Delta Delay Mechanism: Interview Drills

Interview drills

Interview Drills 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.

PROMPT
You see delta delay on critical paths, aggressor count, SI WNS impact on Delta Delay Mechanism. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Coupling capacitance between adjacent nets changes effective delay; aggressors slow or speed victims depending on switching alignment.
3. Requests SI report, coupled net list, delta delay histogram.
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

Aggressor / victim coupling

VICTIM NET  ~~~~~~~~~~~~ (quiet)
AGGRESSOR   ^   ^   ^   ^  (switching)

Aligned switching -> delta delay on victim
SI analysis adds coupling to effective RC

Root-cause tree to narrate

ROOT-CAUSE TREE — Delta Delay Mechanism

delta delay on critical paths, aggressor count, SI WNS impact 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

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.

Principal STA review addendum

Coupling capacitance between adjacent nets changes effective delay; aggressors slow or speed victims depending on switching alignment.

Metric: delta delay on critical paths, aggressor count, SI WNS impact