STA Mastery · All levels

Graph Traversal & Slack: Interview Drills

Interview Drills for Graph Traversal & Slack.

Interview drills

Interview Drills 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.

diagram

PROMPT
You see WNS, TNS, path slack distribution, failing endpoint count on Graph Traversal & Slack. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains STA builds a timing graph of cell and net arcs; setup slack = required time − arrival time along the worst path.
3. Requests report_timing -path_type full, slack histogram, path schematic.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to upsizing cells, widening routes, or blaming tools without reading constraints and path delay.

Diagram to draw on the 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).

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Graph Traversal & Slack

WNS, TNS, path slack distribution, failing endpoint count looks wrong
        |
   same netlist tag?
     /        \
   no          yes
   |            |
 view/SDC     real path or
 mismatch     constraint bug
 /    \           |
lib   SPEF    delay breakdown
diff  diff    cell vs net

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

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