STA Mastery · All levels

Cell vs Net Delay Budgets: Step-by-Step Walkthrough

Step-by-Step Walkthrough for Cell vs Net Delay Budgets.

Step-by-step analysis walkthrough

Use when you own Cell vs Net Delay Budgets in a timing war room.

Open signoff scenario list — note worst WNS corner.
report_timing -max/-min on failing group.
Trace launch and capture clocks on critical path.
Check exceptions and generated clocks on path.
Break down cell vs net delay.
Compare implementation vs signoff view if delta suspicious.
Assign owner: RTL, synth, PD, CTS, SI.
Propose ECO with hold/SI check plan.
Run regression matrix.
Update closure dashboard and memo.

Artifacts to collect

report_timing -cap -tran -nets, delay breakdown report, RC extraction log
SDC file
SPEF tag
MMMC scenario list
ECO ticket

Decision memo template

diagram

STA DECISION MEMO — Cell vs Net Delay Budgets
corner/mode:
WNS/TNS:
hypothesis:
ECO:
regression:
owners: PD owner, STA owner, extraction owner

Reference visuals

Delay breakdown bar

diagram

PATH DELAY BREAKDOWN

cell delay  ████████████████  62%
net delay   ██████████        38%

If net-heavy -> routing/RC fix.
If cell-heavy -> sizing/VT/upsize.

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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