Cell vs Net Delay Budgets: Interview Drills

Interview drills

Interview Drills for Cell vs Net Delay Budgets focuses on cell delay %, net delay %, high fanout net count, transition violations. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

PROMPT
You see cell delay %, net delay %, high fanout net count, transition violations on Cell vs Net Delay Budgets. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Path delay splits between library cell arcs and interconnect parasitics; implementation fixes target the dominant contributor.
3. Requests report_timing -cap -tran -nets, delay breakdown report, RC extraction log.
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

Delay breakdown bar

PATH DELAY BREAKDOWN

cell delay  ████████████████  62%
net delay   ██████████        38%

If net-heavy -> routing/RC fix.
If cell-heavy -> sizing/VT/upsize.

Root-cause tree to narrate

ROOT-CAUSE TREE — Cell vs Net Delay Budgets

cell delay %, net delay %, high fanout net count, transition violations 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

Slack is a path story — read the graph, not just the number.

Concept diagram

TIMING GRAPH

FF --cell-- logic --net-- logic --cell-- FF
launch                              capture

Metric graph

SLACK TAIL

paths |    ***
      |  *******
      |*********  <- fix WNS first

Reports and artifacts

• WNS/TNS

• path slack histogram

• cell vs net %

• path group summary

Mini case study

WNS -90ps but 80% net delay on one congested route — PD route fix beat upsizing three cells.

Debug branches

• Cell-heavy -> sizing/VT

• Net-heavy -> route/layer

• Check launch/capture clocks

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

Path delay splits between library cell arcs and interconnect parasitics; implementation fixes target the dominant contributor.

Metric: cell delay %, net delay %, high fanout net count, transition violations