What is CPPR (Common Path Pessimism Removal)? How is crosstalk considered in it for setup and hold?

o   OCV analysis (like AOCV/POCV or simple derating) applies different delay values for 'early' (fast) and 'late' (slow) conditions.

o   For a setup check, the launch clock path uses late delays, and the capture clock path uses early delays. For a hold check, it's reversed.

o   However, both clock paths often share a common segment starting from the clock root before diverging. Applying both early and late derates simultaneously to this same physical common path introduces artificial pessimism because the common path cannot physically be both fast and slow at the exact same instant.

o   Solution (CPPR): STA tool calculates delay difference of this common path by applying late and early derate. And this delay difference is credited in slack. Called CPPR. It helps in relaxing timing removing unnecessary pessimism.

o   Crosstalk Consideration in CPPR:

o   Crosstalk Impact on Paths: Crosstalk delay/noise is considered separately during the main timing calculation for both the data path and the clock paths (launch and capture) before CPPR is applied. Crosstalk can add delay (hurting setup, helping hold) or speed up signals (helping setup, hurting hold) depending on the relative switching directions of victim and aggressor nets.

o   Interaction:

For Setup Analysis:
Crosstalk can add extra delay, and STA tools often double-count it on both launch and capture paths.
This leads to an overly pessimistic result (sometimes assuming CRPR = 0).

For Hold Analysis:
Since hold checks happen on the same clock edge, the crosstalk effect is identical for both launch and capture paths.
This allows the extra pessimism to be removed, giving a more accurate result.

o    CPPR Overview: https://vlsi.pro/common-path-clock-reconvergence-pessimism-removal/