Path Group Prioritization: Worked Example

Worked example

Worked Example 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.

A signoff run shows WNS by path group, reg2reg vs in2reg vs reg2out slack. 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 Path Group Prioritization.

Sequence under inspection

TIMING PATH — Path Group Prioritization

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: WNS by path group, reg2reg vs in2reg vs reg2out slack

Path group map

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.

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 path group summary, group_policy SDC, endpoint classification report.

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

Did the fix work?

BEFORE / AFTER ECO — Path Group Prioritization

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

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