Welcome to Day 22! Today’s topic is Routing in VLSI Design, an essential step in the physical design flow. Routing establishes electrical connections between components on a chip while meeting performance, power, and area constraints.
What is Routing?
Routing is the process of creating physical interconnections (wires or metal layers) between the terminals of cells and blocks as defined by the netlist. This step ensures signal integrity and meets design constraints.
Goals of Routing
Connectivity:
- Ensure all required nets (connections) are established.
Signal Integrity:
- Minimize crosstalk, noise, and electromagnetic interference.
Timing Optimization:
- Reduce delays to meet timing constraints.
Power Efficiency:
- Optimize power delivery and minimize IR drops.
Area Utilization:
- Use metal layers efficiently to avoid congestion.
Types of Routing
1. Global Routing:
Roughly determines the paths for nets across the chip.
Divides the chip into grids and assigns nets to specific tracks.
2. Detailed Routing:
Finalizes the exact physical paths of interconnects.
Aligns wires to specific tracks within the assigned grids.
3. Special Routing:
- Dedicated to critical nets like clock and power/ground signals.
Routing Challenges
Congestion:
- High-density designs can lead to routing blockages.
Timing Violations:
- Long or inefficient routes can cause delays.
Signal Integrity Issues:
- Crosstalk and noise can degrade signal quality.
Manufacturing Constraints:
- Must adhere to design rules for manufacturability (e.g., spacing, width).
Routing Steps in VLSI
1. Pre-Routing:
Define metal layers and routing resources.
Perform congestion analysis to identify hotspots.
2. Global Routing:
Assign coarse paths for each net across the chip.
Balance routing congestion and timing requirements.
3. Detailed Routing:
Place wires on specific tracks.
Ensure design rule checks (DRCs) are met.
4. Post-Routing Optimization:
- Address timing, congestion, and signal integrity issues.
Key Concepts in Routing
1. Multi-Layer Routing:
Uses multiple metal layers (e.g., M1, M2, M3) for efficient interconnects.
Lower layers: Short, local interconnects.
Upper layers: Long, global interconnects.
2. Design Rules:
- Minimum wire width, spacing, and via sizes to ensure manufacturability.
3. Crosstalk and Shielding:
- Use shielding techniques to minimize interference between parallel wires.
4. Vias:
- Connect different metal layers. Efficient via placement is crucial for performance.
Practical Experiment: Routing a Digital Design
Scenario:
A digital design with 100k gates and a clock frequency of 1 GHz.
Goal: Perform routing to minimize timing violations and congestion.
Tools Used:
Cadence Innovus: For global and detailed routing.
Synopsys IC Compiler: For post-routing optimization.
Steps Taken:
Defined metal layers and design rules.
Performed global routing to assign paths for all nets.
Completed detailed routing and verified DRC compliance.
Optimized routing to reduce critical path delays.
Results:
Achieved 99% routing completion.
Reduced timing violations by 25%.
Learning Resources
Books:
“VLSI Physical Design Automation” by Sadiq M. Sait and Habib Youssef.
“Principles of CMOS VLSI Design” by Neil Weste and David Harris.
Courses:
VLSI Physical Design Flow (Udemy).
Advanced Routing Techniques (NPTEL).
Videos:
What’s Next?
Tomorrow, on Day 23, I’ll explore Timing Analysis and Closure, diving into how designs are verified to meet timing constraints before manufacturing.
Conclusion
Routing plays a critical role in ensuring a manufacturable and high-performance VLSI design. By balancing connectivity, timing, and area constraints, routing transforms the logical design into a physically realizable chip.
See you on Day 23!