LIB File in Physical Design | Liberty File | .lib file

Lib file is a short form of Liberty Timing file. Liberty syntax is followed to write a .lib file. LIB file is an ASCII representation of timing and power parameter associated with cells inside the standard cell library of a particular technology node. Lib file is basically a timing model file which contains cell delay, cell transition time, setup and hold time requirement of the cell. So Lib file basically contains the timing and electrical characteristics of a cell or macros. Lib file is generated and provided to ASIC designer by a standard cell library vendor or Foundry if the foundry provides a standard cell library. 

The information inside the Lib file can be divided into two main parts, in the first part, it contains some information which is common for all the standard cells. To understand it better have a look in the following snapshot of the Lib file.

The common part of Lib file contains

  • Library name and technology name
  • Units (of time, power, voltage, current, resistance and capacitance)
  • Value of operating condition ( process, voltage and temperature) – Max, Min and Typical

Based on operating conditions there are three different lib files for Max, Min and Typical corners. In the second part of Lib file, it contains cell-specific information for each cell. The part of Lib file which contains cell-specific information is shown below.

Cell-specific information in Lib file is mainly

  • Cell name
  • PG Pin name
  • Area of cell
  • Leakage power in respect of input pins logic state
  • Pins details
    • Pin name
    • Pin direction
    • Internal power
    • Capacitance
    • Raise capacitance
    • Fall Capacitance
    • Fanout load

A snapshot of Lib file for the pin part is given below.

Timing and power parameter of a cell is obtained by simulating the cell in a variety of operating conditions and data are represented in the Lib file. There are two main techniques to characterize a cell and generated the Lib file.
      • CCS (Composite Current Source)
      • NLDM (Non-Linear Delay Model)
In CCS technique current source is used whereas in NLDM technique voltage source is used to model and derive the Lib parameters. Based on CCS and NLDM technique used for characterizing the cell, we call the corresponding Lib file is CCS Lib file and NLDM Lib file. Since CCS technique is having more controlling parameters as compare to NLDM technique, So CCS Lib file is more accurate. NLDM Lib file has lesser run time mean fast run compare to CCS Lib and also the size of the NLDM file is lesser than that of CCS Lib file.
 
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