Signoff vs Implementation Views: Worked Example

Worked example

Worked Example for Signoff vs Implementation Views focuses on view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

A signoff run shows view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap. The first review mistake is to blame implementation without checking constraints. A better review starts by pinning one critical path, proving where slack was lost, and checking whether the SDC rule is correct for Signoff vs Implementation Views.

Sequence under inspection

TIMING PATH — Signoff vs Implementation Views

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap

View comparison

SAME NETLIST — TWO VIEWS

impl_view SPEF (typical)     WNS = +50ps
signoff_view SPEF (cworst)   WNS = -30ps

Delta is expected; panic only if delta exceeds historical bound.

1. Capture report_timing and the relevant SDC excerpt.

2. Tag corner, mode, view, and path group.

3. Find the first cell or net that dominates critical delay.

4. Compare against view comparison report, SPEF tag manifest, delta slack log.

5. Choose one reversible ECO and write the regression matrix before editing netlist or constraints.

Did the fix work?

BEFORE / AFTER ECO — Signoff vs Implementation Views

WNS (ps)
  0 |                    --- target
-50 |    ● regressed
-80 |         ● before ECO
-20 |              ● after ECO
    +-------------------------------> ECO iteration
Prove setup AND hold moved safely.

STA deep dive

Signoff WNS is the minimum across the full scenario matrix.

Concept diagram

MMMC

corners (ss/tt/ff) x modes (func/test) x views (signoff RC)
= scenario set -> min(WNS) is signoff

Metric graph

WNS HEATMAP BY CORNER

        ss    tt    ff
setup  -80   +10   +5
hold   -15   -2    -8

Reports and artifacts

• per-corner WNS

• mode summary

• lib map audit

• view delta

Mini case study

Passed TT typical but failed SS hold — signoff matrix was incomplete; added low-voltage mode.

Debug branches

• Isolate failing scenario first

• Never compare unlike SPEF tags

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

Implementation and signoff use different parasitic corners and analysis views; comparing them without mapping causes false panic or false confidence.

Metric: view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap