OpenROAD - Routing

Routing connects all placed standard cells, macros, and pins according to the logical netlist. It is performed in two main stages: global routing and detailed routing, which progressively refine the physical paths of interconnections. In OpenROAD, these tasks are handled by FastRoute (global) and TritonRoute (detailed).

Official documentation:

• FastRoute (Global Routing) README
• TritonRoute (Detailed Routing) README
• OpenROAD-flow-scripts Routing Tutorial

1. Overview of Routing Flow

Stage	Engine	Description
Global Routing	FastRoute	Estimates wire paths and assigns routing regions for each net.
Track Assignment	TritonRoute-prepass	Assigns nets to specific routing tracks on each metal layer.
Detailed Routing	TritonRoute	Creates exact wire geometries, vias, and layer transitions, ensuring DRC compliance.

2. Basic Routing Example

# Read the design
read_lef tech.lef
read_lef stdcells.lef
read_def results/cts.def

# Perform global and detailed routing
route_design

The route_design command executes global routing, track assignment, and detailed routing sequentially.

Equivalent manual sequence:

global_route
detailed_route

👉 Reference example: route1.tcl

3. Global Routing (FastRoute)

During global routing, OpenROAD computes approximate paths between pins, divides the chip into routing grids, and estimates wire congestion.

Example

global_route \
  -guide_file results/guides.guide \
  -overflow_iterations 150 \
  -congestion_iterations 50

Options

Option	Description
-guide_file	File storing routing guides for detailed routing
-overflow_iterations	Number of iterations to resolve congestion
-congestion_iterations	Iterations to refine congested regions

Output: guides.guide — routing guide file defining regions for each net.

👉 More details: FastRoute documentation

4. Detailed Routing (TritonRoute)

Detailed routing creates actual metal paths and vias that connect all nets, following the guides produced by FastRoute. It also performs DRC checks and reports violations.

Example

detailed_route \
  -output_drc results/drc.rpt \
  -output_maze results/route.maze

Options

Option	Description
-output_drc	Saves the DRC violation report
-output_maze	Saves the routing maze graph for debugging
-verbose	Enables detailed logging of route progress

👉 Reference: TritonRoute README

5. Antenna and Via Checks

After detailed routing, OpenROAD can automatically fix antenna violations (caused by long wires connected to gates) and verify via rules.

repair_antennas
check_antennas

Optionally run after routing:

detailed_route -output_drc results/drc_final.rpt

6. Example: Full Routing Script

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

# === Routing ===
global_route -guide_file results/guides.guide
detailed_route -output_drc results/drc.rpt

# === Post-routing checks ===
repair_antennas
check_antennas

# Export routed design
write_def results/routed.def
write_guides results/guides.guide
write_sdc results/final.sdc

7. Visualization and Debugging

Launch GUI mode to view routing layers and congestion maps:

openroad -gui
% read_def results/routed.def
% gui::show_routing_layers
% gui::show_congestion_heatmap

Tips:

• Toggle routing layers using Layer Visibility Panel.
• Use “Heatmap → Routing Congestion” to inspect critical regions.
• Export screenshots for documentation or analysis.

8. Outputs

File	Description
routed.def	Fully routed layout ready for extraction
guides.guide	Routing guides from global router
drc.rpt	DRC violations report from detailed router
antenna.rpt	Optional antenna violation report
routed.odb	OpenDB snapshot after routing

9. Troubleshooting Common Issues

Issue	Cause	Mitigation
Unrouted nets	Overlaps or missing guides	Re-run global_route with more overflow iterations
High congestion	Density too high	Reduce utilization or adjust macro spacing
DRC errors	Layer constraints or pin obstructions	Tune routing layers or fix pin locations
Antenna violations	Long metal wires	Use repair_antennas

10. Summary of Key Commands

Command	Description
route_design	Run full routing sequence
global_route	Perform global routing using FastRoute
detailed_route	Perform detailed routing using TritonRoute
repair_antennas	Fix antenna-related violations
check_antennas	Report antenna issues
write_guides	Export routing guides
write_def	Save routed DEF

11. Next Step

After routing is completed and the layout is DRC-clean, proceed to Parasitic Extraction to generate the RC networks used for post-layout timing verification and sign-off.

👉 See also: {doc}parasitic_extraction