Saturday 12 December 2015

Useful Skew

Useful skew-If clock is skewed intentionally to resolve violations, it is called useful skew.

For example there is setup violation in the design, Then we add some skew along the clock path in order to eliminate the setup violation.

You have three registers A B C each getting the same clock A.
Now, the latency of clockA in A is 1 (slack 12 say) clockA in B 12 (slack -12) So clock B is setup violated. There are many ways of fixing setup violation like adding buffers to speed up the data path etc... but let us assume that we cannot use any of this and the only possible way to get this done is tweak the clock path (quite dangerous). Since Setup slack = required time (clock) - arrival time (data) if i reduce the required time my slack violation will cease. Now, i add delay buffers in clock path after clock A in A (not before) so that only the clock A for B is delayed.
Interesting to note here is that delay in clock path for B will also delay data path for the flop to which the output of flop B goes (assume C) If this C has setup margin such that B can add that delay only then can you use the skew. This is called useful skew which is used to fix violations by taking slack from some and giving it to the other

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