Which units are used in physical design, timing, and power?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A number without its unit is just noise — PD juggles distance, time, capacitance, resistance, voltage, current, and power, often in different units across different files. Distance is usually micrometers (um) or nanometers (nm) — 1 um = 1000 nm. Time is nanoseconds or picoseconds — 1 ns = 1000 ps. Capacitance is picofarads or femtofarads — 1 pF = 1000 fF.
Technical Explanation
- A number without its unit is just noise — PD juggles distance, time, capacitance, resistance, voltage, current, and power, often in different units across different files.
- Distance is usually micrometers (um) or nanometers (nm) — 1 um = 1000 nm. Time is nanoseconds or picoseconds — 1 ns = 1000 ps. Capacitance is picofarads or femtofarads — 1 pF = 1000 fF.
- Each conversion is trivial on its own, but silently mixing them can shift a result by a factor of 1000 without any error message.
- Every format declares its own units: Liberty declares timing/capacitance units, DEF declares database units per micron, SPEF declares its own parasitic units — and the tool's display settings are separate from how the source file actually encodes data.
- A classic gotcha: "0.040" in a library with a 1 ns time unit means 40 ps; "0.002" with a 1 pF unit means 2 fF. Always read the declared unit before trusting the number.
- The same discipline applies to power and resistance — you can't drop a kilohm value into an ohms equation without converting it first.
Visual Verification
Physical design ties microscopic spatial dimensions (µm, nm) directly to electrical parasitics (fF, Ω) and picosecond timing delays; matching unit conventions across libraries is vital to prevent 1000× calculation errors.
Command Checks & Actions
set_units -time 1.0ns -capacitance 1000.0fFDeclares the units that every unqualified numeric value in the SDC is interpreted in -- the actual mechanism behind "which units does physical design use."
report_libConfirms which library is attached and its native units, since a mismatch between a library's characterization units and the SDC's set_units is a real, if uncommon, source of silently wrong numbers.
Common Mistake
The Trap: Assuming that all numeric values in SDC, Liberty, DEF, and SPEF use the same units.
Follow-up Question & Model Response
"How would you compare a 0.08 ns report with an 80 ps specification?"
Candidate Model Response: Convert both to one unit first; they represent the same time.
Practical Example
Tapeout Scenario: For a simple RC time constant, 2 kohm multiplied by 3 fF gives 6 ps. Converting first gives 2000 ohm times 0.000000000000003 farad = 0.000000000006 second. This is an RC time constant, not a complete cell or path delay.
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