Delta Delay Mechanism: Reports & Metrics

Reports and metrics

Reports & Metrics for Delta Delay Mechanism focuses on delta delay on critical paths, aggressor count, SI WNS impact. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

The job of a report is to turn delta delay on critical paths, aggressor count, SI WNS impact into a closure decision. A single WNS number is almost never enough; you need path group breakdown, cell vs net split, and a clear gap between current slack and signoff target.

Metric movement

METRIC GRAPH — delta delay on critical paths, aggressor count, SI WNS impact

WNS (ps)
  ^
  |                         target (0)
  |                       - - - - - - -
  |                  o after ECO + hold check
  |              o
  |         o baseline run
  |    o failing regression
  +--------------------------------------> closure iteration
    view audit     path group fix     MMMC signoff

Readout:
  - explain why WNS/TNS moved and which path group drove the change

Slack distribution

SLACK HISTOGRAM — Delta Delay Mechanism

path count
  |      ***
  |    *******
  |  ***********
  |*************  <- WNS (worst path)
  +--------------------> slack (ns)
  -0.15  0.0  0.10

Fix the left tail first; average slack misleads.

• Track delta delay on critical paths, aggressor count, SI WNS impact by path group, corner, and analysis view.

• Report WNS/TNS and failing endpoint count together.

• Include setup and hold splits; they often diverge after ECO.

• Always store the metric next to the artifact that produced it.

STA deep dive

Coupling makes delay a function of neighbors, not just wire RC.

Concept diagram

SI ON VICTIM

victim net delay = base_RC + coupling_from_aggressors

Metric graph

DELTA DELAY ON CRITICAL PATHS

no SI   ████████  1.2ns
with SI ██████████  1.35ns  (+150ps)

Reports and artifacts

• delta delay report

• aggressor list

• noise peaks

• SI WNS delta

Mini case study

Setup fixed by upsize but SI worsened — spacing ECO on bus bits recovered margin.

Debug branches

• Re-run SI after every timing ECO

• Shield wide switching buses

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.

Reading reports