Graph Traversal & Slack: Worked Example

Worked example

Worked Example for Graph Traversal & Slack focuses on WNS, TNS, path slack distribution, failing endpoint count. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

A signoff run shows WNS, TNS, path slack distribution, failing endpoint count. 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 Graph Traversal & Slack.

Sequence under inspection

TIMING PATH — Graph Traversal & Slack

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: WNS, TNS, path slack distribution, failing endpoint count

Timing graph arcs

TIMING GRAPH

FF1 --cell--> U1 --net--> U2 --cell--> FF2
 |                                      |
 launch point                           capture point

STA walks worst path (max delay for setup, min for hold).

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 report_timing -path_type full, slack histogram, path schematic.

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

Did the fix work?

BEFORE / AFTER ECO — Graph Traversal & Slack

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

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.

Principal STA review addendum

STA builds a timing graph of cell and net arcs; setup slack = required time − arrival time along the worst path.

Metric: WNS, TNS, path slack distribution, failing endpoint count