STA Mastery · All levels

Graph Traversal & Slack

Delay Graph & Path Analysis: STA builds a timing graph of cell and net arcs; setup slack = required time − arrival time along the worst path.

What this topic teaches

Graph Traversal & Slack is about converting constraints and path rules into measurable slack. STA builds a timing graph of cell and net arcs; setup slack = required time − arrival time along the worst path. 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 WNS, TNS, path slack distribution, failing endpoint 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 — Graph Traversal & Slack

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.

Timing graph arcs

diagram

TIMING GRAPH

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

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

Slack distribution

diagram

SLACK HISTOGRAM

paths
  |        *
  |      ****
  |    ********
  |  *************  <- WNS (left tail)
  +--------------------> slack (ns)
   -0.2  0.0  0.2

WNS = worst negative slack (most critical failing path).

Timing path sequence

diagram

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

Who owns which layer

diagram

STA OWNERSHIP LAYERS — Graph Traversal & Slack

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: WNS, TNS, path slack distribution, failing endpoint count.
Primary artifact: report_timing -path_type full, slack histogram, path schematic.
Owners to bring into review: STA owner, implementation 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 — Graph Traversal & Slack

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

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.

Practice this lesson