Why must all libraries in the scaling groups have CCS noise data specifically, beyond CCS timing?
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Because SMVA is a full signoff feature that includes noise across voltage combinations, and voltage-dependent noise response - crosstalk bumps and immunity - is captured only in the CCS noise models. CCS timing alone cannot produce it.
Complete Technical Explanation
The reason comes down to what each model contains. CCS timing gives you delay behaviour, essentially how a cell drives a load. CCS noise gives you something different: the current-versus-disturbance waveform models that describe how a cell responds to and propagates a noise event. Crosstalk bumps and noise immunity are voltage-dependent phenomena, and their voltage dependence lives in those noise models, not in the delay tables. SMVA is not a timing-only feature. It is a full signoff capability, and that includes noise and SI analysis across voltage combinations - update_noise and report_noise are scenario-aware precisely because noise is analyzed per DVFS scenario. So the tool has to evaluate each cell's noise response at each voltage it might be operating at, which it can only do by scaling between library characterisations that actually carry noise data. That is why the requirement lands on every library in the scaling groups rather than most of them. A scaling group's job is to interpolate cell behaviour across voltage; if one member library lacks CCS noise data, there is nothing to interpolate for that portion of the voltage range and the noise computation across voltages cannot be performed. The distinction to hold onto for the interview is that having CCS libraries is not the same as having CCS noise libraries - the requirement is specifically about the noise models, beyond CCS timing.
In Pd / Signoff
This turns into a library-qualification gate before an SMVA run. When you assemble define_scaling_lib_group for each Vt flavour, verify CCS noise data is present in every member library at every corner, not just in the nominal one. A library set that passes a timing-only sanity check will still block the noise half of SMVA signoff.
Common Trap
Assuming CCS-qualified libraries are automatically adequate for SMVA. CCS timing satisfies delay scaling but not noise, so a scaling group missing CCS noise data in even one library leaves the voltage-dependent noise computation impossible - discovered late, when the noise stage of signoff is reached.
Expect Next
What specifically can CCS noise models represent about a cell's behaviour that CCS timing models cannot?
Topic And Primetime Commands
SMVA / Rail Voltages. update_noise, report_noise, define_scaling_lib_group
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