What is a timing library, really?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
A timing library (Liberty .lib / compiled .db) is the SPICE-characterized rulebook for every cell in a library at one specific PVT point — specifying its logic function, pin capacitances, propagation delays, output transition tables, sequential timing checks (setup, hold, recovery, removal), and power.
Mentor explanation
A timing library provides the SPICE-characterized electrical model of standard cells and macros at a single PVT condition.
Key terms
- Characterization — the foundry-side process of simulating every cell across many input-slew/output-load combinations in SPICE and recording the results.
- NLDM (Non-Linear Delay Model) — classic voltage-threshold-based 2D table lookup for delay and slew.
- CCS/ECSM (Composite Current Source) — current-source-based models that capture nonlinear driver behavior more accurately for modern crosstalk and noise analysis.
- Setup/hold/recovery/removal/min-pulse-width — the sequential-cell checks stored in the library for flip-flops and latches.
Because a .lib file is characterized at exactly one PVT point, a signoff-grade MMMC flow needs a separate library file per corner you check — you cannot reuse one library across both SS/125°C and FF/-40°C corners.
Practical example
Liberty (.lib) Timing Group Snippet:
pin(Y) {
direction : output;
function : "(!A)";
timing() {
related_pin : "A";
timing_type : combinational;
cell_rise(delay_template_7x7) {
values("0.018, 0.024, 0.038", \
"0.028, 0.035, 0.051", \
"0.046, 0.056, 0.074");
}
rise_transition(delay_template_7x7) { ... }
}
}Interview trap
Assuming a single .lib covers every corner. Each library represents one PVT point only; MMMC requires a complete library set per corner, pointing to consistently characterized files.
Key takeaways
- A timing library is a SPICE-characterized database of cell delays, slews, capacitances, and constraints.
- Each library file represents exactly one PVT operating point.
- Advanced nodes use Composite Current Source (CCS) models rather than NLDM to capture non-linear driver behavior.
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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