Pessimism due to different transition sense in the common clock path
the case when a positive-edge triggered FF is talking to a
negative-edge triggered FF. In this case, the transition sense for the
cells constituting the common clock path would be different for launch
and capture. Rise and fall times of the cells can be different. Hence we
need to take into account the difference between the derated rise and
fall times of the cells while calculating CPPR.
Figure 4: Different transition sense for launch and capture clock path at BUF1
Now, consider two possibilities:
Take the late delay for launch (at rise edge, in the above case) and
early delay for capture (at fall edge, in the above case).
Setup Slack would be: -1.1 + 1.8 = 0.7
the premise for this calculation is the assumption that BUF1 exhibits
late and early derates simultaneously. This is not a plausible scenario
and hence adds extra pessimism in the timing analysis.
To avoid extra pessimism in timing analysis, for the same cell in the
common clock path, one should take either late or early derates for both
the launch and capture paths.
Two possible cases:
Assuming Late derates for both launch and capture:
case for setup would correspond to the one with minimum clock skew. In
the above example, early case presents are more critical scenario with
Setup Slack would now be: -0.9 + 1.8 = 0.9.
Hence Case II has lesser pessimism than Case I.