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Cell vs Net Delay Budgets: Comparison Matrix

Comparison Matrix for Cell vs Net Delay Budgets.

Comparison matrix

Path group and optimization focus determine closure efficiency.

diagram

+------------------+----------------+----------------+----------------+
| Approach         | Strength       | Weakness       | Best when      |
+------------------+----------------+----------------+----------------+
| Conservative     | safe           | slow closure   | risk-averse    |
| Balanced         | practical      | needs discipline | production     |
| Aggressive       | fast           | regression risk | tapeout crunch |
| RTL change       | structural     | long loop      | architectural  |
+------------------+----------------+----------------+----------------+

When to choose each approach

Match approach to schedule and silicon risk

Interview traps

One-size ECO playbook
Ignoring hold

Evidence comparison

diagram

STA EVIDENCE MATRIX — Cell vs Net Delay Budgets

+-------------------+------------------------+--------------------------+-------------------------+
| Evidence           | Tells you              | Does not prove           | Next action             |
+-------------------+------------------------+--------------------------+-------------------------+
| report_timing      | critical path slack    | root cause class         | cell vs net split       |
| clock report       | clock reachability     | path delay               | fix generated_clock     |
| SDC diff           | constraint change      | physical delay           | re-run all scenarios    |
| SPEF tag compare   | parasitic view         | constraint quality       | isolate view mismatch   |
| MMMC dashboard     | worst scenario         | single-path fix          | assign path group owner |
+-------------------+------------------------+--------------------------+-------------------------+

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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