Noise Glitch Analysis: Interview Drills

Interview drills

Interview Drills for Noise Glitch Analysis focuses on noise peak, glitch width, functional fail rate on SI-critical nets. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

PROMPT
You see noise peak, glitch width, functional fail rate on SI-critical nets on Noise Glitch Analysis. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Beyond delta delay, aggressors can inject glitches on quiet victims; noise analysis checks functional thresholds on sensitive inputs.
3. Requests noise report, victim waveform, fix list (spacing/shielding).
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

Glitch on quiet victim

VOLTAGE
  ^     glitch peak
  |    /\
  |___/  \_____ victim (should stay quiet)
      ^ may cause false toggle on input FF

Root-cause tree to narrate

ROOT-CAUSE TREE — Noise Glitch Analysis

noise peak, glitch width, functional fail rate on SI-critical nets 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

Beyond delta delay, aggressors can inject glitches on quiet victims; noise analysis checks functional thresholds on sensitive inputs.

Metric: noise peak, glitch width, functional fail rate on SI-critical nets