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Cell vs Net Delay Budgets: Pitfalls & Red Flags

Pitfalls & Red Flags for Cell vs Net Delay Budgets.

Pitfalls and red flags

Pitfalls & Red Flags for Cell vs Net Delay Budgets focuses on cell delay %, net delay %, high fanout net count, transition violations. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

A path looks clean in one view but fails in signoff SPEF.
The SDC allows behavior that the implementation team assumed was illegal.
A generated_clock definition shifts capture edge silently.
Hold fails on short paths after setup-focused ECO.
The fix improves one corner but regresses another MMMC scenario.

Layer responsibility check

diagram

STA OWNERSHIP LAYERS — Cell vs Net Delay Budgets

layer              owns                         failure mode
----------------   --------------------------   ---------------------
constraints        clocks, IO, exceptions       false violations
implementation     cells, nets, placement       real path delay
extraction         SPEF/RC corners              view mismatch
signoff policy     derate, MMMC matrix          margin disputes
closure           ECO order, regression        fix breaks other corner

STA deep dive

Slack is a path story — read the graph, not just the number.

Concept diagram

diagram

TIMING GRAPH

FF --cell-- logic --net-- logic --cell-- FF
launch                              capture

Metric graph

diagram

SLACK TAIL

paths |    ***
      |  *******
      |*********  <- fix WNS first

Reports and artifacts

WNS/TNS
path slack histogram
cell vs net %
path group summary

Mini case study

WNS -90ps but 80% net delay on one congested route — PD route fix beat upsizing three cells.

Debug branches

Cell-heavy -> sizing/VT
Net-heavy -> route/layer
Check launch/capture clocks

Senior review question

Ask: what corner/mode/view proves this topic is closed or failing?

Key takeaways

State corner, mode, view, and database tag with every slack claim.
Run setup and hold plus MMMC regression after every ECO.

Common pitfalls

Comparing STA runs with different SPEF or SDC tags.
Broad false_path to green-wash violations.
Setup-only ECO without hold check.

Principal STA review addendum

Path delay splits between library cell arcs and interconnect parasitics; implementation fixes target the dominant contributor.

Metric: cell delay %, net delay %, high fanout net count, transition violations

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