Cell vs Net Delay Budgets

What this topic teaches

Cell vs Net Delay Budgets is about converting constraints and path rules into measurable slack. Path delay splits between library cell arcs and interconnect parasitics; implementation fixes target the dominant contributor. The hard part is never the textbook inequality; it is proving, under the right corner/mode/view, which layer — constraint, view, or implementation — broke the contract.

The senior-engineer question

When cell delay %, net delay %, high fanout net count, transition violations moves, can you identify the failing path group, the STA layer, the responsible owner, and the smallest experiment that proves the root cause?

STA CLOSURE FLOW — Cell vs Net Delay Budgets

constraints (SDC)
        |
        v
timing graph (cell + net delays)
        |
        v
MMMC scenarios (corner x mode x view)
        |
        v
variation + SI adjustments
        |
        v
slack (WNS/TNS) -> ECO -> regression

Debug rule: always state corner, mode, view, and database tag with slack.

Picture the timing closure flow

Start every study session by drawing the behavior before reading signals. The diagrams below are the mental models to reproduce on a whiteboard.

Delay breakdown bar

PATH DELAY BREAKDOWN

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

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

Timing path sequence

TIMING PATH — Cell vs Net Delay Budgets

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: cell delay %, net delay %, high fanout net count, transition violations

Who owns which layer

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

Evidence to collect

• Primary metric: cell delay %, net delay %, high fanout net count, transition violations.

• Primary artifact: report_timing -cap -tran -nets, delay breakdown report, RC extraction log.

• Owners to bring into review: PD owner, STA owner, extraction owner.

• Spec clause or requirement ID for every constraint claim.

• One failing report_timing excerpt and one reduced path that isolates the rule.

Ownership map

OWNERSHIP MAP — Cell vs Net Delay Budgets

artifact              owner
----------------      -----------------
timing report       PD owner
SDC/constraint      STA owner
implementation      extraction owner

Every WNS cluster needs a named owner before ECO.

Subpages in this topic

Each topic is taught across mechanism, inputs/outputs, reports, debug, worked example, pitfalls, interview, checklist, theory, design space, expanded case study, walkthrough, comparison matrix, software view, and silicon PPA impact.

Key takeaways

• State corner, mode, and view with every slack number.

• Separate constraint bugs, view mismatches, and real path delays before ECO.

• Run setup and hold plus full MMMC after every change.

Common pitfalls

• Comparing runs with different SPEF or SDC tags.

• Broad false_path to hide violations.

• Setup-only ECO without hold regression.

STA deep dive

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

Concept diagram

TIMING GRAPH

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

Metric graph

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.