29 March 2008

PVT, Derarting and STA

What is the derate value that can be used?
  • For setup check derate data path by 8% to 15%, no derate in the clock path.
  • For hold check derate clock path by 8% to 15%, no derate in the data path.

What are the corners you check for timing sign-off? Is there any changes in the derate value for each corner?
  • Corners: Worst, Best, Typical.
  • Same derating value for best and worst.For typical it can be less.

Write Setup and Hold equtions?
  • Setup equation: Tlaunch clock + Tclk-q_max + Tcombo_max <= Tcapute clock - (Tsetup+skew)
  • Hold equation: Tlaunch clock + Tclk-q_min + Tcombo_min >= Tcapture clock + (Thold-skew)

Where do you get the WLM's? Do you create WLM's? How do you specify?
  • Wire Load Models (WLM) are available from the library vendors.
  • We dont create WLM.
  • WLMs can be specified depending on the area.

Where do you get the derating value? What are the factors that decide the derating factor?
  • Based on the guidelines and suggestions from the library vendor and previous design experience derating value is decided.
  • PVT variation is the factor that decides the derating factor.

What factors decides the setup time of flip-flop?
  • D- pin transition and clock transition.

Why dont you derate the clock path by -10% for worst corner analysis?
  • We can do. But it may not be accurate as the data path derate.

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