STA Mastery · All levels

View Mismatch Triage

STA Debug Playbook: Different SPEF, RC corners, or parasitic modes between runs explain apparent regressions without any design change.

What this topic teaches

View Mismatch Triage is about converting constraints and path rules into measurable slack. Different SPEF, RC corners, or parasitic modes between runs explain apparent regressions without any design change. 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 slack delta between views, SPEF tag mismatch count moves, can you identify the failing path group, the STA layer, the responsible owner, and the smallest experiment that proves the root cause?

diagram

STA CLOSURE FLOW — View Mismatch Triage

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.

Run diff checklist

diagram

COMPARE TWO STA RUNS
same?  netlist tag
same?  SDC tag
same?  SPEF tag / corner
same?  lib corner map
same?  tool version
first NO -> explain slack delta

Timing path sequence

diagram

TIMING PATH — View Mismatch Triage

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: slack delta between views, SPEF tag mismatch count

Who owns which layer

diagram

STA OWNERSHIP LAYERS — View Mismatch Triage

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: slack delta between views, SPEF tag mismatch count.
Primary artifact: view diff report, SPEF metadata, run comparison script.
Owners to bring into review: STA owner, extraction owner, CAD 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

diagram

OWNERSHIP MAP — View Mismatch Triage

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

Debug hygiene separates senior STA leads from report readers.

Concept diagram

diagram

DEBUG ORDER

1. same tags?  2. constraints?  3. view?  4. real path?  5. ECO

Metric graph

diagram

ROOT CAUSE MIX (typical)

constraint bugs  ████████  35%
view mismatch    ██████    25%
real path        ██████    25%
methodology      ████      15%

Reports and artifacts

run manifest
SDC diff
SPEF tag
scenario isolation

Mini case study

Mystery regression: SPEF corner tag typo in run script — not a design bug.

Debug branches

Manifest every run
War room by path group

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.

Practice this lesson