Liberty Format

Liberty format (also called .lib) is the industry-standard file format for describing the electrical characteristics of standard cells, I/O pads, and macro blocks — providing the timing, power, and other data that synthesis, place-and-route, and static timing analysis tools use to implement and verify digital designs.

What Liberty Files Contain

• Cell Definitions: Each cell (AND2, INV, DFFQ, etc.) is defined with:
• Pin declarations (input, output, inout)
• Pin functions (Boolean logic function)
• Pin capacitance values
• Timing arcs (every input→output delay path)
• Power data (switching, internal, leakage)

• Timing Information:
• Cell Delay: Propagation delay from each input to each output, stored as 2D lookup tables indexed by input transition time and output load capacitance.
• Output Transition: Rise/fall time at the output, also 2D tables.
• Setup/Hold Time: Sequential cell (flip-flop, latch) timing constraints — the minimum time data must be stable before/after the clock edge.
• Clock-to-Q Delay: Output delay of a flip-flop after the clock edge.

• Power Information:
• Dynamic Power: Switching energy per transition for each arc.
• Internal Power: Short-circuit power during switching.
• Leakage Power: Static power for each input state combination.

• Operating Conditions: The PVT corner (process, voltage, temperature) for which the data applies.

Liberty File Organization

library (my_lib_ss_0p75v_125c) {
  technology (cmos) ;
  delay_model : table_lookup ;
  nom_voltage : 0.75 ;
  nom_temperature : 125 ;
  cell (INV_X1) {
    pin (A) { direction: input; capacitance: 0.5; }
    pin (Y) { direction: output; function: "!A";
      timing () {
        related_pin: "A";
        cell_rise (lut_7x7) { index_1(...); index_2(...); values(...); }
        cell_fall (lut_7x7) { ... }
      }
    }
  }
}

Liberty Variants

• NLDM (Non-Linear Delay Model): Basic Liberty — 2D delay tables. Sufficient for most digital design.
• CCS (Composite Current Source): Adds current waveform data for more accurate delay computation. Used at advanced nodes.
• ECSM (Effective Current Source Model): Cadence's alternative advanced model.
• LVF (Liberty Variation Format): Extension that adds per-cell variation data for POCV analysis.

One Library, Many Corners

• A separate Liberty file is generated for each PVT corner (SS/TT/FF × voltage × temperature).
• A typical design uses 10–50+ Liberty files across all corners for MCMM analysis.

Liberty format is the universal language between library providers and EDA tools — it is the single most important data format in the digital design flow.