OpenROAD - Placement

Placement arranges all standard cells inside the defined core area, minimizing total wire length, congestion, and timing violations. In OpenROAD, placement is performed in multiple stages — global, legalization, and detailed — using the integrated placement engines RePlAce and OpenDP.

Official documentation:

• OpenROAD Placement (RePlAce) README
• OpenROAD Detailed Placement (OpenDP) README
• OpenROAD-flow-scripts Placement Tutorial

1. Overview of Placement Flow

Placement in OpenROAD is executed in three main phases:

Phase	Tool	Description
Global placement	RePlAce	Distributes cells to minimize wire length and congestion (overlaps allowed).
Legalization	OpenDP	Snaps cells to legal rows and removes overlaps.
Detailed placement	OpenDP / RePlAce refine	Final local adjustments for better timing and routability.

2. Basic Placement Example

# Read design and tech data
read_lef tech.lef
read_lef stdcells.lef
read_def results/floorplan.def

# Perform placement
place_design

The place_design command automatically performs global placement, legalization, and detailed placement in sequence.

Equivalent manual execution:

global_placement
detailed_placement

👉 Reference script: place1.tcl

3. Global Placement

During global placement, OpenROAD minimizes the total estimated wire length and balances cell density using RePlAce.

Example:

# Global placement with specific density and iterations
global_placement \
  -density 0.7 \
  -init_density_penalty 8.0 \
  -pad_left 2.0 -pad_right 2.0

Options

Option	Description
-density	Target density (0–1). Higher = tighter packing.
-pad_left / -pad_right	Adds whitespace for routability.
-init_density_penalty	Adjusts spreading strength.

👉 More examples: RePlAce documentation

4. Legalization

Legalization aligns all cells onto legal row sites, removes overlaps, and respects blockages or halos defined during floorplanning.

Example:

# Legalize placement
detailed_placement

To restrict legalization to specific regions:

detailed_placement -region {100 100 800 800}

👉 Reference module: OpenDP (Detailed Placement)

5. Incremental and Timing-Driven Placement

After initial placement, the design can be re-optimized to improve timing or congestion before CTS.

Example:

# Incremental placement to fix timing
resizer::repair_timing -incremental

Alternatively:

global_placement -timing_driven

6. Filler, Tap, and Endcap Cells

To maintain uniform well connections and avoid DRC errors, OpenROAD can insert additional filler and tap cells after placement.

tapcell -distance 14
filler_placement {FILLCELL_X1 FILLCELL_X2 FILLCELL_X4}

👉 Documentation: Tapcell Insertion

7. Example: Full Placement Script

read_lef tech.lef
read_lef stdcells.lef
read_def results/floorplan.def

# === Placement ===
place_design

# or step-by-step:
global_placement -density 0.65
detailed_placement
tapcell -distance 14
filler_placement {FILLCELL_X1 FILLCELL_X2 FILLCELL_X4}

write_def results/placement.def

8. Outputs

File	Description
placement.def	DEF file with placed standard cells
placement.log	Log file with placement statistics
placement.odb	OpenDB database snapshot
report_place.txt	Optional timing and congestion report

9. Visualization (Optional)

Launch OpenROAD GUI to inspect cell distribution and density heatmaps:

openroad -gui
% read_def results/placement.def
% gui::show_density_heatmap

You can toggle display layers (e.g., macro outlines, congestion maps) for debugging routability.

10. Summary of Key Commands

Command	Description
place_design	Perform full placement flow automatically
global_placement	Analytical placement (wirelength & density)
detailed_placement	Legalization and local refinement
tapcell	Insert well and endcap cells
filler_placement	Fill remaining empty sites

11. Next Step

After placement is complete, proceed with Clock Tree Synthesis (CTS), where buffers are added to distribute the clock signal evenly across the design.

👉 See also: {doc}cts_and_optimization