STA Mastery · All levels

Useful Skew Tradeoffs: Interview Drills

Interview Drills for Useful Skew Tradeoffs.

Interview drills

Interview Drills for Useful Skew Tradeoffs focuses on skew budget used, hold impact, clock tree power. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see skew budget used, hold impact, clock tree power on Useful Skew Tradeoffs. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Intentional clock skew can borrow setup time but increases hold risk and clock tree complexity; useful skew is a negotiated margin.
3. Requests CTS skew report, useful skew map, hold regression.
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

Useful skew borrow

diagram

CLOCK AT CAPTURE FF delayed intentionally

launch edge  |
data path    |-------->
capture edge |    +skew borrow
             more setup margin, hold risk increases

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Useful Skew Tradeoffs

skew budget used, hold impact, clock tree power 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

Intentional clock skew can borrow setup time but increases hold risk and clock tree complexity; useful skew is a negotiated margin.

Metric: skew budget used, hold impact, clock tree power

Practice this lesson