ExpertQuestion 5 of 10Source page 17

Why can a change in RC alone also change cell delay, even when the cell itself is untouched?

From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series

Direct answer

Because a cell's delay depends on its output slew, and output slew depends on the load it drives — which includes the interconnect. Change the RC, and you change the effective load and the receiver's input slew, which changes the delay the next cell reports, even though that next cell never physically changed.

Mentor explanation

This is one of the more counterintuitive feedback loops in timing, and it's a genuinely strong interview question because it forces you to connect the wire domain and the cell domain instead of treating them as separate stories. A change in RC alters the receiver's incoming waveform — its slew. That slew is one of the two axes (along with load) that every cell delay lookup depends on. So the next cell in the chain, with the exact same characterization data and the exact same physical instance, reports a different delay simply because the signal arriving at its input looks different. Nobody touched the cell. The wire feeding it changed the story anyway.

Example or conceptual chain

RC changes -> receiver waveform and load change -> slew changes -> the next cell uses a different delay-table lookup point -> cell delay changes.

Key takeaways

  • Because a cell's delay depends on its output slew, and output slew depends on the load it drives — which includes the interconnect.
  • Change the RC, and you change the effective load and the receiver's input slew, which changes the delay the next cell reports, even though that next cell never physically changed.
Visual explanationMMMC context: Why can a change in RC alone also change cell delay, even…
MMMC context: Why can a change in RC alone also change cell delay, even…A three-step concept map summarizes the focus, core answer, and practical verification for Why can a change in RC alone also change cell delay, even when the cell itself is untouched?Question focusWhy can a change in RCalone also change celldelay, even…Core answerBecause a cell's delaydepends on its outputslew, and output slew…Verify in practiceValidate the configuredview, active objects,and report context.Understand → explain the mechanism → verify the assumptions
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  1. State the direct answer.
  2. Explain the timing or physical reason.
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  4. Say how you would verify it in a real flow.

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