Re: skew value
hi ,
Let us first Understand what positive skew and negative skew mean.
Suppose we have a register to register path with a combi logic (combi cloud ) between them.
Irrespective of setup or hold analysis , if the 2nd register receives clock after 1st register ,then it is case of positve skew .
If the 2nd register receives clock before the 1st register ,then it is the case of negative skew.
Based on this concept let us modify the basic for setup and hold equations so that there are no violations:
setup analysis
ignoring launch clock path and capture clock path)
+skew:
T(clk_q )+T(combi)+Tsetup<=Time period+Tskew.
-skew:
T(clk_q )+T(combi)+Tsetup<=Time period -Tskew.
Hold analysis
ignoring launch clock path and capture clock path).
+skew:
T(clk_q )+T(combi)>=T hold+Tskew.
-skew:
T(clk_q )+T(combi)>=T hold-Tskew