STA Mastery · All levels

Setup / Hold Closure Order: Interview Drills

Interview Drills for Setup / Hold Closure Order.

Interview drills

Interview Drills for Setup / Hold Closure Order focuses on setup WNS, hold WNS, simultaneous violation count. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see setup WNS, hold WNS, simultaneous violation count on Setup / Hold Closure Order. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Setup and hold compete through delay changes; closure order and ECO type must avoid fixing one at the expense of the other.
3. Requests setup/hold trend chart, ECO sequence log, path replay.
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

Setup vs hold tradeoff

diagram

DELAY CHANGE EFFECT

increase cell delay:
  setup slack  improves  (more time before capture)
  hold slack   worsens    (data arrives later vs hold edge)

ECO must check BOTH sides.

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Setup / Hold Closure Order

setup WNS, hold WNS, simultaneous violation count 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

ECO without regression is a coin flip.

Concept diagram

diagram

ECO LOOP

failing path -> fix -> setup check -> hold check -> SI -> MMMC matrix

Metric graph

diagram

ECO PROGRESS

WNS
 0 |              o target
-50|        o
-100|   o baseline
    +-----------------> ECO #

Reports and artifacts

pre/post WNS
hold delta
leakage delta
ECO ticket log

Mini case study

48-hour tapeout crunch: VT swap fixed setup; hold fixed with route on short path — order mattered.

Debug branches

Never ship setup-only proof
Logical ECO needs equivalence

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

Setup and hold compete through delay changes; closure order and ECO type must avoid fixing one at the expense of the other.

Metric: setup WNS, hold WNS, simultaneous violation count

Practice this lesson