BeginnerQuestion 7 of 20Source page 5

Why doesn't a standard cell have just one fixed delay?

From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series

Direct answer

Because delay isn't a stored number — it's the output of a function that depends on the PVT condition, the input transition (slew), the output load, which specific timing arc is being measured, and the model used to compute it.

Mentor explanation

It's tempting to think of a library as a lookup table of "this cell equals this many picoseconds," but that's not how it works. The same physical inverter can report a very different delay depending on how fast its input is switching and how much capacitance it's driving on its output, on top of the PVT condition it was characterized under. That's why a netlist alone tells you nothing about actual delay — it tells STA which cell exists, and the library plus the surrounding context tells STA how that instance of the cell actually behaves right now.

Example or conceptual chain

Cell delay = f(PVT, input slew, output load, transition direction, timing arc, characterization model).

Key takeaways

  • Because delay isn't a stored number — it's the output of a function that depends on the PVT condition, the input transition (slew), the output load, which specific timing arc is being measured, and the model used to compute it.
Visual explanationMMMC context: Why doesn't a standard cell have just one fixed delay?
MMMC context: Why doesn't a standard cell have just one fixed delay?A three-step concept map summarizes the focus, core answer, and practical verification for Why doesn't a standard cell have just one fixed delay?Question focusWhy doesn't a standardcell have just one fixeddelay?Core answerBecause delay isn't astored number — it's theoutput of a…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.
  3. Name one caveat.
  4. Say how you would verify it in a real flow.

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Related MMMC questions

BeginnerWhat does PVT actually stand for, and what does each letter mean?BeginnerWhat is a timing library, really?BeginnerWhat is input slew, and why does it change a cell's delay?ExpertWhy can a change in RC alone also change cell delay, even when the cell itself is untouched?