STA Mastery · All levels
Useful Skew Tradeoffs: Comparison Matrix
Comparison Matrix for Useful Skew Tradeoffs.
Comparison matrix
Logical vs physical ECO differ in cost, risk, and turnaround.
+------------------+----------------+----------------+----------------+
| Approach | Strength | Weakness | Best when |
+------------------+----------------+----------------+----------------+
| Conservative | safe | slow closure | risk-averse |
| Balanced | practical | needs discipline | production |
| Aggressive | fast | regression risk | tapeout crunch |
| RTL change | structural | long loop | architectural |
+------------------+----------------+----------------+----------------+When to choose each approach
Match approach to schedule and silicon risk
Interview traps
One-size ECO playbook
Ignoring hold
Evidence comparison
STA EVIDENCE MATRIX — Useful Skew Tradeoffs
+-------------------+------------------------+--------------------------+-------------------------+
| Evidence | Tells you | Does not prove | Next action |
+-------------------+------------------------+--------------------------+-------------------------+
| report_timing | critical path slack | root cause class | cell vs net split |
| clock report | clock reachability | path delay | fix generated_clock |
| SDC diff | constraint change | physical delay | re-run all scenarios |
| SPEF tag compare | parasitic view | constraint quality | isolate view mismatch |
| MMMC dashboard | worst scenario | single-path fix | assign path group owner |
+-------------------+------------------------+--------------------------+-------------------------+STA deep dive
ECO without regression is a coin flip.
Concept diagram
ECO LOOP
failing path -> fix -> setup check -> hold check -> SI -> MMMC matrixMetric graph
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