Why can a change in RC alone also change cell delay, even when the cell itself is untouched?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
Cell delay depends on output capacitive load (C_{load}) and input transition time (slew). A higher RC corner increases the capacitive load on the cell's output (slowing it down) and degrades interconnect slew, delivering a slower transition to all downstream receiver cells.
Mentor explanation
Interconnect parasitics directly modulate standard cell propagation delays.
Key terms
- Effective Capacitance (C_{eff}) — the portion of distributed wire capacitance actively seen by the driving gate.
- Input slew degradation — wire resistance and capacitance filter signal transitions, delivering slower ramps to downstream cells.
- NLDM/CCS lookup modulation — higher load and degraded slew index higher delay entries in the cell's Liberty table.
- Slew-delay feedback loop — a degraded wire slew increases cell delay, which further degrades the cell's output slew for the next stage.
Wire RC does not just add interconnect delay — it directly increases the internal propagation delays of standard cells throughout the datapath.
Practical example
RC Slew Feedback Chain:
RC Extraction Change (Metal Thickness Variations)
│
▼
Altered Wire Resistance & Capacitance (R_wire, C_wire)
│
▼
Modified Effective Load (C_eff) on Driver Gate U1
│
▼
Degraded Output Slew on Net 'net_A' (35ps -> 68ps)
│
▼
Slower Input Slew at Receiver Gate U2
│
▼
Receiver U2 indexes a different Liberty LUT entry -> Cell Delay increases by +25ps!Interview trap
Assuming that wire RC changes only affect net delay. Interconnect parasitics directly alter driving cell loads and downstream input slews, significantly increasing cell internal delays.
Key takeaways
- Wire RC alters resistive shielding (C_eff), directly changing driver output transition times.
- Degraded output slew travels to the receiver, causing it to look up higher cell delay in its Liberty tables.
- Cell delays and interconnect RC delays are dynamically coupled through slew propagation.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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