Explain how CRPR, crosstalk, and PBA interact for an SI-critical clock path, end to end.
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Three layers: base CRPR removes the OCV double-count on the shared clock portion for final slack; CRPR plus crosstalk removes the coupling delta on the common segment only for zero-cycle checks; and in PBA, pba_enable_xtalk_delay_ocv_pessimism_reduction extends CRPR to the arrival windows.
Complete Technical Explanation
There are three layers, and they stack. The base layer is ordinary CRPR: it removes the OCV double-count on the shared clock portion when final slack is computed, and it does so on every path whenever it is enabled. The second layer is CRPR interacting with crosstalk, governed by the zero-cycle rule. On the common segment, crosstalk-induced delay is removed only for zero-cycle checks, where the same edge launches and captures, because only then does the aggressor affect the launch and the capture signal identically. For any other check the two events are different edges at different times, the delta cannot be assumed identical, and it stays in. The third layer is arrival-window pessimism in path-based analysis. During PBA the arrival windows used for crosstalk timing do not get CRPR by default, so derating on a shared clock segment can falsely widen the aggressor and victim windows and manufacture overlaps that cannot physically occur. Setting pba_enable_xtalk_delay_ocv_pessimism_reduction to true applies CRPR to those windows as well. Getting an SI-limited clock path genuinely accurate can require understanding all three and, where appropriate, enabling all three - at escalating runtime cost, since each layer moves the CRPR accounting into a more expensive part of the computation.
In Pd / Signoff
This is how you triage an SI-limited clock path at post-route signoff. You establish that base CRPR is on, decide whether the check is zero-cycle to know if the coupling delta should be credited, and only then reach for arrival-window pessimism reduction in PBA. The layering tells you which runtime you are about to spend and what accuracy you get back.
Common Trap
Treating CRPR as one switch, so that enabling it is assumed to have removed all shared-clock pessimism including crosstalk deltas and arrival-window widening. Each layer is separately scoped, and the crosstalk layer is further restricted to zero-cycle checks, so a path can be CRPR-enabled and still carry two kinds of pessimism.
Expect Next
For a non-zero-cycle SI-limited clock path, which of the three layers can still recover pessimism and which cannot?
Topic And Primetime Commands
Operating Conditions / On-Chip Variation. pba_enable_xtalk_delay_ocv_pessimism_reduction
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