Useful Skew

 Useful Skew

• By intentionally adding some delay to in the clock path at clock pin of capture path or launch path is called as Useful skew

Consider the above figure

• The above figure has two timing paths from F1 to F2 and From F2 to F3
• There is timing violation in first timing path , We have setup margin in F2 to F3
• By adding some delay to the clock pin of the Flop F2 , it will fix the setup time violation

• Example DRT is 500 ps ,DAT is 600 ps , setup is violated with 100 ps

• consider From Flop F1 to Flop F2, there is timing violation of -100 pico sec, setup slack = DRT-DAT, hold is 200ps
• From Flop F2 to Flop F3 ,there is a setup margin of 500ps and hold margin of 100ps

• Obsevartions

1. After adding the delay to the clock pin of the flop F2 , setup got fixed between the F1 and F2 but hold time may failure we should take care of that, based on the margin and hold slack we can add the delay 
2. Between the F2 and F3 setup may violate but hold may benefit,  if we add more delay to the clock pin of the F2

Care fully observe the below figure