Variable Migration Guides

These are migration guides for some particularly complex variables from OpenLane 1.

Migrating FP_PDN_VOFFSET/FP_PDN_HOFFSET/FP_PDN_VPITCH/FP_PDN_HPITCH for smaller designs

When working with smaller designs, OpenLane used to scale down the PDN pitch/offset values by a factor of 4 so at least one set of PDN straps would be able to connect to the design.

This was a bit too magical and not entirely deterministic, and thus LibreLane will no longer automatically do this scaling down for you. You will have to manually calculate the scale.

See Power Distribution Networks for more info.

Migrating DIODE_INSERTION_STRATEGY

As they were extremely complex, the OpenLane diode insertion strategies were replaced by two flags in LibreLane’s “Classic” flow:

• GRT_REPAIR_ANTENNAS: Attempts to repair antennas by enabling the step OpenROAD.RepairAntennas that invokes OpenROAD’s repair_antennas function

• RUN_HEURISTIC_DIODE_INSERTION: Runs a custom script by Sylvain Munaut that inserts diodes based on a net’s Manhattan distance at global placement time.

The mapping for the strategies is as follows:

• Strategies 1, 2 and 5 will throw an error.

  • 1 is an unreasonable brute force approach that adds a diode to every single net. 2 and 5 utilize replaceable “fake diode” fill cells, which are a hack that will not be available in all PDKs.

• Strategy 0:

  • Both flags are set to false.

• Strategy 3:

  • GRT_REPAIR_ANTENNAS is set to true.

• Strategy 4:

  • RUN_HEURISTIC_DIODE_INSERTION is set to true.

• Strategy 6:

  • Both flags are set to true.

Although for now LibreLane will attempt the conversion for you automatically, it is recommended you update your designs as this feature will get removed.

Migrating MACROS (Optional, but highly recommended)

LibreLane allows you to configure macros you use in a configuration object called MACROS.

This object collects all information about macros and instances of macros into one object.

For example, where an SPM macro in OpenLane would be defined across multiple variables in this manner:

{
  "VERILOG_FILES_BLACKBOX": ["dir::macros/spm/nl/spm.nl.v"],
  "EXTRA_GDS_FILES": ["dir::macros/gds/spm.gds"],
  "EXTRA_LEFS": ["dir::macros/lef/spm.lef"],
  "EXTRA_SPEFS": "spm dir::macros/spm/spef/nom_/spm.min.spef dir::macros/spm/spef/nom_/spm.nom.spef dir::macros/spm/spef/nom_/spm.max.spef",
  "MACRO_PLACEMENT_CFG": "dir::macro_placement.cfg"
}

This information and more is laid out in the MACROS object as follows:

{
  "MACROS": {
    "spm": {
      "instances": {
        "spm_inst_0": {
          "location": [10, 150],
          "orientation": "N"
        },
        "spm_inst_1": {
          "location": [150, 150],
          "orientation": "N"
        }
      },
      "gds": ["dir::macros/spm/gds/spm.magic.gds"],
      "lef": ["dir::macros/spm/lef/spm.lef"],
      "nl": ["dir::macros/spm/nl/spm.nl.v"],
      "spef": {
        "nom_*": "dir::macros/spm/spef/nom_/spm.nom.spef",
        "min_*": "dir::macros/spm/spef/min_/spm.min.spef",
        "max_*": "dir::macros/spm/spef/max_/spm.max.spef"
      }
    }
  }
}

You will notice a number of things:

• Instances and their locations are no longer declared in a separate file.

• Multiple SPEF files can be declared to match various IPVT corner, i.e.

  • nom_* will match nom_tt_025C_1V80 and nom_ff_n40C_1v95

  • min_* will match min_ss_100C_1v60

  • etc

• The previous bullet point also applies to .lib files (if available)

This configuration object helps keep all data related to instantiated macros in the same place. The EXTRA_ variables still exist and are loaded indiscriminately in all operations where the appropriate files are loaded, but the MACROS object give you a little bit more control.

Tip

During STA, a Macro’s .lib view or a combination of its Verilog netlist and .spef view could be used to provide the necessary timing information.

By default, netlist and .spef are prioritized over the .lib files because the characterized .lib files produced by OpenSTA are not the most accurate, but you may override this behavior by setting STA_MACRO_PRIORITIZE_NL to false.

As the name implies, this is merely a priority and if one view is available, that one will always be used.