Explain the interaction between CCS receiver modeling and max-transition/max-capacitance DRC characterization.
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
CCS adds receiver_capacitance1/2 and driver tables, so safe max_capacitance is the lowest maximum load index across all of them and safe max_transition the lowest maximum slew index across NLDM, CCS driver and receiver_capacitance2. Otherwise DRC passes while delay calc extrapolates.
Complete Technical Explanation
The purpose of the design-rule limits, from a delay-calculation point of view, is to guarantee that delay calculation never has to extrapolate beyond characterized data. CCS complicates that guarantee because it consults more tables than NLDM does: receiver_capacitance1 and receiver_capacitance2 tables for the load, plus driver tables for the slew, in addition to the NLDM tables. So the safe max_capacitance is the lowest maximum load index across all of them, and the safe max_transition is the lowest maximum slew index across the NLDM tables, the CCS driver tables and receiver_capacitance2. Set the limit from the NLDM table alone and you get a specific silent failure: a CCS table with a smaller max index will be extrapolated - raising RC-011 - on nets whose NLDM-based DRC check reports a clean pass. The companion setting closes the other half of the loop. With report_capacitance_use_ccs_receiver_model true, the max-capacitance checks reflect the CCS loads that delay calculation actually uses rather than the smaller lumped pin caps, so the check and the model agree on what the load is. The two features are coupled by design: correct DRC limits are what keep CCS delay calculation inside its own tables.
In Pd / Signoff
This belongs in library qualification, not in block-level timing. When a library is brought up, derive max_transition and max_capacitance from the minimum of the maxima across NLDM and CCS tables, and enable report_capacitance_use_ccs_receiver_model true in the signoff script. A passing DRC report is only meaningful if the limits were derived that way.
Common Trap
Deriving the DRC limits from the NLDM tables alone. If a CCS table has a smaller max index, delay calc extrapolates and raises RC-011 while max_transition and max_capacitance both report clean - so the design looks DRC-legal on nets whose delays are extrapolated and inaccurate.
Expect Next
Why does max_transition include receiver_capacitance2 in the set of tables you take the minimum maximum slew index over?
Topic And Primetime Commands
Delay Calculation / CCS Modelling. report_capacitance_use_ccs_receiver_model
Keep building interview depth
Continue practising STA
Continue learning free
Get a practical low-power chapter
Receive the existing “Low Power and Multivoltage Fundamentals” PDF chapter and its download link by email.
Continue practising