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1 ECE 551: Digital System Design & Synthesis Spring 2003 Lecture Materials Prepared by: Charles Kime, Kewal Saluja and Michael Schulte
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1/16/2003 ECE 551 Spring 2003 2 ECE 551: Digital System Design & Synthesis Lecture Set 1: oIntroduction oOverview of Contemporary Digital Design oPragmatics 1
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1/16/2003 ECE 551 Spring 2003 3 ECE 551 - Digital System Design & Synthesis Lecture 1.1 - Introduction Overview oCourse Purpose oCourse Topics oCourse Tools oCourse Info
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1/16/2003 ECE 551 Spring 2003 4 Course Purpose To provide knowledge and experience in performing contemporary logic design based on oHardware description languages (HDLs) oHDL simulation oAutomated logic synthesis oTiming analysis With consideration for oPractical design and test issues oChip layout issues oDesign reuse for system-on-a-chip (SoC)
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1/16/2003 ECE 551 Spring 2003 5 Course Topics Pragmatics of Digital Design Hardware Modeling with the Verilog HDL Event-Driven Simulation and Testbenches Verilog Language Constructs and Delay Behavioral Descriptions in Verilog An Overview of VHDL Logic Synthesis and Timing Physical Design and Design Reuse
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1/16/2003 ECE 551 Spring 2003 6 Course Tools Modelsim HDL Simulation Tools (Mentor) Design Analyzer Synthesis Tools (Synopsys) G11 Technology Library (LSI Logic)
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1/16/2003 ECE 551 Spring 2003 7 Course Information Course Conduct Course Conduct Standard Reference Standard Reference The above plus all other course material can be found at http://courses.engr.wisc.edu/ecow/g et/ece/551/kime/ http://courses.engr.wisc.edu/ecow/g et/ece/551/kime/ Be familiar with all!
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1/16/2003 ECE 551 Spring 2003 8 Lecture 1.2 – Contemporary Digital Design Overview oLayout Lite oApplication Specific Integrated Circuit (ASIC) Technologies oIC Costs oASIC Design Flows The Role of HDLs and Synthesis The Role of IP Cores and Reuse The Role of Physical Design oSummary
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1/16/2003 ECE 551 Spring 2003 9 Layout Lite - 1 IC are produced from masks that correspond to geometric layouts produced by the designer or by EDA tools. In CMOS, a typical IC cross-section: Substrate Oxide Transistor Metal 3 Metal 2 Metal 1 Polysilicon Diffusion Channel
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1/16/2003 ECE 551 Spring 2003 10 Layout Lite - 2 The layout corresponding to the cross-section: oThe transistor is outlined in broad yellow lines. oEverything else is interconnect. Channel Transistor
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1/16/2003 ECE 551 Spring 2003 11 IC Implementation Technologies STANDARD IC FULL CUSTOM SEMI - CUSTOM FIELD PROGRAMMABLE STANDARD CELL GATE ARRAY, SEA OF GATES ASICFPGAPLD
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1/16/2003 ECE 551 Spring 2003 12 Distinguishing Features of IC Technologies - 1 Implementation technologies are distinguished by: oThe levels of the layout 1) transistors and 2) interconnect that are: Common to distinct IC designs (L1) Different for distinct IC designs (L2) oThe use of predesigned layout cells Predesigned cells are used (P1) Predesigned cells are not used (P2)
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1/16/2003 ECE 551 Spring 2003 13 Distinguishing Features of IC Technologies - 2 Implementation technologies are distinguished by: oMechanism used for instantiating distinct IC designs: Metallization (M) Fuses or Antifuses (F) Stored Charge (C) Static Storage (R)
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1/16/2003 ECE 551 Spring 2003 14 Technologies in Terms of Distinguishing Features - 1 Full Custom – P2, M oTransistors – L2, Interconnects – L2 Standard Cell – P1, M oTransistors – L2, Interconnects – L2 Gate Array, Sea of Gates – P1, M oTransistors – L1, Interconnects – L2
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1/16/2003 ECE 551 Spring 2003 15 Technologies in Terms of Distinguishing Features - 2 FPGA – P1, F or R oTransistors – L1, Interconnects – L1 PLD – P1, F or C oTransistors – L1, Interconnects – L1
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1/16/2003 ECE 551 Spring 2003 16 Technologies in Terms of Shared Fabrication Steps Custom Fabricated Layers oFull Custom and Standard Cells – all layers are custom fabricated oGate Arrays and Sea of Gates – only interconnect (metallization) layers custom fabricated oFPGAs and PLDs – nothing is custom fabricated Consequences due to economy-of-scale: oFab costs reduced for Gate Arrays and Sea of Gates oFab costs further reduced for FPGAs and PLDs
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1/16/2003 ECE 551 Spring 2003 17 Layout Styles - 1 Technologies in terms of layout styles: Adjustable Spacing Megacells Standard Cell Gate Array - Channeled … … Fixed Spacing Base Cell
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1/16/2003 ECE 551 Spring 2003 18 Layout Styles - 2 Technologies in terms of layout styles: … Base Cell Gate Array - Channel-less (Sea of Gates) Gate Array - Structured … … Fixed Embedded Block
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1/16/2003 ECE 551 Spring 2003 19 IC Costs - 1 An example: 10,000 gate circuit [1] oFixed costs Standard Cell - $146,000 Gate Array - $86,000 FPGA - $21,800 oVariable costs Standard Cell - $8 per IC Gate Array - $10 per IC FPGA - $39 per IC
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1/16/2003 ECE 551 Spring 2003 20 IC Costs - 2 An example: 10,000 gate circuit
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1/16/2003 ECE 551 Spring 2003 21 IC Costs – 3 Why isn’t FPGA cheaper per unit due to economy-of-scale? oThe chip area required by each of the successive technologies from Full Custom to FPGAs increases for a fixed-sized design. oThe larger the chip area, the poorer the yield of working chips during fabrication oAlso, due to increased sales, FPGA prices have declined since the mid-90’s much faster than the other technologies.
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1/16/2003 ECE 551 Spring 2003 22 Draw Datapath Schematics * ASIC Design Flow - Traditional Write Specifications Define System Architecture Partition - Data- path &Control Define State Diag/Tables Draw Control Schematics * Integrate Design* Do Physical Design* Implement* * Steps followed by validation and refinement
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1/16/2003 ECE 551 Spring 2003 23 Traditional Flow Problems Schematic Diagrams oLimited descriptive power State Diagrams and Algorithmic State Machines oLimited portability oLimited complexity oDifficult to describe parallelism oLimited complexity Time-Intensive and Hard to Update
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1/16/2003 ECE 551 Spring 2003 24 How about HDLs Instead of Diagrams? - 1 Hardware description languages (HDLs) oComputer-based programming languages oModel and simulate the functional behavior and timing of digital hardware oSynthesizable into a technology-specific netlist Two main HDLs used by industry oVerilog HDL (C-based, industry-driven) oVHSIC HDL or VHDL (Ada-based, defense/industry/university-driven).
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1/16/2003 ECE 551 Spring 2003 25 How about HDLs Instead of Diagrams? - 2 Advantages of HDLs oHighly portable (text) oDescribes multiple levels of abstraction oRepresents parallelism oProvides many descriptive styles Structural Register Transfer Level (RTL) Behavioral oServe as input for synthesis
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1/16/2003 ECE 551 Spring 2003 26 How about Synthesis instead of Manual Design? Increased design efficiency Reduces verification/validation problem Ability to explore more of overall design space Are there disadvantages? Potential for better optimization
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1/16/2003 ECE 551 Spring 2003 27 HDL/Synthesis Design Flow - 1 Pre-Synthesis Sign-Off Verification: Functional Design Specification Design Partition Design Entry: HDL Behavioral Integration To next page Verification: Functional Synthesis and Technology Map
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1/16/2003 ECE 551 Spring 2003 28 HDL/Synthesis Design Flow - 2 Extract Parasitics Test Generation & Fault Simulation Verification: Post-Synthesis Timing Verification: Post-Synthesis Physical Design From prior page Verification: Physical & Electrical Design Sign-Off
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1/16/2003 ECE 551 Spring 2003 29 An Example from Industry A G3 wireless processor was designed using the following methodology: oEntire processor modeled and tested using VHDL and C-based test programs oProcessor functionality verified by synthesizing to an FPGA and running 3G wireless applications at 25 MHz oProcessor timing and design feasibility verified by synthesizing to a standard cell library and running applications at 500 MHz. oFinal version of processor implemented using a mix of standard cell and custom logic to achieve low- power and 800 MHz clock speed.
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1/16/2003 ECE 551 Spring 2003 30 Newer Technologies and Design Flows - SOC System-on-a-Chip (SoC) oDesigners use (Intellectual Property – IP) cores RISC Core, DSP, Microcontroller, Memory The main function is to glue many cores and generate/design only those components for which cores and designs may not be available Used in ASIC as well as custom design environment The issues relevant to this will be discussed near the end of the course
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1/16/2003 ECE 551 Spring 2003 31 Synthesis and Technology Map Contemporary Design Flow - 1 Pre-Synthesis Sign-Off Design Specification Design Partition Verification: Functional To HDL/Synth Design Flow -2 Integration & Verification: Functional Select IP Cores Design Entry: HDL Behavioral Preliminary Phys. Design
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1/16/2003 ECE 551 Spring 2003 32 Lecture 1.2 Summary Application Specific Integrated Circuit (ASIC) Technologies oProvides a basis for what we will design IC Costs oGives a basis for technology selection ASIC Design Flows oShows the role of HDLs and synthesis oProvides a structure for what we will learn What we will do
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1/16/2003 ECE 551 Spring 2003 33 References 1) Smith, Michael J. S., Application-Specific Integrated Circuits, Addison-Wesley, 1997.
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1/16/2003 ECE 551 Spring 2003 34 Lecture 1.3 Pragmatics 1 Pragmatics refers to practical design choices and techniques Topics oCell Libraries oAsynchronous Circuits oThree-State Logic and Hi-Z State
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1/16/2003 ECE 551 Spring 2003 35 Cells and Cell Libraries What is a cell? What is a cell library? What appears in the cell library for each ASIC cell?
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1/16/2003 ECE 551 Spring 2003 36 What is a Cell? Cells are the building blocks for digital designs Come in different sizes, shapes and functions varying from transistors to large memory arrays or even a processor Typically cells: oSmall Scale: AND, OR, NAND, NOR, NOT, AOI, OAI, Flip-Flops, Latches oMedium Scale: Multiplexers, Decoders, Adders oLarge Scale: Memories, Processors Provided by ASIC vendors
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1/16/2003 ECE 551 Spring 2003 37 What is a Cell Library? A database specifying and describing the target technology in the form of pre- designed objects called cells. Synthesis target technology. In-Class Discussion: What are typical components in the database for each cell?
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1/16/2003 ECE 551 Spring 2003 38 Asynchronous Techniques Delay-dependent design Combinational hazards Combinational hazard prevention Asynchronous design
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1/16/2003 ECE 551 Spring 2003 39 Delay-Dependent Design 1 LA PA A A LA PA Example: Level-to-Pulse Converter(Delay-Based)
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1/16/2003 ECE 551 Spring 2003 40 Delay-Dependent Design 2 Sometimes useful But should be avoided Time delays vary and so may: oFail oProduce variable results, e. g. pulse length
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1/16/2003 ECE 551 Spring 2003 41 Delay-Dependent Design 3 LA PA D QC Clock Level on LA must be longer than a clock period and must not rise close to the positive clock edge. Ideally, synchronous with Clock. Level to Pulse Converter (Synchronous)
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1/16/2003 ECE 551 Spring 2003 42 Combinational Hazards 1 Example - Hazard in a Multiplexer A C F B 1 1 B F
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1/16/2003 ECE 551 Spring 2003 43 Combinational Hazards - 2 A circuit has a hazard if there exists an assignment of delays such that an unwanted signal transition (glitch), can occur. Types of changes on combinational circuit inputs : oSingle-input change (SIC) oMultiple-input change (MIC) A SIC static hazard exists on a circuit output if in response to a SIC, the output momentarily changes to the opposite value. oStatic 1-hazard – output value to remain at 1 oStatic 0-hazard – output value to remain at 0
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1/16/2003 ECE 551 Spring 2003 44 Combinational Hazards - 3 Classification of Combinational Hazards oStatic – SIC/MIC – output changes when it should remain fixed - output value within the “transition region of input changes is fixed. oDynamic – SIC/MIC – output changes three or more times when it should change only once. oEssential – MIC – output changes when it should remain fixed – output value within the “transition region” of input changes not fixed.
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1/16/2003 ECE 551 Spring 2003 45 Combination Hazards - 4 In-class Example: Illustration of static, dynamic and essential hazards
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1/16/2003 ECE 551 Spring 2003 46 Combinational Hazards - 5 Consequences of Hazards oSignals with hazards within or entering asynchronous circuits (note that a flip-flop is an asynchronous circuit with respect to its clock signal!) oCause incorrect state behavior Extra state changes Incorrect state changes In-Class Example: Prevention of Hazards oRedundant Logic oDelay Dependence
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1/16/2003 ECE 551 Spring 2003 47 Asynchronous Design - 1 Which of the following sequential circuits involve asynchronous design? oA circuit that has no global clock signal involved in its operation – state changes occur in response to input changes only. oA D flip-flop circuit oA circuit using clock gating on flip-flop clock inputs oA circuit with a clock which uses the clear and preset inputs on the flip-flops for other than initialization.
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1/16/2003 ECE 551 Spring 2003 48 Asynchronous Design - 2 Because of the difficulty of eliminating hazards, it is very difficult to insure correct operation under all timing possibilities Design must be done manually or by use of very specialized synthesis tools. Therefore, avoid it if you can! If you truly need it, investigate some of the more contemporary approaches[1] which avoid some of the many difficulties.
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1/16/2003 ECE 551 Spring 2003 49 Three-State and Other Hi-Z States Three-state conflicts Floating three-state nets and inputs Pull-ups and Pull-downs Bus keepers
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1/16/2003 ECE 551 Spring 2003 50 Three-State Conflicts - 1 What are they and what are their effects? oStatic – Chip damage or static power consumption oDynamic – Dynamic or static power consumption 1 1 1 0 D0 D1 E0 E1 OUT E0 E1 E0 E1 1 1 1 1 0 0
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1/16/2003 ECE 551 Spring 2003 51 Three-State Conflicts - 2 How can conflicts be avoided? oStatic – Decoded enable signals oDynamic – Delay control 1 1 0 D0 D1 E0 E1 OUT E0 E1 E0 E1 1 1 0 1 0 0
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1/16/2003 ECE 551 Spring 2003 52 Floating Inputs and Three- State Nets - 1 Floating input values on gates can cause: ostatic power dissipation ohigh-frequency switching that induces power supply noise Floating input values arise from: oGate inputs, e. g., for example on exterior of IC, that are not connected oLines driven by 3-state buffer or gate outputs, all of which are in the Hi-Z state.
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1/16/2003 ECE 551 Spring 2003 53 Floating Inputs and Three- State Nets – 2 How can floating inputs and nets be avoided? oUse a pull-up or pull-down resistor or transistor with a fixed gate voltage value. Advantage – simple Disadvantages – static power dissipation and loading of node oOn internal lines, particularly buses, use a bus keeper (weak buffer)
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1/16/2003 ECE 551 Spring 2003 54 Non-D flip-flops D Flip-Flops oUnique characteristic – the typical master-slave DFF is also functionally an edge-triggered DFF. Non- D Flip-Flops (JK, T, etc.) oIn the cell libraries, these flip-flop may be full-custom designs or may simply consist of a DFF with added logic. oIf it is just a DFF with added logic, you might as well design for a DFF to give the logic optimization software more flexibility.
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1/16/2003 ECE 551 Spring 2003 55 References [1] Chris J. Myers, Asynchronous Circuit Design, John Wiley & Sons, Inc., New York, 2001.
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