ECE 551: Digital System Design & Synthesis

Motivation and IntroductionLectures Set 1

(3 Lectures)

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Overview• Course Introduction• Overview of Contemporary Digital Design

– Layout– Application Specific Integrated Circuit (ASIC)

Technologies– IC Costs– ASIC Design Flows– The Role of HDLs and Synthesis

• Summary

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Course Introduction• Purpose:

– To provide knowledge and experience in performing contemporary logic design based on 1) hardware description languages (HDLs), 2) HDL simulation, 3) automated logic synthesis and 4) timing analysis with consideration for a) pragmatic design and test issues, b) chip layout issues, and c) design reuse in the context of the ASIC (Application Specific Integrated Circuit) and SOC (System-On-a-Chip) technologies.

• Conduct and Outline– Go through the course homepage for 551– http://www.cae.wisc.edu/~ece551/spring02/general/conduct.html

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Layout• IC are produced from masks that correspond

to geometric layouts produced by the designer or automatically.

• In CMOS, a typical IC cross-section:

Substrate

Oxide

Transistor

Metal 3

Metal 2

Metal 1

PolysiliconDiffusion

Channel

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Layout (continued)• The layout corresponding to the cross-section:

– The transistor is outlined in broad yellow lines.– Everything else is interconnect.

Channel

Transistor

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IC Implementation Technologies• Implementation technologies are distinguished by:

– The levels of the layout 1) transistors and 2) interconnect that are:• Common to distinct IC designs (L1) • Different for distinct IC designs (L2)

– The use of predesigned layout cells• Predesigned cells are used (P1)• Predesigned cells are not used (P2)

– Mechanism used for instantiating distinct IC designs:• Metallization (M)• Fuses or Antifuses (F)• Stored Charge (C)• Static Storage (R)

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IC Implementation Technologies (continued)

STANDARDIC

FULLCUSTOM

SEMI-CUSTOM

FIELDPROGRAMMABLE

STANDARDCELL

GATE ARRAY,SEA OF GATES

ASIC

FPGA PLD

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IC Implementation Technologies (continued)

• Technologies in terms of Distinguishing Features:– Full Custom – P2, M

• Transistors – L2, Interconnects – L2– Standard Cell – P1, M

• Transistors – L2, Interconnects – L2– Gate Array, Sea of Gates – P1, M

• Transistors – L1, Interconnects – L2– FPGA – P1, F or R

• Transistors – L1, Interconnects – L1– PLD – P1, F or C

• Transistors – L1, Interconnects – L1

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IC Implementation Technologies (continued)

• Technologies in terms of shared fabrication steps (can be used for common transistors/interconnects):– Full Custom and Standard Cells – all layers are custom fabricated– Gate Arrays and Sea of Gates – only interconnect (metallization) layers custom fabricated– FPGAs and PLDs – nothing is custom fabricated

• Consequences due to economy-of-scale:– Fab costs reduced for Gate Arrays and Sea of Gates– Fab costs further reduced for FPGAs and PLDs

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IC Implementation Technologies (continued)

• Technologies in terms of layout styles:

Adjustable Spacing

Megacells

Standard Cell

Gate Array - Channeled

…

…Fixed Spacing

Base Cell

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IC Implementation Technologies (continued)

• 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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IC Costs• An example: 10,000 gate circuit [1]

– Fixed costs• Standard Cell - $146,000• Gate Array - $86,000• FPGA - $21,800

– Variable costs• Standard Cell - $8 per IC• Gate Array - $10 per IC• FPGA - $39 per IC

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IC Costs (continued)

0500,000

1,000,0001,500,0002,000,0002,500,0003,000,0003,500,0004,000,000

100 1000 10000 100000

StandardCellGateArrayFPGA

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IC Costs (continued)

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CELLS

• Can of different sizes, shapes and functions varying from transistors, on-chip resistors to large memory arrays or even a processor (often referred to as IP “intellectual property” core)

• Typically cells in todays cell libraries are:– Gates – AND, OR, NAND, NOR, NOT– Complex gates – AOI, OAI– Storage elements – Flip-Flops, Lateches

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ASIC Cell Libraries

• Sources –– ASIC vendor

• use tools supplied by the vendor, often details are not provided (proprietary)

– Third party• Details are provided, often general enough to be used with a

known technology, testing may not be necessary

– Build your own• Complete the layout and test to verify the design and

performance, often complex and expensive

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ASIC Cell Libraries (contd.)

• Cells– Transistors and transistors as resisitors– Combinational logic cells

• Gates – number of inputs, performance (drive capability and width)

• Complex gates

– Sequential cells• Flip-flops, latches

– Datapath logic cells• Multiplexors, adders, ALU, mutipliers

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Draw DatapathSchematics *

Traditional ASIC Design Flow

WriteSpecifications

Define SystemArchitecture

Partition - Data-path &Control

Define StateDiag/Tables

Draw ControlSchematics *

IntegrateDesign*

Do Physical Design*

Implement**Steps followed by validation and refinement

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Traditional Flow Problems • Schematic Diagrams

– Limited descriptive power

• State Diagrams and Algorithmic State Machines

– Limited portability

– Limited complexity – Difficult to describe parallelism

– Limited complexity

Tedious and/or Repetitive Detail

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How about HDLs Instead of Diagrams?

• HDLs – Highly portable (text)

– Describes multiple levels of abstraction

– Represents parallelism

– Provides many descriptive styles• Structural • Register Transfer Level (RTL)• Behavioral

– Serve as input for synthesis

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How about Synthesis instead of Manual Design?

• Increas ed de sign e ffic iency

• Reduces verification/validation problem

• Ability to explore more of overall design space

• Are there disadvantages?

• Potential for better optimization

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Refine to RTL(HDL)*

Physical Design*

Synthesis Design FlowWrite

Specifications

Define Architec-ture (HDL)*

Implement*

*Steps followed by validation and refinement

Partition Arch-itecture

SynthesizeRTL*

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SynthesizeRTL*

Contemporary Design FlowWrite

Specifications

Define Architec-ture (HDL)*

Implement*

*Steps followed by validation and refinement

Partition Arch-itecture

Refine to RTL*

PreliminaryPhysical Design

Select IPCores

Physical Design*

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Newer technologies and design flows

• System on Chip (SOC)– Designers are provided with IP cores– 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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Summary– Course Conduct – Be familiar with it– Application Specific Integrated Circuit (ASIC)

Technologies – provides a basis for understanding what we are designing

– IC Costs – Gives a basis for technology selection– ASIC Design Flows

• The role of HDLs and synthesis• Provides a structure for what we are to learn

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References

1) Smith, Michael J. S., Application-Specific Integrated Circuits, Addison-Wesley, 1997.

2) For layout refer to texts for ECE 555 and ECE 755