STA Mastery · All levels

Graph Traversal & Slack: Comparison Matrix

Comparison Matrix for Graph Traversal & Slack.

Comparison matrix

Path group and optimization focus determine closure efficiency.

diagram

+------------------+----------------+----------------+----------------+
| Approach         | Strength       | Weakness       | Best when      |
+------------------+----------------+----------------+----------------+
| max delay        | setup          | miss hold      | setup closure  |
| min delay        | hold           | miss setup     | hold closure   |
| both             | complete       | 2x runtime     | signoff        |
| path group       | focused        | miss others    | debug          |
+------------------+----------------+----------------+----------------+

When to choose each approach

Match approach to schedule and silicon risk

Interview traps

One-size ECO playbook
Ignoring hold

Evidence comparison

diagram

STA EVIDENCE MATRIX — Graph Traversal & Slack

+-------------------+------------------------+--------------------------+-------------------------+
| Evidence           | Tells you              | Does not prove           | Next action             |
+-------------------+------------------------+--------------------------+-------------------------+
| report_timing      | critical path slack    | root cause class         | cell vs net split       |
| clock report       | clock reachability     | path delay               | fix generated_clock     |
| SDC diff           | constraint change      | physical delay           | re-run all scenarios    |
| SPEF tag compare   | parasitic view         | constraint quality       | isolate view mismatch   |
| MMMC dashboard     | worst scenario         | single-path fix          | assign path group owner |
+-------------------+------------------------+--------------------------+-------------------------+

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

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

Practice this lesson