Using ECO Steps

Engineering Change Orders (ECOs) are a powerful tool you can use to resolve issues with the circuit that:

  • Are discovered late into PnR, where an RTL change would put you behind schedule

  • Were not successfully resolved using automatic PnR tools

Classically, ECOs were done by the user manually opening the design database and then re-running a part of the flow. LibreLane of course lets you do that if you want:

$ librelane --run-tag my_run ./config.yaml
[…]
[ERROR] Hold violations found.
$ openroad -gui ./runs/my_run/*-openroad-resizertimingpostgrt/*.odb
$ librelane --run-tag my_run_eco --with-initial-state ./runs/my_run/*-openroad-resizertimingpostgrt/state_out.json --from openroad.steamidpnr-3 ./config.yaml
[…]
[INFO] Flow successful.

Tip

To know the order of the steps in the default flow, invoke librelane.help Classic. Steps are case-insensitive.

Though for issues that occur regularly on a particular pin or instance, LibreLane actually provides a set of steps that let you:

  • Insert a buffer on a specific driver or sink pin to help with hold violation

  • Insert a diode on a specific sink pin to help with antenna violations

The two steps are Odb.InsertECOBuffers and Odb.InsertECODiodes. They are not part of the default flow, so you will need to inject those steps manually.

This tutorial will show Odb.InsertECOBuffers, but diodes work broadly similarly (except the diode step only supports sinks and not drivers.)

Example

Consider this quaint demonstrative design:

module hold_violation(
  input clk,
  input d,
  output q
);
  wire intermediate;
  wire clk_delayed;

  sky130_fd_sc_hd__clkbuf_4 dly (
    .A(clk),
    .X(clk_delayed)
  );

  sky130_fd_sc_hd__dfrtp_4 u_ff1 (
    .CLK(clk),
    .D(d),
    .RESET_B(1'b1),
    .Q(intermediate)
  );

  sky130_fd_sc_hd__dfrtp_1 u_ff2 (
    .CLK(clk_delayed),
    .D(intermediate),
    .RESET_B(1'b1),
    .Q(q)
  );
endmodule

Here, the clock for u_ff2 is delayed, forcing a hold violation into existence. In a more realistic scenario, the resizer will usually be able to fix something so simple, but for the sake of demonstration, let’s just turn it off.

So running this config:

DESIGN_NAME: hold_violation
CLOCK_PORT: clk
CLOCK_PERIOD: 5
VERILOG_FILES: dir::hold_violation.v
RUN_POST_CTS_RESIZER_TIMING: false
RUN_POST_GRT_RESIZER_TIMING: false
FP_SIZING: absolute
DIE_AREA: [0, 0, 100, 100]

…will invariably end up with hold violations at the typical corner:

Hold violations found in the following corners:
* max_ff_n40C_1v95
* min_ff_n40C_1v95
* nom_ff_n40C_1v95

Now, some designs are huge and it’s something of a hassle to re-run routing just to fix this issue. So what can we do in this case?

That’s where LibreLane’s ECO steps come in.

Adding it to your flow

You can use the meta.flow and meta.substituting_steps keys to modify the flow conveniently from your configuration file as follows:

JSON YAML 
{
  "meta": {
    "flow": "Classic",
    "substituting_steps": {
      "+OpenROAD.DetailedRouting": "Odb.InsertECOBuffers",
      "+Odb.InsertECOBuffers": "OpenROAD.DetailedRouting"
    }
  }
}

meta:
  flow: Classic
  substituting_steps:
    "+OpenROAD.DetailedRouting": "Odb.InsertECOBuffers"
    "+Odb.InsertECOBuffers": "OpenROAD.DetailedRouting"

See By Substituting Steps for more info.

Configuring buffers

But adding the step alone is not enough: you also need to tell it where you need an ECO buffer inserted.

JSON YAML 
{
  "INSERT_ECO_BUFFERS": [
    {
      "target": "u_ff1/Q",
      "buffer": "sky130_fd_sc_hd__buf_1"
    }
  ]
}

INSERT_ECO_BUFFERS:
  - target: u_ff1/Q
    buffer: sky130_fd_sc_hd__buf_1

Then find the state_out.json for detailed routing:

runs/RUN_2025-09-16_00-11-02/42-openroad-detailedrouting/state_out.json

…and run this command:

$ librelane ./config.yaml --with-initial-state runs/RUN_2025-09-16_00-11-02/42-openroad-detailedrouting/state_out.json --from odb.insertecobuffers

You’ll now find that the hold violations at the typical corner have been solved, but at the fast corner, there’s still a violation as the buffer is too fast. So what’s the solution?

Well, just add another buffer.

JSON YAML 
{
  "INSERT_ECO_BUFFERS": [
    {
      "target": "u_ff1/Q",
      "buffer": "sky130_fd_sc_hd__buf_1"
    },
    {
      "target": "u_ff1/Q",
      "buffer": "sky130_fd_sc_hd__buf_1"
    }
  ]
}

INSERT_ECO_BUFFERS:
  - target: u_ff1/Q
    buffer: sky130_fd_sc_hd__buf_1
  - target: u_ff1/Q
    buffer: sky130_fd_sc_hd__buf_1

You will notice that the new OpenROAD.DetailedRouting step added has a much faster runtime; and that’s because it only has to route nets that have been modified as part of the ECO.