ASIC Design Flow

ECE 448Lecture 19

Sources

• Jamie Bernard, Physical Level Design using Synopsys,Scholarly Paper, GMU, 2005

• Cory Ellinger,VLSI Design Automation,Independent Research Project,GMU, 2005.

IntroductionIntroduction

Introduction• Technological Advances

– 19th Century - Steel– 20th Century – Silicon

• Growth in Microelectronic (Silicon) Technology– Moore’s Law (# of transistors double/18 months)– One Transistor– Small Scale Integration (SSI)

• Multiple Devices (Transistor / Resistor / Diodes)• Possibility to create more than one logic gate (Inverter, etc)

– Large Scale Integration (LSI)• Systems with at least 1000 logic gates (Several thousand transistors)

– Very Large Scale Integration• Millions to hundreds of millions of transistors (Microprocessors)

– Intel indicates that dual core processors will soon exist that contain 1 billion transistors

Introduction• Manual (Human) design can occur with small number of

transistors

• As number of transistors increase through SSI and VLSI, the amount of evaluation and decision making would become overwhelming (Trade-offs)

– Maintaining performance requirements (Power / Speed / Area)– Design and implementation times become impractical

• How does one create a complex electronic consisting of millions of transistors?

Automate the Process using Computer-Aided Design (CAD) ToolsAutomate the Process using Computer-Aided Design (CAD) Tools

Introduction• CAD tools provide several advantages

– Ability to evaluate complex conditions in which solving one problem creates other problems

– Use analytical methods to assess the cost of a decision– Use synthesis methods to help provide a solution – Allows the process of proposing and analyzing solutions to occur

at the same time

• Electronic Design Automation– Using CAD tools to create complex electronic designs (ECAD)– Several companies who specialize in EDA

• Cadence® Design Systems• Magma® Design Automation Inc.• Synopsys®

CAD Tools Allow Large Problems to be SolvedCAD Tools Allow Large Problems to be Solved

Design FlowDesign Flow

Top Level Digital Design Flow

RTL Design

Place + Route

Physical Verification

Synthesis

Design Inception

Design Complete

Macro Development

RTL DesignDesign Function Digital Tool

RTL Design

Testbench Developement

Mixed Mode Simulation

FPGA Verification(users disgression)

Lint Checking(users digression)

Code Coverage(users disgression)

Formal Verification

Cadence NC VerilogMentor Graphis ModelSim

Cadence NC VerilogMentor Graphics ModelSim

Cadence AMS Designer

Xilinx ISE

Cadence Hal

Cadence ICT

Agilent ADSMatlab

Design Inception Design Inception

Synthesis

Synthesis + Macro Development

System Interface Simulation

Cadence Conformal

Synthesis + Macro Development

Synthesis + Macro DevelopmentDesign Function Digital Tool

Synthesis

Static Timing Analysis

Logical Equivalency

DFT

Place + Route

Gate-Level Simulation

RTL

Synopsys DC Cadence RC

Synopsys PrimeTime

Cadence Conformal

Synopsys DFT CompilerCadence RC

Place + Route

Cadence NC VerilogMentor Graphics Modelsim

RTL

Macro Generation

Macro Verification

Macro Rules Generation / Library Generation

Mentor Graphics Calibre

Artisan/Cadence DFII

Artisan

Verification Verification

Place + Route

Floorplan

Macro Placement / Std Cell Placement

Placement-Based Optimization

Clock Tree Synthesis

Route

RC Extraction

Signal Integrity

Design Function Digital Tool

Static Timing

Analysis

Cadence NanoRoute

Cadence Fire&Ice QX

Cadence CeltIC / Voltage Storm

SynopsysPrime-Time

Verification Verification

Cadence Encounter

Synthesis Synthesis

ATPG Mentor Graphics FastScan

Cadence EncounterMetal Fill

Spare Cells / Decoupling Cap Filler Cells Cadence Encounter

Physical VerificationDesign Function Digital Tool

GDSII Preparation / Schematic Preparation

DRC

LVS

ERC

Simulation Preparation

Back Annotated SimulationLayout / Chip Finishing

Cadence DFII Cadence DFII

Cadence NC VerilogCadence Virtuoso

Placed + Routed Design

Placed + Routed Design

Design Complete Design Complete

Mentor Graphics Calibre

Top-Level Simulation Synopsys NanosimCadence AMS Designer

Design Flow - Overview

• Generic VLSI Design Flow from System Specification to Fabrication and Testing

• Steps prior to Circuit/Physical design are part of the FRONT-END flow

• Physical Level Design is part of the BACK-END flow

– Physical Design is also known as “Place and Route”

• CAD tools are involved in all stages of VLSI design flow

– Different tools can be used at different stages due to EDA common data formats*

• Synopsys® CAD tool for Physical Design is called Astro™

Front-End Design FlowFront-End Design Flow

Synthesis using Design Compiler

Wireload model basics (1)

Wireload model basics (2)

Back-End Design FlowBack-End Design Flow

Physical Level Design using Synopsys®

What does Astro™ do?

Where does the Gate Level Netlist come from?1st Input to Astro™

Standard Cell Library2nd Input to Astro™

• Pre-designed collection of logic functions

– OR, AND, XOR, etc

• Contains both Layout and Abstract views

– Layout (CEL) contains drawn mask layers required for fabrication

– Abstract (FRAM) contains only minimal data needed for Astro™

– Timing information• Cell Delay / Pin Capacitance

• Common height for placement purposes

Timing Constraints3rd Input to Astro™

• Derived from system specifications and implementation of design

• Identical to timing constraints used during logic synthesis

• Common constraints in electronic designs– Clock Speed/Frequency– Input / Output Delays associated with I/O signals– Multicycle Paths– False Paths

• Astro™ uses these constraints to consider timing during each stage of the place and route process

Concept of Place and Route

• Location of all standard cells is automatically chosen by the tool during placement (Based upon routing and timing)

• Pins are physically connected during routing (Based upon timing)

Concepts of Placement

• Standard cells are placed in “placement rows”

• Cells in a timing-critical path are placed close together to reduce routing related delays (Timing Driven)

• Placement rows can be abutting or non-abutting

Concepts of Routing

• Connecting between metal layers requires one or more “vias”

• Metal Layers have preferred routing directions

– Metal 1 (Blue) Horizontal– Metal 2 (Yellow) Vertical– Metal 3 (Red) Horizontal

Design SetupDesign Setup

Design Flow – Design Setup

• The three main inputs for Astro™ need to be combined into a common database

• Design Setup needs to be completed prior to performing place and route

• This environment (database) allows Astro™ to use both the logical and physical information of the design

• Goal of Design Setup is to prepare design for Floorplanning

Design Setup Step #1 Creating a Design Library

Technology File

• Layer and via Definitions

• Process design rules (Minimum metal widths and spacing)

• Resistance / Capacitance parasitic models

• Units (Time / Capacitance / Distance)

• GUI display information (Colors and fill template for layers)

• This file is stored in the design library (Common Database)

Design Setup Step #2 Attach Reference Libraries

• Reference libraries contain cells used by many other designs

• They are referenced by pointers in the design library for memory efficiency

Design Setup Step #3 Read Netlist

• Netlist is generated from logic synthesis stage

• Netlist can be in several different formats– Verilog– VHDL– EDIF (Electronic Design Interchange Format)

• Parsed for syntax errors and other errors that may cause problems during physical design

Design Setup Step #4 Expand Netlist

• Netlist must be flattened or “Expanded

• During this process, Astro™ verifies that an “abstract” view is available for each leaf cell (Reference Library Check)

Design Setup Step #5 Create Starting Cell

• UNIX structure for design library• CEL directory will contain starting cell

(beginning point for place and route)– CEL (Graphical)– NETL (Netlist)– EXP (Expanded)

Design Setup Step #6 Bind Netlist to Cell

• Logical and physical representations are merged into Starting Cell

Design Setup Step #7 Preserve the Hierarchy

• After Place and Route, the design must be functionally verified

– Goal of reusing existing test benches and stimulus files

• Astro™ only operates on a flat design– By default output netlist will be flat– Test “probe” points on sub-block boundaries may

have been moved or removed, and the test bench can not be reused

• Solution: Preserve the Hierarchy– Astro™ can maintain the ports and function of the

ports at the hierarchical boundaries of the sub-blocks.

– Astro™ can then operate on flat design and reconstruct the hierarchy if needed

FloorplanFloorplan

Design Flow – Floorplan

• Layout design done at the chip level– Defining layout hierarchy– Estimation of required design area

• A blueprint showing the placement of major components in the design (non-standard cell)

– Inputs / Output (I/O)– RAMs / ROMs/– Reusable Intellectual Property (IP) macros

• Approaches to Floorplanning (Automatic or Manual)– Constructive– Iterative– Knowledge-Based

Design Must Be Floorplanned Before P&R

• Floorplan of design:– Core area defined with large macros placed– Periphery area defined with I/O macros placed– Power and Ground Grid (Rings and Straps) established

• Utilization:– The percentage of the core that is used by placed standard cells and

macros– Goal of 100%, typically 80-85%

I/O Placement and Chip Package Requirements

• Some Bond Wire requirements:

– No Crossing

– Minimum Spacing

– Maximum Angle

– Maximum Length

Guidelines for a Good Floorplan

• A few quick iterations of place and route with timing checks may reveal the need for a different floorplan

Defining the Power/Ground Grid and Blockages

• Purpose of Grid is to take the VDD and VSS received from the I/O area and distribute it over the core area

• Blockages can also be added in the floorplan to prohibit standards cells from being placed in those areas

Timing Driven PlacementTiming Driven Placement

Design Flow – Timing Driven Placement• Astro™ optimizes, places, and

routes the logic gates to meet all timing constraints

• Balancing design requirements– Timing– Area– Power– Signal Integrity

Timing Constraints• Astro™ needs constraints to

understand the timing intentions

– Arrival time of inputs– Required arrival time at outputs– Clock period

• Constraints come from the Logic Synthesis tool

– SDC (Synopsys Design Constraints) format

Cell and Net Delays

• Astro™ calculates delay for every cell and every net

• To calculate delays, Astro™ needs to know the resistance and capacitance of each net

– Uses geometry of net and Look Up Tables to estimate the resistances and capacitances

Timing Sanity Check• Prior to placement, Astro™ can check the design

timing constraints for feasibility

• The process is called a “Zero-Interconnect” check

• Astro™ performs timing analysis based upon design constraints (SDC) and cell delay only, while ignoring the RC delay

• If major timing violations exist, the problem will probably get worse with the added RC delay (routing)

Zero-Interconnect Check Saves Time and ResourcesZero-Interconnect Check Saves Time and Resources

Timing Driven Placement

• Timing Driven Placement places critical path cells close together to reduce net RC

• Prior to routing, RC are based on Virtual Routes

• What if critical paths do not meet timing constraints with placement?

Logic Optimizations

• These optimizations can be done during pre-place, in-place, or post-place stages of placement

• Each optimization can be done separately or all done concurrently during placement (none – one – all)

Clock Tree SynthesisClock Tree Synthesis

Design Flow – Clock Tree Synthesis

• All clock pins are driven by a single clock source

• Large delay and transition time due to length of net

• Clock signal reach some registers before others (Skew)

Clock Tree Topologies

• Clock source is connected to center of the network

• Networks are distributed in a H or X shape until clock pin of register is driven by a local buffer

H-Tree and X-Tree Topologies Solve Single Clock Pin ProblemH-Tree and X-Tree Topologies Solve Single Clock Pin Problem

After Clock Tree Synthesis

• A clock (buffer) tree is built to balance the output loads and minimize the clock skew

• A delay line can be added to the network to meet the minimum insertion delay (clock balancing)

Gated - CTS

• Clocks may not be generated directly from I/O

• Power saving techniques such as clock-gating are used to turn of the clock to sections of the design

• Astro™ can interpret gated clocks and can build clock trees “through” the logic to the registers

Effects of CTS

• Several (Hundreds/Thousands) of clock buffers added to the design

• Placement / Routing congestion may increase

• Non-clock cells may have been moved to less ideal locations

• Timing violations can be introduced

RoutingRouting

Design Flow – Routing

• Routing is a fundamental step in the place and route process

• Create metal shapes that meet the requirements of a fabrication process

– The physical connection between cells in the design

• Virtual routes used during placement and CTS need to become reality

– Timing of design needs to be preserved– Timing data such as signal transitions and clock skew needs to

match the virtual route estimates

Process of Routing Can Be Timing DrivenProcess of Routing Can Be Timing Driven

Timing Driven Routing

• Routing along the timing-critical path is given priority– Creates shorter, faster connections

• Non-critical paths are routed around critical areas– Reduces routing congestion problems for critical paths– Does not adversely impact timing of non-critical paths

Concept of Routing Tracks

• Metal routes must meet minimum width and spacing “design rules” to prevent open and short circuits during fabrication

• In grid based routing systems, these design rules determine the minimum center-to-center distance for each metal layer (Track/Grid spacing)

• Congestion occurs if there are more wires to be routed than available tracks

Grid-Based Routing System

• Metal traces (routes) are built along and centered around routing tracks

• Each metal layer has its own tracks and preferred routing direction

– Metal 1 – Horizontal– Metal 2 – Vertical

• Track and pitch information can be located in the technology file

– Design Rules

Routing Operations

• Astro™ performs 4 stages during routing– 1) Global Routing– 2) Track Assignment– 3) Detailed Routing– 4) Search and Repair

• After stages 1-3, all clock/signal nets will be completely routed and should meet all timing and design rule (DRC) requirements

• Any remaining DRC violations can be fixed by Search and Repair

Global Routing

• First step in routing process

• Each net receives a broad routing plan determining how the net will be routed in the design

– Determine open channels for routing

• Provides the background information for the next step of Track Assignment

• Routing congestion is resolved– If congestion is severe, extreme measures may be taken such as

moving large macros or re-floorplanning the design to relieve congestion

Globally Connecting All Point A’s to All Point B’sGlobally Connecting All Point A’s to All Point B’s

Track Assignment

• Second step in routing process

• Assigns each net to a specific track and creates the physical metal traces

• Attempts are made to make long, straight traces and to reduce the numbers of vias

• Physical DRC rules are not checked

Detailed Routing

• Third step in routing process• Attempts to clear DRC violations using a

fixed size Sbox• All violations may not be fixed due to fixed

Sbox size

Search and Repair

• Last step in routing process

• Fixes the remaining DRC violations through multiple loops using a progressively larger Sbox size

• Design must have 0 DRC violations– DRC rules for Astro™ are only a subset of the complete

technology DRC rules– Must use Hercules™ for sign-off DRC check

VerificationVerification

What Happens After Place and Route?

Verification

Formal Verification

• New standard cells have been added to the design through timing optimizations and clock tree synthesis

• The final netlist created by Astro™ needs to be compared to the original gate-level netlist

• Formal verification ensures the functional equivalency at the logic level between the two implementations (original vs. final) of the design

– The intended function was maintained throughout the physical design process

Formality® is the Sign-Off Tool for Formal VerificationFormality® is the Sign-Off Tool for Formal Verification

Timing Verification• Star-RCXT™ performs the layout parasitic extraction of

the resistances and capacitances of all routes in the design

• Results in a format such as SPEF (Standard Parasitic Extended Format)

– SPEF is an smaller, extended format of Standard Parasitic Format(SPF), which enables the transfer of design specific resistancesand capacitances from physical design to timing analysis and simulation tools

• Primetime® performs static timing analysis– Detects timing violations by combining SPEF from Star-RCXT™

and netlist from Astro™ and checks against the design timing constraints (clock frequencies)

Star-RCXT™ and Primetime®are the Sign-Off Tools for Timing Verification

Star-RCXT™ and Primetime®are the Sign-Off Tools for Timing Verification

Physical Verification• Checks the design for fabrication feasibility and physical

defects that could result in the design to not function properly

– 3 checks (DRC, ERC, and LVS)

• Design Rule Checks (DRC)– Verifies that design does not violate any fabrication rules

associated with the target process technology (metal width/space, antenna ratio, etc)

• Electrical Rules Checks (ERC)– Verifies that there are no short or open circuits with power and

ground as well as resistors/capacitors/transistors with floatingnodes (part of LVS)

• Layout Versus Schematic (LVS)– Final physical design matches the logical (schematic) version in

terms of correct connectivity and number of electrical devices

Hercules™ is the Sign-Off Tool for Physical VerificationHercules™ is the Sign-Off Tool for Physical Verification

Fabrication

• Physical Design process is complete upon successful completion of timing, functional, and physical verification

• The design can be “Taped-Out” and GDSII created for the manufacturer

– GDSII (Graphic Design System II) is a binary format containing the physical geometry information of the design.

– The shapes are assigned numeric attributes in the form of “Layer Number”and “Data Type” (Metal 1 => 100:0)

• Fabrication and Test determine which chips can be implemented into the system (yield)

Future• Technology continues to push the envelope with each new

process node– Requirements of faster speeds, lower power and cost, and all within

the smallest area possible creates problems during physical design

• Challenges in physical design are dramatic at 130nm/90nm– Voltage Drop, Crosstalk and Signal Integrity, Reliability

(Electromigration)– PD tools can still be implemented to address these issues

• 65nm/45nm Concerns– Leakage power of transistors could reach the level of the dynamic

power of the design– Wiring delays outweighing gate delays (130nm and beyond)– Cross coupling capacitance could begin to dominate over the

capacitance of the wire itself

• These challenges and concerns will force logic designers to think physically and physical designers to think electrically

EDA Tool Advancement Needed for 45nm EDA Tool Advancement Needed for 45nm

Conclusion

• Designing a sophisticated VLSI design is complicated– Number of transistors combined with challenges at each processing

node

• Design time can be over 1 year from system conception to fabricated chips

• No one person or design team could manually design and complete a system in a time frame to be market competitive

Task of Designing, Placing,Routing, and Fabricating Complex VLSI Designs

Must Become Automated

Task of Designing, Placing,Routing, and Fabricating Complex VLSI Designs

Must Become Automated

QUESTIONS ?QUESTIONS ?