STA Mastery · All levels

Path Group Prioritization: Interview Drills

Interview Drills for Path Group Prioritization.

Interview drills

Interview Drills for Path Group Prioritization focuses on WNS by path group, reg2reg vs in2reg vs reg2out slack. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see WNS by path group, reg2reg vs in2reg vs reg2out slack on Path Group Prioritization. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Path groups classify timing intent so optimization and debug focus on the paths that matter for signoff and product modes.
3. Requests path group summary, group_policy SDC, endpoint classification report.
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

Path group map

diagram

PATH GROUPS

reg2reg   : FF -> logic -> FF     (core closure)
in2reg    : input -> FF           (interface)
reg2out   : FF -> output          (interface)
reg2cg    : FF -> clock gate      (special hold risk)

Optimize and report WNS per group — not global average only.

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Path Group Prioritization

WNS by path group, reg2reg vs in2reg vs reg2out slack 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

Path groups classify timing intent so optimization and debug focus on the paths that matter for signoff and product modes.

Metric: WNS by path group, reg2reg vs in2reg vs reg2out slack

Practice this lesson