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Timing analysis and CDC

On a path with a clock period of 1.25 ns (800 MHz), the clock-to-output delay plus combinational logic delay totals 1.05 ns, the setup time is 0.05 ns, and the setup clock uncertainty is set to 0.05 ns. What is the setup slack?

  1. A0.15 ns
  2. B0.20 ns
  3. C0.10 ns
Show answer and explanation

Answer: C. 0.10 ns

Clock uncertainty is a margin subtracted from the available time to account for jitter and skew that is not yet determined. The required time is 1.25 − 0.05 (uncertainty) − 0.05 (setup time) = 1.15 ns and the arrival time is 1.05 ns, so the slack is 0.10 ns. 0.15 ns leaves out the uncertainty, and 0.20 ns subtracts neither the setup time nor the uncertainty. Before place and route there is no clock tree, so it is common to set a larger uncertainty that also covers skew.

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More Timing analysis and CDC questions

  1. In clock uncertainty settings, period jitter is often included for setup but not for hold. Why?
  2. In STA, which combination correctly gives the corner conditions mainly used to check setup and hold?
  3. For OCV derating in setup analysis, the launch clock path and data path delays are multiplied by 1.1 and the capture clock path delay by 0.9. With a launch clock path of 1.0 ns, a data path (including clock-to-output) of 1.5 ns, a capture clock path of 1.0 ns (no common portion), a period of 3.0 ns and a setup time of 0.1 ns, what is the setup slack?
  4. In STA with OCV derating, when the launch and capture clock paths share a common portion, which correctly describes CRPR (common path pessimism removal)?
  5. In a circuit running on a 2.0 ns clock, a path has a data arrival time (clock-to-output plus combinational logic delay) of 3.3 ns and a setup time of 0.1 ns. Since this path only needs to capture once every two cycles, a setup multicycle path of 2 is set. What is the setup slack?