Parasitics estimation

Parasitics estimation#

Parasitics estimation commands are described below.

Commands#

Note

  • Parameters in square brackets [-param param] are optional.

  • Parameters without square brackets -param2 param2 are required.

Set Wire RC#

The set_wire_rc command sets the resistance and capacitance used to estimate delay of routing wires. Separate values can be specified for clock and data nets with the -signal and -clock flags. Without either -signal or -clock the resistance and capacitance for clocks and data nets are set.

# Either run 
set_wire_rc -clock ... -signal ... -layer ...

# Or
set_wire_rc -resistance ... -capacitance ...

set_wire_rc 
    [-clock] 
    [-signal]
    [-data]
    [-corner corner]
    [-layers layers_list]

or 
set_wire_rc
    [-h_resistance res]
    [-h_capacitance cap]
    [-v_resistance res]
    [-v_capacitance cap]

or
set_wire_rc 
    [-clock] 
    [-signal]
    [-data]
    [-corner corner]
    [-layer layer_name]
or 
set_wire_rc
    [-resistance res]
    [-capacitance cap]

Options#

Switch Name

Description

-clock

Enable setting of RC for clock nets.

-signal

Enable setting of RC for signal nets.

-layers

Use the LEF technology resistance and area/edge capacitance values for the layers. The values for each layers will be used for wires with the prefered layer direction, if 2 or more layers have the same prefered direction the avarege value is used for wires with that direction. This is used for a default width wire on the layer.

-layer

Use the LEF technology resistance and area/edge capacitance values for the layer. This is used for a default width wire on the layer.

-resistance

Resistance per unit length, units are from the first Liberty file read.

-capacitance

Capacitance per unit length, units are from the first Liberty file read.

-h_resistance

Resistance per unit length for horizontal wires, units are from the first Liberty file read.

-h_capacitance

Capacitance per unit length for horizontal wires, units are from the first Liberty file read.

-v_resistance

Resistance per unit length for vertical wires, units are from the first Liberty file read.

-v_capacitance

Capacitance per unit length for vertical wires, units are from the first Liberty file read.

Set Layer RC#

The set_layer_rc command can be used to set the resistance and capacitance for a layer or via. This is useful if these values are missing from the LEF file, or to override the values in the LEF.

set_layer_rc 
    [-layer layer]
    [-via via_layer]
    [-resistance res]
    [-capacitance cap]
    [-corner corner]

Options#

Switch Name

Description

-layer

Set layer name to modify. Note that the layer must be a routing layer.

-via

Select via layer name. Note that via resistance is per cut/via, not area-based.

-resistance

Resistance per unit length, same convention as set_wire_rc.

-capacitance

Capacitance per unit length, same convention as set_wire_rc.

-corner

Process corner to use.

Report Layer RC#

The report_layer_rc command reports the layer resistance and capacitance values used for parasitics estimation. These values were previously set with the set_layer_rc command or they originate from the LEF.

report_layer_rc
    [-corner corner]

Options#

Switch Name

Description

-corner

Process corner to report.

Estimate Parasitics#

Estimate RC parasitics based on placed component pin locations. If there are no component locations, then no parasitics are added. The resistance and capacitance values are per distance unit of a routing wire. Use the set_units command to check units or set_cmd_units to change units. The goal is to represent “average” routing layer resistance and capacitance. If the set_wire_rc command is not called before resizing, then the default_wireload model specified in the first Liberty file read or with the SDC set_wire_load command is used to make parasitics.

After the global_route command has been called, the global routing topology and layers can be used to estimate parasitics with the -global_routing flag.

The optional argument -spef_file can be used to write the estimated parasitics using Standard Parasitic Exchange Format.

estimate_parasitics
    -placement|-global_routing
    [-spef_file spef_file]

Options#

Switch Name

Description

-placement or -global_routing

Either of these flags must be set. Parasitics are estimated based after placement stage versus after global routing stage.

-spef_file

Optional. File name to write SPEF files. If more than one corner is available for the design, the files will be written as filename_corner.spef.

Useful Developer Commands#

If you are a developer, you might find these useful. More details can be found in the source file or the swig file.

Command Name

Description

check_parasitics

Check if the estimate_parasitics command has been called.

check_corner_wire_caps

Check wire capacitance for corner.

dblayer_wire_rc

Get layer RC values.

set_dblayer_wire_rc

Set layer RC values.

Example scripts#

Examples scripts demonstrating how to run parasitics estimation on a sample design as follows:

./test/make_parasitics1.tcl

Regression tests#

There are a set of regression tests in ./test. For more information, refer to this section.

Simply run the following script:

./test/regression

Limitations#

FAQs#

Check out GitHub discussion about this tool.

License#

BSD 3-Clause License. See LICENSE file.

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