the skew, and −0.08 ns treats the skew in the opposite direction (as if the capture clock arrived early). Note that the same skew works against hold.

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