ECE 551: Digital System
Design & Synthesis
Spring 2003
Lecture Materials Prepared
by: Charles Kime, Kewal
Saluja and Michael Schulte
1
ECE 551: Digital System
Design & Synthesis
Lecture
Set 1:
oIntroduction
oOverview of Contemporary Digital
Design
oPragmatics 1
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ECE 551 - Digital System Design & Synthesis
Lecture 1.1 - Introduction
 Overview
o Course
o Course
o Course
o Course
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Purpose
Topics
Tools
Info
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Course Purpose


To provide knowledge and experience in performing
contemporary logic design based on
o Hardware description languages (HDLs)
o HDL simulation
o Automated logic synthesis
o Timing analysis
With consideration for
o Practical design and test issues
o Chip layout issues
o Design reuse for system-on-a-chip (SoC)
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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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Course Tools
 Modelsim
HDL Simulation Tools (Mentor)
 Design Analyzer Synthesis Tools
(Synopsys)
 G11 Technology Library (LSI Logic)
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Course Information
 Course
Conduct
 Standard Reference
 The above plus all other course
material can be found at
http://courses.engr.wisc.edu/ecow/ge
t/ece/551/kime/
 Be familiar with all!
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Lecture 1.2 – Contemporary
Digital Design
 Overview
o Layout Lite
o Application Specific Integrated Circuit (ASIC)
Technologies
o IC Costs
o ASIC Design Flows
 The Role of HDLs and Synthesis
 The Role of IP Cores and Reuse
 The Role of Physical Design
o Summary
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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:
Metal 3
Oxide
Metal 2
Metal 1
Transistor
Polysilicon
Diffusion
Substrate
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Channel
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Layout Lite - 2

The layout corresponding to the cross-section:
Transistor
Channel
o The transistor is outlined in broad yellow lines.
o Everything else is interconnect.
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IC Implementation
Technologies
STANDARD
IC
ASIC
FULL
CUSTOM
STANDARD
CELL
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SEMICUSTOM
GATE ARRAY,
SEA OF GATES
FIELD
PROGRAMMABLE
FPGA
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PLD
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Distinguishing Features of IC
Technologies - 1
 Implementation
technologies are
distinguished by:
o The levels of the layout 1) transistors and 2)
interconnect that are:
 Common to distinct IC designs (L1)
 Different for distinct IC designs (L2)
o The use of predesigned layout cells
 Predesigned cells are used (P1)
 Predesigned cells are not used (P2)
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Distinguishing Features of IC
Technologies - 2
 Implementation
technologies are
distinguished by:
o Mechanism used for instantiating
distinct IC designs:
 Metallization (M)
 Fuses or Antifuses (F)
 Stored Charge (C)
 Static Storage (R)
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Technologies in Terms of
Distinguishing Features - 1
 Full
Custom – P2, M
o Transistors – L2, Interconnects – L2
 Standard
Cell – P1, M
o Transistors – L2, Interconnects – L2
 Gate
Array, Sea of Gates – P1, M
o Transistors – L1, Interconnects – L2
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Technologies in Terms of
Distinguishing Features - 2
 FPGA
– P1, F or R
o Transistors – L1, Interconnects – L1
 PLD
– P1, F or C
o Transistors – L1, Interconnects – L1
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Technologies in Terms of
Shared Fabrication Steps

Custom Fabricated Layers
o Full Custom and Standard Cells – all layers are custom
fabricated
o Gate Arrays and Sea of Gates – only interconnect
(metallization) layers custom fabricated
o FPGAs and PLDs – nothing is custom fabricated

Consequences due to economy-of-scale:
o Fab costs reduced for Gate Arrays and Sea of Gates
o Fab costs further reduced for FPGAs and PLDs
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Layout Styles - 1
 Technologies
in terms of layout styles:
Standard Cell
Adjustable Spacing
…
Megacells
Gate Array - Channeled
Fixed Spacing
…
Base Cell
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Layout Styles - 2
 Technologies
in terms of layout styles:
Gate Array - Channel-less
…
(Sea of Gates)
Base Cell
Gate Array - Structured
Fixed Embedded Block
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…
…
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IC Costs - 1
 An
example: 10,000 gate circuit [1]
o Fixed costs
 Standard Cell - $146,000
 Gate Array - $86,000
 FPGA - $21,800
o Variable costs
 Standard Cell - $8 per IC
 Gate Array - $10 per IC
 FPGA - $39 per IC
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IC Costs - 2
 An
example: 10,000 gate circuit
4,000,000
3,500,000
Standard
Cell
Gate
Array
FPGA
3,000,000
2,500,000
2,000,000
1,500,000
1,000,000
500,000
0
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100
1000
10000
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100000
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IC Costs – 3

Why isn’t FPGA cheaper per unit due to
economy-of-scale?
o The chip area required by each of the successive
technologies from Full Custom to FPGAs increases for
a fixed-sized design.
o The larger the chip area, the poorer the yield of
working chips during fabrication
o Also, due to increased sales, FPGA prices have
declined since the mid-90’s much faster than the
other technologies.
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ASIC Design Flow - Traditional
Write
Specifications
Draw Datapath
Schematics *
Define System
Architecture
Define State
Diag/Tables
Partition - Datapath &Control
Draw Control
Schematics *
Do Physical
Design*
*Steps followed by validation
Implement*
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and refinement
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Integrate
Design*
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Traditional Flow Problems

Schematic Diagrams
o Limited descriptive power
o Limited portability
o Limited complexity
 State Diagrams and Algorithmic State
Machines
o Limited complexity
o Difficult to describe parallelism
 Time-Intensive and Hard to Update
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How about HDLs Instead
of Diagrams? - 1

Hardware description languages (HDLs)
o Computer-based programming languages
o Model and simulate the functional behavior and
timing of digital hardware
o Synthesizable into a technology-specific netlist

Two main HDLs used by industry
o Verilog HDL (C-based, industry-driven)
o VHSIC HDL or VHDL (Ada-based,
defense/industry/university-driven).
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How about HDLs Instead
of Diagrams? - 2
 Advantages
of HDLs
o Highly portable (text)
o Describes multiple levels of abstraction
o Represents parallelism
o Provides many descriptive styles
 Structural
 Register Transfer Level (RTL)
 Behavioral
o Serve as input ECE
for551synthesis
Spring 2003
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How about Synthesis
instead of Manual Design?
 Increased
 Potential
 Ability
space
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for better optimization
to explore more of overall design
 Reduces
 Are
design efficiency
verification/validation problem
there disadvantages?
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HDL/Synthesis Design Flow - 1
Design
Specification
Design
Partition
Design Entry:
HDL Behavioral
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Verification:
Functional
Pre-Synthesis
Sign-Off
Integration
Synthesis and
Technology Map
Verification:
Functional
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To next page
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HDL/Synthesis Design Flow - 2
From prior page
Verification:
Post-Synthesis
Timing Verification:
Post-Synthesis
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Test Generation
& Fault Simulation
Extract
Parasitics
Physical Design
Verification:
Physical &
Electrical
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Design Sign-Off
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An Example from Industry

A G3 wireless processor was designed using the
following methodology:
o Entire processor modeled and tested using VHDL and
C-based test programs
o Processor functionality verified by synthesizing to an
FPGA and running 3G wireless applications at 25 MHz
o Processor timing and design feasibility verified by
synthesizing to a standard cell library and running
applications at 500 MHz.
o Final version of processor implemented using a mix
of standard cell and custom logic to achieve lowpower and 800 MHz clock speed.
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Newer Technologies and
Design Flows - SOC
System-on-a-Chip (SoC)
o Designers 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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ECE 551 Spring 2003

Contemporary Design Flow - 1
Design
Specification
Design
Partition
Select IP
Cores
Preliminary
Phys. Design
Design Entry:
HDL Behavioral
Verification:
Functional
Integration &
Verification:
Functional
Pre-Synthesis
Sign-Off
Synthesis and
Technology Map
To HDL/Synth Design Flow -2
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Lecture 1.2 Summary

Application Specific Integrated Circuit (ASIC)
Technologies
o Provides a basis for what we will design

IC Costs
o Gives a basis for technology selection

ASIC Design Flows
o Shows the role of HDLs and synthesis
o Provides a structure for
 what we will learn
 What we will do
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References
1)
Smith, Michael J. S., Application-Specific
Integrated Circuits, Addison-Wesley,
1997.
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Lecture 1.3 Pragmatics 1
 Pragmatics
refers to practical design
choices and techniques
 Topics
o Cell Libraries
o Asynchronous Circuits
o Three-State Logic and Hi-Z State
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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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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:

o Small Scale: AND, OR, NAND, NOR, NOT, AOI, OAI,
Flip-Flops, Latches
o Medium Scale: Multiplexers, Decoders, Adders
o Large Scale: Memories, Processors

Provided by ASIC vendors
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What is a Cell Library?
A
database specifying and describing the
target technology in the form of predesigned objects called cells. Synthesis
target technology.
 In-Class Discussion: What are typical
components in the database for each cell?
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Asynchronous Techniques
 Delay-dependent
design
 Combinational hazards
 Combinational hazard prevention
 Asynchronous design
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Delay-Dependent Design 1
LA
PA
A
LA
A
PA
Example: Level-to-Pulse Converter(Delay-Based)
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Delay-Dependent Design 2
 Sometimes
useful
 But should be avoided
 Time delays vary and so may:
o Fail
o Produce variable results, e. g. pulse length
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Delay-Dependent Design 3

Level to Pulse Converter (Synchronous)
LA
PA
D
Clock

C
Q
Level on LA must be longer than a clock
period and must not rise close to the positive
clock edge. Ideally, synchronous with Clock.
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Combinational Hazards 1
 Example
- Hazard in a Multiplexer
A
1
B
F
1
C
B
F
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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 :

o Single-input change (SIC)
o Multiple-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.
o Static 1-hazard – output value to remain at 1
o Static 0-hazard – output value to remain at 0
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Combinational Hazards - 3

Classification of Combinational Hazards
o Static – SIC/MIC – output changes when it should
remain fixed - output value within the “transition
region of input changes is fixed.
o Dynamic – SIC/MIC – output changes three or more
times when it should change only once.
o Essential – MIC – output changes when it should
remain fixed – output value within the “transition
region” of
input changes not fixed.
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Combination Hazards - 4
 In-class
Example: Illustration of static,
dynamic and essential hazards
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Combinational Hazards - 5

Consequences of Hazards
o Signals with hazards within or entering asynchronous
circuits (note that a flip-flop is an asynchronous
circuit with respect to its clock signal!)
o Cause incorrect state behavior
 Extra state changes
 Incorrect state changes

In-Class Example: Prevention of Hazards
o Redundant Logic
o Delay Dependence
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Asynchronous Design - 1

Which of the following sequential circuits involve
asynchronous design?
o A circuit that has no global clock signal involved in its
operation – state changes occur in response to input
changes only.
o A D flip-flop circuit
o A circuit using clock gating on flip-flop clock inputs
o A circuit with a clock which uses the clear and preset
inputs on the flip-flops for other than initialization.
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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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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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Three-State Conflicts - 1

What are they and what are their effects?
o Static – Chip damage
or static power consumption
o Dynamic – Dynamic or static
power consumption
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1
D0
1
E0
0
D1
1
E1
E0 1
E1 1
1
E0 0
E1 01
OUT
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Three-State Conflicts - 2

How can conflicts be avoided?
o Static – Decoded enable
signals
o Dynamic – Delay control
1
0
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1
E0 0
E1 01
D0
E0
1
E0 0
E1 1
OUT
D1
E1
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Floating Inputs and ThreeState Nets - 1

Floating input values on gates can cause:
o static power dissipation
o high-frequency switching that induces power supply
noise

Floating input values arise from:
o Gate inputs, e. g., for example on exterior of IC, that
are not connected
o Lines driven by 3-state buffer or gate outputs, all of
which are in the Hi-Z state.
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Floating Inputs and ThreeState Nets – 2
 How
can floating inputs and nets be
avoided?
o Use 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
o On internal lines, particularly buses, use a bus
keeper (weak buffer)
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Non-D flip-flops

D Flip-Flops
o Unique characteristic – the typical master-slave DFF
is also functionally an edge-triggered DFF.

Non- D Flip-Flops (JK, T, etc.)
o In the cell libraries, these flip-flop may be full-custom
designs or may simply consist of a DFF with added
logic.
o If 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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References
[1] Chris J. Myers, Asynchronous Circuit
Design, John Wiley & Sons, Inc., New
York, 2001.
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