Place and Route (P&R)

Definition: Place and Route (P&R) is a crucial stage in the VLSI physical design process where the circuit elements of a chip are positioned on the die (placement) and the wires connecting them are laid out (routing). This process transforms a logical netlist into a physical layout that can be manufactured.

Key Phases:

  1. Floorplanning:
    • Partitioning the chip area
    • Defining power grid structure
    • Placing macro blocks and I/O pads
  2. Placement:
    • Global Placement: Rough positioning of cells
    • Detailed Placement: Fine-tuning cell positions
  3. Clock Tree Synthesis (CTS):
    • Designing the clock distribution network
    • Balancing clock skew and latency
  4. Routing:
    • Global Routing: Planning approximate paths for nets
    • Detailed Routing: Assigning specific metal layers and tracks
  5. Post-Route Optimization:
    • Timing closure
    • Design rule violation fixes
    • Signal integrity improvements

Key Objectives:

  • Minimize total wirelength
  • Meet timing constraints
  • Optimize power consumption
  • Ensure design rule compliance
  • Maximize routability

Placement Techniques:

  1. Force-Directed Placement:
    • Models cells as masses and connections as springs
  2. Simulated Annealing:
    • Probabilistic technique for approximating global optimum
  3. Quadratic Placement:
    • Formulates placement as a quadratic optimization problem
  4. Analytical Placement:
    • Uses mathematical programming techniques

Routing Algorithms:

  1. Maze Routing (Lee’s Algorithm):
    • Guarantees to find a path if one exists
    • Can be slow for complex designs
  2. Line-Search Routing:
    • Faster than maze routing but may miss some paths
  3. Channel Routing:
    • Efficient for row-based designs
  4. Global Routing:
    • Often uses multicommodity flow techniques

Challenges in P&R:

  1. Timing Closure:
    • Meeting setup and hold time requirements
  2. Congestion Management:
    • Avoiding routing hotspots
  3. Power Distribution:
    • Ensuring uniform power delivery across the chip
  4. Signal Integrity:
    • Managing crosstalk and electromagnetic interference
  5. DFM (Design for Manufacturability):
    • Adhering to manufacturing process constraints

Advanced P&R Techniques:

  1. Multi-Corner Multi-Mode (MCMM):
    • Optimizing for multiple operating conditions simultaneously
  2. Concurrent Clock and Data Optimization (CCDO):
    • Jointly optimizing clock and signal paths
  3. Physical Synthesis:
    • Logic restructuring during P&R for better QoR
  4. Abutment and Channel-less Routing:
    • Maximizing area utilization in advanced nodes

Industry Tools:

  • Cadence Innovus
  • Synopsys IC Compiler
  • Mentor Calibre
  • ANSYS RedHawk

Example: Simple P&R Flow

# Typical P&R flow in TCL (Tool Command Language)

# Read in the design
read_netlist design.v
read_sdc constraints.sdc

# Floorplanning
create_floorplan -die_size {1000 1000} -core_utilization 0.7

# Placement
place_design

# Clock Tree Synthesis
clock_tree_synthesis

# Routing
route_design

# Post-Route Optimization
optimize_design -post_route

# Design Rule Checking
check_design

# Generate outputs
write_def final_layout.def
write_gds final_layout.gds

Future Trends in P&R:

  1. Machine Learning for predictive P&R
  2. Handling ultra-large-scale integration (ULSI)
  3. 3D IC and chiplet-based design flows
  4. Quantum computing-aware P&R algorithms
  5. Enhanced support for photonic integrated circuits

P&R remains a critical and computationally intensive step in chip design, often requiring significant time and resources. As chip complexities increase and process nodes shrink, advancements in P&R technologies continue to be crucial for enabling next-generation electronic devices.