Useful Skew Tradeoffs: Worked Example

Worked example

Worked Example 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.

A signoff run shows skew budget used, hold impact, clock tree power. The first review mistake is to blame implementation without checking constraints. A better review starts by pinning one critical path, proving where slack was lost, and checking whether the SDC rule is correct for Useful Skew Tradeoffs.

Sequence under inspection

TIMING PATH — Useful Skew Tradeoffs

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: skew budget used, hold impact, clock tree power

Useful skew borrow

CLOCK AT CAPTURE FF delayed intentionally

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

1. Capture report_timing and the relevant SDC excerpt.

2. Tag corner, mode, view, and path group.

3. Find the first cell or net that dominates critical delay.

4. Compare against CTS skew report, useful skew map, hold regression.

5. Choose one reversible ECO and write the regression matrix before editing netlist or constraints.

Did the fix work?

BEFORE / AFTER ECO — Useful Skew Tradeoffs

WNS (ps)
  0 |                    --- target
-50 |    ● regressed
-80 |         ● before ECO
-20 |              ● after ECO
    +-------------------------------> ECO iteration
Prove setup AND hold moved safely.

STA deep dive

ECO without regression is a coin flip.

Concept diagram

ECO LOOP

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

Metric 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