20. Baseline OpenLane2 Flow

🧪 Minimal OpenLane2 Classic Flow Without SRAM

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🎯 Purpose

This document records a validated baseline OpenLane2 flow
without any macros.

The purpose of this step is to:

• ✅ Confirm that OpenLane2 itself works end-to-end
• 🔗 Validate the environment, PDK linkage, and CLI usage
• 🧱 Establish a known-good reference run before introducing SRAM macros

All later macro integration work is built on top of this baseline.

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⚠️ Important Note

OpenLane2 vs OpenLane (v1)

OpenLane2 is not CLI-compatible with OpenLane (v1).

Key differences relevant to this document:

• ❌ OpenLane2 does not use openlane run
• 📄 Designs are driven by explicit configuration files
• 🚫 Passing only a design directory is not supported
• 🔒 OpenLane2 validates configuration keys strictly

This document reflects actual working OpenLane2 behavior,
not v1 conventions or assumptions.

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📋 Prerequisites

• OpenLane2 installed in a Python virtual environment
  (see docs/10_env.md)
• PDK available locally (sky130)
• Environment variables set:

export PDK_ROOT=$HOME/pdks
export PDK=sky130A

• Virtual environment activated:

source venv/bin/activate

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🧱 Baseline Design Choice

A very small and simple design is used intentionally.

Design constraints:

• Single clock domain
• No macros
• Minimal logic
• Standard-cell only

This minimizes noise and isolates pure OpenLane2 flow behavior.

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📁 Using the Reference Design (spm)

Instead of creating custom RTL, this baseline uses the
known-good reference design provided with OpenLane2:

openlane2-designs/designs/spm

This avoids RTL-related issues and keeps the focus on the toolchain.

Verified contents:

spm/
├─ config.json
├─ src/
│  ├─ spm.v
│  └─ spm.sdc
├─ pin_order.cfg
└─ run_config.json

This design is:

• ✅ Macro-free
• ✅ Minimal
• ✅ Suitable as a baseline reference

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▶ Running the Baseline Flow (Classic)

From the OpenLane2 repository root:

cd ~/openlane2
source venv/bin/activate

Run OpenLane2 using an explicit configuration file:

poetry run openlane \
  --design-dir ../openlane2-designs/designs/spm \
  ../openlane2-designs/designs/spm/config.json

Key points

• --design-dir explicitly points to the design directory
• The configuration file path must exist
• OpenLane2 automatically selects the Classic flow

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🔄 Expected Flow Stages

A successful run executes the full Classic flow:

1. Verilator lint
2. Synthesis
3. Floorplanning
4. Placement
5. Clock Tree Synthesis (CTS)
6. Routing
7. DRC
8. GDS generation

Successful completion message:

Flow complete.

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📦 Expected Outputs

After completion, artifacts are generated under:

openlane2-designs/designs/spm/runs/RUN_*/
└─ final/
   ├─ gds/
   │  └─ spm.gds
   ├─ lef/
   ├─ reports/
   └─ views/

Primary success indicator

• ✅ final/gds/spm.gds exists
• ✅ The GDS opens correctly in KLayout

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✅ Verification Checklist

• OpenLane2 runs without CLI errors
• Synthesis completes
• Placement completes
• Routing completes
• DRC stage completes
• Final GDS is generated
• No macros are involved

⚠️ Warnings (lint, routing heuristics, etc.) are acceptable at this stage.

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🧠 Observations

• OpenLane2 enforces strict configuration validation
• v1-style keys (design_name, rtl, clocking) are rejected
• Explicit configuration files are mandatory
• The Classic flow is stable for macro-free designs

This confirms the environment and toolchain are sound.

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🧭 Role of This Baseline

This baseline serves as:

• 🟢 A known-good reference
• 🧪 A control experiment for macro integration
• 🧯 A debugging anchor if later macro runs fail

⚠️ No SRAM-related work should begin until this baseline is reproducible.

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➡ Next Step

Proceed to:

➡ docs/30_macro_sram.md
SRAM hard macro integration: blackbox, LEF/GDS, fixed placement

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Last updated: Baseline OpenLane2 Classic flow completed and GDS verified ✅