Exploring the Liberty File Format (.lib) in ASIC Design

The Liberty file format (.lib) is a widely used format for representing standard cell libraries in ASIC (Application-Specific Integrated Circuit) design. These files provide detailed timing and power information for logic cells, essential for various stages of digital design, such as synthesis, place and route, and timing analysis. the Liberty file format (.lib) has  information's to all logic cells in library ,only logic cells so filler cells , tap cells ,etc. don't have .lib view cause they are metals (they have only a physical view library exchange format(.lef).The (.lib) and (.db) files are equal but the only difference between them that (.lib) is readable.

What is the information that inside liberty file (.lib) ?

1.Cell Characterization:

Function: The logic function of the cell.

Area: The physical area occupied by the cell.

Leakage Power: The power consumed when the cell is not switching.

2.Operating Conditions:

Operating conditions define the conditions under which the cell's timing and power are characterized, such as voltage, temperature, and process corners.

3.Wire Load Models:

Wire Load Models used to estimate the delay introduced by interconnects based on the total cell area and net fanout.

4.Look-Up Tables:

Look-Up Tables that provide timing and power information based on input transition times and output load capacitances.

I'll give you example of  Liberty file format  (.lib ) from

nangate called NangateOpenCellLibrary_ff1p25vn40c.lib

Example: NangateOpenCellLibrary_ff1p25vn40c.lib

Here’s a structured overview of the NangateOpenCellLibrary_ff1p25vn40c.lib file:

•  First section of library liberty file ( .lib )

 General Attributes

This is  section of  Liberty file format (.lib) is General Attributes :

• Technology: Specifies the technology node.
• Delay Model: Indicates the delay model used, typically look-up tables.
• Units: Defines the units for various measurements (e.g., time in nanoseconds, power in microwatts).
• Operating Conditions: Specifies the process, voltage, and temperature conditions (e.g., fastfast process, 1.25V nominal voltage, -40°C temperature).

• Second section of library liberty file ( .lib )

Defaults

      This is section of  Liberty file format (.lib) is Defaults :

• Threshold Voltage: Default threshold voltage for the cells.
• Logical DRC: Design Rule Check parameters like maximum capacitive load, maximum fanout, and maximum transition time. These defaults can be overridden using SDC (Synopsys Design Constraints) commands.

• Third section of library liberty file ( .lib )

             Wire Load Model

 This is section of  Liberty file format (.lib) is Wire Load Model

• Wire Load Models: Different models provided to estimate interconnect delay based on the total cell area. For example, a model named "1k_hvratio_1_2" might be used based on specific criteria.

• Fourth section of library liberty file ( .lib )

Library Cells 

 This is section of  Liberty file format (.lib) is Library Cells 

• Cell Details: Each cell's detailed information including size, pin directions, functions, and power pins. For instance, an AND gate (AND2X2) with two input pins and a specific drive strength.

Timing Look-Up Table

• Look-Up Tables: Timing and power information presented in look-up tables with indices for input transition (index1) and output load capacitance (index2).

Usage of library liberty (.lib) Files 

Timing Verification: Ensures that the design meets timing constraints without running detailed SPICE simulations.

Simplified Calculations: Uses timing models like the Nonlinear Delay Model (NLDM) to abstract cell behavior and simplify timing calculations.

Path Delays: Calculates path delays by combining cell delays and net delays. Net delays are estimated using wire load models based on fanout and design size.

Setting Boundaries for Design Using Nonlinear Delay Model (NLDM)

In digital design, particularly in ASIC (Application-Specific Integrated Circuit) design, the Nonlinear Delay Model (NLDM) is used to calculate cell timing. This involves setting specific input transition times and output load capacitances as boundaries for the design.

Key Concepts

1. Input Transition: The time it takes for the input signal to transition from low to high or high to low.
2. Output Load Capacitance: The capacitance load at the output of the cell, which affects the cell's delay and output transition time.

Using Look-Up Tables

NLDM relies on look-up tables within the Liberty file format (.lib) file to provide timing information based on the input transition and output load capacitance. Here's how it works:

1. Index1 (Input Transition): Represents the time taken for the input signal to transition.
2. Index2 (Output Load Capacitance): Represents the capacitance at the cell's output.

Steps to Calculate Timing with Look-Up Tables Using NLDM

1. Set Input Transition and Output Load Capacitance: Determine the input transition times and output load capacitances for your design. These values act as boundaries and are used to index into the look-up tables.
2. Look-Up Table Extraction: Extract timing values from the look-up tables based on the set input transition and output load capacitance.
3. Calculate Cell Delay / propagation Delay (tpd): Use the extracted values from Look-up tables to calculate the cell delay, which is the time taken for the output to respond to the input transition.
4. Calculate Output Transition: Determine the true rise/fall time of the output signal using the look-up tables. This value is used as the input transition for the next stage in the design.
5. Path Delays: Total path delay is the sum of cell delays and net delays. Wire load models in the Liberty file format (.lib) file provide average R/C values for estimating net delays.

Example Workflow

Here's an example of how EDA tools calculate cell delay , this a good simulation of actual look-up table which mentioned is section four .

Cell Delay/Propagation Delay (tpd) Look-up Tables

1. Determine Boundaries: Set input transition to 0.5 ns and output load capacitance to0.05 pF for your design.
2. Extract Timing Values: Use these values to look up the corresponding delay and transition times in the Liberty file format (.lib) file.
3. Calculate Cell Delay: Extract the cell delay from look-up table provide a cell delay (tpd) of 0.23 ns.
4. Calculate Output Transition: Extract the output transition from look-up table provide time is 0.3 ns.
5. Total Path Delay: Sum the cell delay with net delays to get the total path delay. If the net delay is 0.1 ns, the total path delay is 0.33 ns.

Importance of Setting Boundaries

Setting appropriate input transition and output load capacitance boundaries ensures accurate timing calculations. These boundaries help in:

• Design Optimization: Adjusting cell placements and interconnections to meet timing constraints.
• Timing Verification: Ensuring that the design meets timing requirements without excessive computational resources.
• Predictable Performance: Providing a clear understanding of how the design will perform under specified conditions.

what about the path delay?

Path Delay is Based on Cell + Net Delay, how to calculate Net delay (wire delay).

Total path Delay

In logic synthesis we don't have interconnection delay , so we use Wire load model to calculate this delay.

A Wire Load Model calculates one parasitic R and one C for each net, based on the net’s fanout:

•  Models for various design sizes are Liberty file format (.lib) file .
•  R/C values are average estimates based on data extracted from similar designs which were fabricated using this process.

Net Delay