40. Results and Observations

📊 Final GDS Generation with SRAM Hard Macro

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🎯 Purpose

This document summarizes the final results of the project, marking the successful completion of M3.

It focuses on:

• 🧩 End-to-end RTL → GDS completion using OpenLane2
• 🧱 Integration of an SRAM hard macro as a fixed physical constraint
• 🧠 Practical observations from macro-aware physical design
• ⚠️ Known limitations and lessons learned

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✅ Final Outcome (Summary)

Achieved Results

• OpenLane2 Classic flow completed successfully across all stages
• A final GDS was generated that includes:
  • Standard-cell logic
  • An externally provided SRAM hard macro
• The entire flow is fully reproducible using documented commands and configuration files

This confirms that:

OpenLane2 can reliably handle hard macro integration
in a realistic SoC-style physical design flow.

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📦 Generated Artifacts

OpenLane2 produces per-run artifacts under the design directory:

designs/<design_name>/runs/RUN_<timestamp>/
└─ final/
   ├─ gds/
   │  └─ <design_name>.gds
   ├─ lef/
   ├─ def/
   ├─ views/
   └─ reports/

Primary Deliverable

• final/gds/<design_name>.gds
  Final top-level layout including the SRAM macro

This GDS has been verified by visual inspection.

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🔍 Layout Verification

Visual Inspection

Using KLayout, the following were confirmed:

• The SRAM macro appears as a large, fixed rectangular block
• Standard cells are placed legally around the macro
• Halo / keepout regions are respected
• No overlaps between macro and standard-cell placement

This confirms correct macro placement and floorplan enforcement.

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🧪 DRC and LVS Status

DRC

• Top-level DRC completed within the project scope
• Warnings may appear around:
  • Macro boundaries
  • Power distribution structures
• Internal SRAM DRC is intentionally excluded

All reported issues are understood and acceptable for this project.

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LVS

• LVS is limited to top-level connectivity
• The SRAM macro is treated as a blackbox
• Full sign-off LVS is explicitly out of scope

This matches the stated project policy.

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📈 Metrics and Observations

Observed qualitative trends:

• Area utilization increases significantly due to the macro footprint
• Routing congestion concentrates near macro edges
• Floorplan quality has a larger impact than synthesis optimization

Exact numerical metrics are design- and macro-dependent and are not the primary focus of this work.

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🧠 Key Lessons Learned

1️⃣ Macro Placement Dominates the Design

• Macro placement decisions must be made early
• Small placement shifts can drastically affect routability

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2️⃣ PDN Is the First Real Bottleneck

• SRAM power pins must align with the top-level PDN
• PDN-related issues are the most common cause of failed runs

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3️⃣ Abstract Views Are Essential

• LEF / maglef views dramatically reduce DRC noise
• Attempting full SRAM internal DRC/LVS is impractical

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4️⃣ OpenLane2 Is Macro-Capable but Explicit

• Macro integration is not automatic
• Constraints must be expressed explicitly
• The flow is more transparent and debuggable than OpenLane (v1)

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⚠️ Known Limitations

The following limitations are intentional:

• Single SRAM macro only
• No advanced timing optimization
• CTS behavior around macros not tuned
• No production-grade sign-off

This repository prioritizes:

Clarity · Realism · Reproducibility
over maximum optimization.

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🔬 Recommended Next Experiments

Possible extensions include:

• Multiple SRAM macros
• Different macro orientations and placements
• Manual PDN refinement
• Comparison with OpenLane (v1) macro handling
• Integration of OpenRAM-generated SRAM macros

These can be explored without changing the repository structure.

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🧭 Final Remarks

This project demonstrates a practical, macro-aware OpenLane2 workflow.

Its value lies in:

• Clear separation between logic and hard macros
• Explicit physical constraints
• Fully reproducible, documented steps

It provides a solid foundation for further exploration of:

Macro-based physical design using OpenLane2

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🖼 Layout Evidence

Figure 1: SRAM Macro Block-Level View

The SRAM hard macro is integrated as a fixed block in the final GDS.

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Figure 2: Standard-Cell-Level View

(SRAM Internal Layout Not Visible)

Only standard-cell geometry is visible.
The SRAM macro is treated as a hard macro and its internal transistor-level layout is intentionally abstracted.

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Last updated: Final GDS generation verified and documented ✅