STA Mastery · All levels

Constraint Bug Patterns: Interview Drills

Interview Drills for Constraint Bug Patterns.

Interview drills

Interview Drills for Constraint Bug Patterns focuses on false violation count after SDC fix, constraint audit pass rate. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see false violation count after SDC fix, constraint audit pass rate on Constraint Bug Patterns. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Most 'mystery' STA failures are wrong clocks, missing generated clocks, bad I/O delay, or over-broad exceptions.
3. Requests SDC diff, constraint validator log, before/after path 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

Top constraint bugs

diagram

CONSTRAINT BUG TOP 5
1. missing generated_clock
2. wrong clock on latch/path
3. IO delay wrong direction
4. false_path too broad
5. case_analysis stale

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Constraint Bug Patterns

false violation count after SDC fix, constraint audit pass rate 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

Debug hygiene separates senior STA leads from report readers.

Concept diagram

diagram

DEBUG ORDER

1. same tags?  2. constraints?  3. view?  4. real path?  5. ECO

Metric graph

diagram

ROOT CAUSE MIX (typical)

constraint bugs  ████████  35%
view mismatch    ██████    25%
real path        ██████    25%
methodology      ████      15%

Reports and artifacts

run manifest
SDC diff
SPEF tag
scenario isolation

Mini case study

Mystery regression: SPEF corner tag typo in run script — not a design bug.

Debug branches

Manifest every run
War room by path group

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

Most 'mystery' STA failures are wrong clocks, missing generated clocks, bad I/O delay, or over-broad exceptions.

Metric: false violation count after SDC fix, constraint audit pass rate

Practice this lesson