What is the exact mechanism by which clock reconvergence pessimism removal (CRPR/CPPR) computes its credit?
From PDVerse STA Mentor Guide · pdVerse Mentor Guide
Technical Explanation
CRPR looks at the clock tree from its root down to the point where the launch and capture paths diverge, computes how much the early and late derated delays differ across that shared segment, and adds that difference back into the timing slack as a credit.
Timing-path Reasoning
Because the shared trunk is physically one and the same piece of clock network for both the launching and capturing flop, it cannot simultaneously be at its slowest (late derate) for one path and its fastest (early derate) for the other. CRPR computes the early/late delay difference on that shared segment up to the branch point and credits it back to slack, effectively canceling out the portion of derate spread that was applied to a segment both paths actually experience identically.
Expected Result
Slack calculations that would otherwise be artificially pessimistic due to derating a shared clock segment in opposite directions are corrected, recovering slack up to the amount of the computed shared-segment delta.
Failure Symptoms
Without this credit, designs with deep shared clock trees show worse (falsely pessimistic) slack than physically justified, sometimes producing reported violations that are pure derate artifacts rather than real risk.
Root Cause
OCV derating the shared clock trunk both ways at once (slow for late-side use, fast for early-side use) creates artificial pessimism since the trunk cannot be both slow and fast simultaneously.
Mentor Note — Debugging Procedure
Identify the common clock trunk from root to the branch point separating launch and capture paths, and confirm the tool is computing and applying the early/late delta on exactly that shared segment as a slack credit. CRPR's credit is not a heuristic fudge — it is a direct computation of the delay difference on the physically shared clock segment, restoring physical realism to setup/hold slack on paths with common clock ancestry.
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