Liberty format defines delays and power of cells. It’s important part of ASIC flow for delay calculation and power estimation.

Syntax Link to heading

Liberty defines 3 types statements

• group statement
• attribute statement
• define statement

Group Statements Link to heading

A group is a named collection of statements that defines a library, a cell, a pin, a timing arc, and so forth. Braces ({}), which are used in pairs, enclose the contents of the group.

group_name (name) {
... statements ...
}

Attribute Statements Link to heading

An attribute statement defines characteristics of specific objects in the library. Attributes applying to specific objects are assigned within a group statement defining the object, such as a cell group or a pin group.

attribute_name : attribute_value ;

Define Statements Link to heading

You can create new simple attributes with the define statement.

define (attribute_name, group_name, attribute_type) ;

Writing liberty library Link to heading

library Group Link to heading

The library group statement defines the name of the library you want to describe. This statement must be the first executable line in your library.

library (my_library) {
...
}

library attributes Link to heading

• technology Attribute
• delay_model Attribute
• bus_naming_style Attribute

Delay and Slew Attributes Link to heading

The spec defines two concepts:

• Delay
• Slew

The spec describes it very well. I will just quote it.

Delay Link to heading

Delay is the time it takes for the output signal voltage, which is falling from 1 to 0, to fall to the threshold point set with the output_threshold_pct_fall attribute after the input signal voltage, which is falling from 1 to 0, has fallen to the threshold point set with the input_threshold_pct_fall attribute

The following commands define the threshold

• input_threshold_pct_fall
• input_threshold_pct_rise
• output_threshold_pct_fall
• output_threshold_pct_rise

Slew Link to heading

Slew is the time it takes for the voltage value to fall or rise between two designated threshold points on an input, an output, or a bidirectional port. The designated threshold points must fall within a voltage falling from 1 to 0 or rising from 0 to 1.

Use the following attributes to enter the two designated threshold points to model the time for voltage falling from 1 to 0:

slew_lower_threshold_pct_fall

slew_upper_threshold_pct_fall

Use the following attributes to enter the two designated threshold points to model the time for voltage rising from 0 to 1:

slew_lower_threshold_pct_rise

slew_upper_threshold_pct_rise

Defining Units attributes Link to heading

Use these library-level attributes to define units:

• time_unit
• voltage_unit
• current_unit
• pulling_resistance_unit
• capacitive_load_unit
• leakage_power_unit