VLSI Chip Physical Design is crucial as it directly impacts the chip’s performance, reliability, and cost. However, a well-designed layout can significantly reduce power consumption, improve signal integrity, and lower the chip’s overall size, resulting in cost savings. Moreover, the physical design is critical in meeting the circuit’s timing constraints, which determine the maximum operating frequency of the chip.
Designing an IC’s physical layout is a complex process that involves many challenges. One of the major challenges is to ensure that the circuit meets timing constraints while minimizing power consumption and area.
In contrast, achieving this requires a deep understanding of the circuit’s functionality and optimization techniques.
Let’s take a closer look at the VLSI physical design process.
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VLSI Physical Design Flow
The VLSI physical design flow consists of several steps that include synthesis, Partitioning, floor planning, placement, stating timing analysis (STA), cock tree synthesis (CTS), routing, and physical verification. Each step in the flow plays a critical role in ensuring that the chip meets the design specifications and constraints.
Synthesis
The synthesis step involves taking the RTL (Register Transfer Level) design as input and generating a gate-level netlist. The goal is to optimize the design for area, power, and timing constraints.
Partitioning
The partitioning step involves dividing the design into smaller modules or partitions based on design hierarchy, functionality, and physical constraints. Partitioning helps to simplify the design and improve the efficiency of subsequent steps.
Floor Planning
The floor planning step involves dividing the chip into functional blocks and determining the location of each block on the chip. The goal of floor planning is to minimize the chip’s area and to ensure that the blocks are placed in a way that facilitates efficient routing and meets the timing constraints.
Placement
The placement step involves determining the exact position of each cell on the chip and optimizing their locations to meet the timing, power, and area constraints. Placement is a critical step that affects the overall performance of the design.
Stating Timing Analysis (STA)
The STA step involves analyzing the timing of the design after placement to ensure that the design meets the timing constraints. The analysis takes into account the delays in the interconnects, buffers, and other components of the design.
Cock Tree Synthesis (CTS)
The CTS step involves inserting buffer cells in the design to balance the timing of the clock signals and reduce clock skew. The goal is to ensure that the clock signals arrive at all the flops at the same time.
Routing
The routing step involves determining the optimal routes for connecting the cells in the circuit. This step is critical to ensure meeting the timing constraints and maintaining the signal integrity of the circuit.
Physical Verification
The physical verification step involves validating the physical design against the original design specifications and constraints. This step includes several checks, such as Design Rule Checking (DRC), Layout vs. Schematic (LVS) verification, and Electrical Rule Checking (ERC). The goal is to ensure that the chip meets the desired performance, reliability, and functionality.
Signoff
The signoff step involves final checks and approval of the design before the fabrication process. This step includes several analyses, such as Static Timing Analysis (STA), Signal Integrity (SI) Analysis, and Power Analysis. The goal is to ensure that the design meets all the specifications and constraints and that it can be fabricated with high yield and low cost. Once the design passes this step, it is ready for tape-out, which means the design is sent to the foundry for fabrication.
VLSI Physical Design Techniques
There are several techniques used in VLSI physical design to optimize the performance, power consumption, and area of the chip. Some of the commonly used techniques include:
Clock Tree Synthesis (CTS)
Undoubtedly, CTS is a technique to ensure the distribution of clock signals evenly across the chip while minimizing the skew. The goal of CTS is to ensure that the clock signal arrives at all parts of the chip at the same time, which is critical in meeting the timing constraints of the circuit.
Power Optimization
Power optimization techniques, such as voltage scaling are used to reduce the power consumption of the chip. This involves reducing the voltage and frequency of the circuit without affecting its performance. Techniques such as voltage scaling, clock gating, and power gating are commonly used to achieve power optimization.
Timing Optimization
Timing optimization techniques, such as buffering are used to ensure that the circuit meets the timing constraints. This involves adjusting the delay of the cells in the circuit to ensure that the critical path meets the timing constraints. Techniques such as buffering, sizing, and gate cloning are commonly used to achieve timing optimization.
Advanced VLSI Physical Design Techniques
With the increasing demand for high-performance and low-power ICs, advanced VLSI physical design techniques have emerged. These techniques aim to optimize the performance, power, and area of the chip while minimizing the design cycle time, cost, and complexity.
Design for Manufacturing (DFM)
which involves optimizing the design for manufacturability. DFM techniques ensure that the layout of the chip is manufacturable, and the manufacturing process is optimized for high yield and low cost. DFM techniques include lithography simulation, critical dimension control, and process variation analysis.
Design for Testability (DFT)
It involves optimizing the design for testing. DFT techniques ensure efficiency, reducing the time and cost of testing. DFT techniques include scan insertion, boundary scan, and built-in self-test.
Design Rule Checking (DRC)
It’s an important technique in VLSI physical design. DRC is a process that checks the layout against a set of design rules to ensure that the layout is correct and manufacturable.
Layout Versus Schematic (LVS)
LVS is a process that compares the layout against the schematic to ensure that the layout matches the design specifications.
VLSI Physical Design Tools
Physical Design Tools provide a range of capabilities for VLSI physical design, including floor planning, placement, routing, timing analysis, power optimization, and physical verification. Designers can choose the tools that best fit their needs and budget to achieve an efficient and reliable chip design.
Here are some of VLSI physical design tools:
Cadence:
- Innovus Implementation System
- Encounter Digital Implementation System
- Tempus Timing Signoff Solution
- Voltus IC Power Integrity Solution
- Conformal Equivalence Checker
- Physical Verification System (PVS)
Synopsys:
- IC Compiler II
- PrimeTime
- StarRC
- VC Formal
- SpyGlass
- TetraMAX ATPG
Mentor Graphics:
- Olympus-SoC Place and Route System
- Calibre Physical Verification System
- Tessent TestKompress
- Questa Simulation and Debugging Platform
- Nitro-SoC Timing Constraints Solution
- HyperLynx Signal Integrity Analysis
