IR Drop Analysis

The IR Drop Analysis module in OpenROAD (psm) is based on PDNSim, an open-source static IR analyzer.

Features:

• Report worst IR drop.

• Report worst current density over all nodes and wire segments in the power distribution network, given a placed and PDN-synthesized design.

• Check for floating PDN stripes on the power and ground nets.

• Spice netlist writer for power distribution network wire segments.

Commands

Note

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

• Parameters without square brackets -param2 param2 are required.

Analyze Power Grid

This command analyzes power grid.

analyze_power_grid
    -net net_name
    [-corner corner]
    [-error_file error_file]
    [-voltage_file voltage_file]
    [-enable_em]
    [-em_outfile em_file]
    [-vsrc voltage_source_file]
    [-source_type FULL|BUMPS|STRAPS]
    [-allow_reuse]

Options

Switch Name	Description
-net	Name of the net to analyze, power or ground net name.
-corner	Corner to use for analysis.
-error_file	File to write power grid error to.
-vsrc	File to set the location of the power C4 bumps/IO pins. Vsrc_aes.loc file for an example with a description specified here.
-enable_em	Report current per power grid segment.
-em_outfile	Write the per-segment current values into a file. This option is only available if used in combination with -enable_em.
-voltage_file	Write per-instance voltage into the file.
-source_type	Indicate the type of voltage source grid to model. FULL uses all the nodes on the top layer as voltage sources, BUMPS will model a bump grid array, and STRAPS will model power straps on the layer above the top layer.
-allow_reuse	Allow the analysis to reuse a previous solution, if one exists.

Check Power Grid

This command checks power grid.

check_power_grid
    -net net_name
    [-floorplanning]
    [-error_file error_file]
    [-dont_require_terminals]

Options

Switch Name	Description
-net	Name of the net to analyze. Must be a power or ground net name.
-floorplanning	Ignore non-fixed instances in the power grid, this is useful during floorplanning analysis when instances may not be properly placed.
-error_file	File to write power grid errors to.
-dont_require_terminals	If specified, this will skip checking if there are terminals on the net.

Write Spice Power Grid

This command writes the spice file for power grid.

write_pg_spice
    -net net_name
    [-vsrc vsrc_file]
    [-corner corner]
    [-source_type FULL|BUMPS|STRAPS]
    spice_file

Options

Switch Name	Description
-net	Name of the net to analyze. Must be a power or ground net name.
-vsrc	File to set the location of the power C4 bumps/IO pins. See Vsrc_aes.loc file for an example and its description.
-corner	Corner to use for analysis.
-source_type	Indicate the type of voltage source grid to model. FULL uses all the nodes on the top layer as voltage sources, BUMPS will model a bump grid array, and STRAPS will model power straps on the layer above the top layer.
spice_file	File to write spice netlist to.

Set PDNSim Net voltage

This command sets PDNSim net voltage.

set_pdnsim_net_voltage
    -net net_name
    -voltage volt
    [-corner corner]

Options

Switch Name	Description
-net	Name of the net to analyze. It must be a power or ground net name.
-voltage	Sets the voltage on a specific net. If this option is not given, the Liberty file’s voltage value is obtained from operating conditions.
-corner	Corner to use this voltage. If not specified, this voltage applies to all corners.

Set PDNSim Instance power

This command sets PDNSim instance power. This should only be used when needing to override the computed power.

set_pdnsim_inst_power
    -inst inst_name
    -power power
    [-corner corner]

Options

Switch Name	Description
-inst	Name of the instance to set the power dor.
-power	Sets the power on a specific instance.
-corner	Corner to use this power. If not specified, this power applies to all corners.

Set PDNSim Power Source Settings

Set PDNSim power source setting.

set_pdnsim_source_settings
    [-bump_dx pitch]
    [-bump_dy pitch]
    [-bump_size size]
    [-bump_interval interval]
    [-strap_track_pitch pitch]
    [-external_resistance resistance]

Options

Switch Name	Description
-bump_dx,-bump_dy	Set the bump pitch to decide the voltage source location. The default bump pitch is 140um.
-bump_size	Set the bump size. The default bump size is 70um.
-bump_interval	Set the bump population interval, this is used to depopulate the bump grid to emulate signals and other power connections. The default bump pitch is 3.
-strap_track_pitch	Sets the track pitch to use for modeling voltage sources as straps. The default is 10x.
-external_resistance	Set to model the resistance of the package or power network outside the chip/block. The default value is 0.0.

Insert Decap Cells

The insert_decap command inserts decap cells in the areas with the highest IR Drop. The number of decap cells inserted will be limited to the target capacitance defined in the -target_cap option. list_of_decap_with_cap is a list of even size of decap master cells and their capacitances, e.g., <cell1> <decap_of_cell1> <cell2> <decap_of_cell2> .... To insert decap cells in the IR Drop of a specific net (power or ground) use -net <net_name>, if not defined the default power net will be used. To use this command, you must first execute the analyze_power_grid command with the net to have the IR Drop information.

insert_decap -target_cap target_cap [-net net_name] -cells list_of_decap_with_cap

Options

Switch Name	Description
-target_cap	Target capacitance to insert os decap cells.
-net	Power or ground net name. The decap cells will be inserted near the IR Drops of the net.
-cells	List of even size of decap master cells and their capacitances.

Source grid options

The source grid models how power is going be delivered to the power grid. The image below illustrate how they can be modeled, the red elements are the source models, the black horizontal boxes represent the top metal layer of the power grid, and the gray boxes indicate these are not used in the modeling.

Bumps with 2x interval	Bumps with 3x interval	Straps	Full

Selectively disconnecting sources

If you need to be able to disconnect some of the terminals in the design, such as in the case of “what-if” analysis or different chip packaging options. This can be done by assigning PSM_DISCONNECT to a terminal or shape in a terminal will cause PDNSim to leave that object disconnected from the analysis.

# Assumes bpin is the block pin to be disconnected
odb::dbBoolProperty_create $bpin PSM_DISCONNECT 1

# Assumes box the box shape in the bpin to be disconnected
odb::dbBoolProperty_create $box PSM_DISCONNECT 1

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
find_net	Get a reference to net name.

Example scripts

Example scripts demonstrating how to run PDNSim on a sample design on aes as follows:

./test/aes_test_vdd.tcl
./test/aes_test_vss.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.

References

1. PDNSIM documentation

2. Chhabria, V.A. and Sapatnekar, S.S. (no date) The-openroad-project/pdnsim: Power Grid Analysis, GitHub. Available at: https://github.com/The-OpenROAD-Project/PDNSim (Accessed: 24 July 2023). (link)

License

BSD 3-Clause License. See LICENSE file.