CAD Algorithms Physical Design Automation of VLSI Systems Mohammad Tehranipoor ECE Department
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Physical Design Automation
Objectives: Obtain general understanding about IC design process Study design automation Study algorithms
Used in designing the layout of a chip
Prepare students for exposure to hard CAD problems
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VLSI Design Cycle
Large number of components:
Optimize requirements for higher performance
The physical design is not practical without the help of computers. Performance relates to speed, power and size.
Time to market competition:
Cost:
Makes the use of computer necessary Using computer makes it cheaper by reducing time-to-market. Manual
System Specifications
Chip Automation
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VSLI Design Cycle (Cont.)
VLSI design cycle can be divided into the following steps:
System Specification:
Goals and constraints of the system Functionality (what will a system do) Performance figures like speed and power Technology constraints like size and space (physical dimensions) Fabrication technology and design techniques
Architectural/Functional Design:
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RISC, CISC, # of ALUs, floating point units, number and structure of pipelines, etc. Functional behavior and functional units to match specification Functional subunits and relationship among them
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VSLI Design Cycle (Cont.)
Logic Design:
Implementation of functional subunits using logic representation
Use of standard building blocks like RAM, ROM, PLA, etc. Logic design should match functional description. Register Transfer Level (RTL) description of subunits.
Boolean expressions, finite state machines or schematics.
RTL is expressed in VHDL or Verilog.
Circuit Design
Logic networks or descriptions are converted into electronic circuits. Circuit elements are designed to meet specifications. Transistors are sized for current capacity and delay requirements. Circuit simulation is used to verify the correctness and timing of each component.
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VSLI Design Cycle (Cont.)
Physical Design: That’s the focus of our course.
The circuit representation is converted into a geometric representation. Geometric representation of a circuit is called layout. The exact details of the layout depends on design rules. Design rules are guidelines based on the limitations of the fabrication process.
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VSLI Design Cycle (Cont.) CAD Tools:
Layout synthesis tools are fast but do have area and performance penalties. Manual layout is very slow but does have better area and performance. Most of the layout of a high performance custom design may be done using manual design. Synopsys, Cadence, Mentor Graphics, Magma, and more
Note that the objective of VLSI CAD tools are to minimize the time for each iteration and the total number of iterations, thus reducing time-to-market.
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VSLI Design Cycle (Cont.)
Fabrication:
After layout and verification, the design is ready for fabrication (called tapeout). Layout data is converted into photo-lithographic masks.
Testing and Debugging:
After fabrication, each die is tested. The wafer is diced into individual chips. Each chip is packaged and tested.
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VLSI Design Cycle System Specifications
Functional Design
X=(AB*CD)+ …
Logic Design
Circuit Design
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VLSI Design Cycle
Physical Design
Fabrication
Packaging
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New Trends in VLSI Design Cycles
Increasing Interconnect Delay:
Interconnect is not scaling at the same rate as the devices.
Almost 60% of path delay may be due to interconnect.
Increasing Interconnect Area:
Almost 30-40% of the area is covered by interconnects in modern designs.
Relative Delay
Interconnect vs. Gate Delay
Technology Node (nm)
ITRS2005
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New Trends in VLSI Design Cycles
Increasing number of Metal Layers:
To meet the increasing need of interconnect, number of metals is increasing.
Up to 12 metal layer in the next few years
In nanometer technology designs:
Many Metal Layers
More silicon area consumed by wires – Miles of Cu wires Increasing wire lengths and interconnect defects, i.e. more bridging faults Increasing number of vias – Metal layers
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New Trends in VLSI Design Cycles Increasing Planning Requirements:
Physical design considerations have to enter into design at much earlier phase.
Synthesis:
The design time can be reduced if layout can be directly generated from a higher level description.
Behavioral Model Physical Layout
New tools can support it.
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Physical Design Process
Physical Design converts circuit description into a geometric description. This description is used to manufacture a chip. The input to a physical design cycle is a circuit diagram/netlist and the output is the layout of the circuit. Required Stages:
Partitioning Floorplanning Placement Routing Compaction Extraction and Verification (post-layout verification)
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New Trends in Physical Design Process
Chip Level Signal Planning Routing of major signals and buses must be planned from early design stages, so that the interconnect distances can be minimized. Interconnects length directly impact circuit delay and layout area
OTC Routing Over-the-cell (OTC) routing allows routing over blocks and active areas.
Reduces the layout area
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Major Challenges
Crosstalk
Cc
Cs
Cc
0.35 µm
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Cs
90 nm
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Cont.
Power Supply Noise and Temperature
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Cont.
Power Supply Noise
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Cont.
Temperature
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Steps of Iterative Design Process
Synthesis:
Analysis:
Synthesis derives or improves a representation at any step. Behavioral synthesis, logic synthesis and layout synthesis are all examples at various steps. Analysis ensures that the design representation matches the requirements at various steps. Requirements like area, power dissipation and speed.
Verification:
Establishes the correctness of design at any given step. Simulation at any step should match specifications.
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Design Process is Iterative Behavioral Simulation-Based Verification Structural Simulation-Based Verification Synthesis Simulation-Based Verification PNR Simulation/Emulation-Based Verification
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Design Styles JC
Preferred style for mass production, Highly optimized layout, Time can be justified
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Selection of Design Style
Selection of design styles depends on many factors:
Type of chip
Volume
Mass production, Medium production volume, …
Cost
High performance, Area, …
Company’s budget, Cost of the chip
Time-to-market
Last two are dominant.
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Full Custom Design Style
Circuit partitioned into sub-blocks. Blocks can be of any size/shape Hierarchical design Placement on any location is allowed Allows very compact designs Difficult automation High performance
The process of automating a fullcustom design style has a much higher complexity than other restricted styles. 7 October 2008
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Standard Cell Design Style
Design process is somewhat simpler than full-custom design style Rectangular cells of same height Library based design Cells arranged in rows Easier automation Inherently nonhierarchical Lower performance Well suited for moderate size designs and medium production volume. Logic synthesis uses standard cell design style.
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Standard Cell
Channel is the space between two rows.
Feedthrough is the empty cells in a row.
Feedthroughs are assigned for the interconnections of non-adjacent cells.
Standard cell design style takes more area than full custom.
Along with semiconductor manufacturing advances, standard cell methodology was responsible for allowing designers to scale ASICs from comparatively simple single-function ICs (of several thousand gates), to complex multi-million gate devices (SoC).
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Standard Cell
2-input NAND or NOR function is sufficient to form any arbitrary Boolean function set
In modern ASIC design, standard cell methodology is practiced with a sizeable library of cells
The library contains multiple implementations of the same logic function, differing in area and speed
This variety enhances the efficiency of automated synthesis, place and route tools
It gives the designer greater freedom to perform implementation tradeoffs
Area vs Speed vs Power Consumption
A complete group of standard cell descriptions is commonly called a technology library.
The technology library is developed and distributed by the foundry operator.
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Gate Array Design Style
A simplification of Standard Cell Design Style
Gate array design is a manufacturing method in which the diffused layers, i.e. transistors and other active devices, are predefined and wafers containing such devices are held in stock prior to metallization, in other words, unconnected
A regular lattice shaped structure Pre-fabricated logic Both vertical and Horizontal channels Only wiring masks need to be defined Rapid fabrication Low performance Easy automation Non-hierarchical structure Logic synthesis can use gate array.
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Gate Array
Photo-lithographic masks are required only for the metal layers
Production cycles are much shorter as metallization is a comparatively quick process
Advantages/Disadvantages:
The steps involved for creating any prefabricated wafer are the same
Only the last few steps in the fabrication process will be used.
Cheaper and easier to produce than full-custom and standard cell.
It offers more area and less time.
Difficulties in routing the interconnect require migration onto a larger array device with consequent increase in the price
Pure, logic-only gate array design is rarely implemented by circuit designers today
FPGA is preferred.
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Gate Array
Today gate arrays are evolving into Structured ASICs Structured ASICs consist of a large IP core like a CPU, DSP unit, peripherals, standard interfaces, integrated memories SRAM, and a block of reconfigurable uncommited logic. This shift is largely because ASIC devices are capable of integrating large blocks of system functionality and "system on a chip" requires far more than just logic blocks.
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FPGA Design Style
Extremely rapid prototyping Re-programmable Lowest performance Easy automation Cells and interconnects are prefabricated. The user simply programs the interconnects. Contains programmable logic blocks and interconnects
Logic blocks can be programmed to perform the function of basic logic gates such as AND, and XOR, or more complex combinational functions such as decoders or mathematical functions. FPGAs contain FFs
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LUT Programming
Source: Fundamentals of Digital Logic , McGraw Hill 2000
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LUT Programming
3-input LUT
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LUT Programming Sequential
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Comparison of Design Styles
Production Volume:
Mass Production Volume
Complexity:
High
Medium Production Volume
Medium Production Volume
Low Production Volume Low
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Physical Design Automation
Physical design implies “physical realization” of integrated circuit layout.
Input to physical design cycle is a circuit design
Circuit netlist (gate level representation) RTL description of the circuit
Output from this stage is a layout of the circuit
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The task from input to output is accomplished in several design stages, each addressing specific goals.
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Physical Design Cycle
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Physical Design Cycle
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Physical Design Cycle
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VLSI Design Automation
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