1
CAD
for Physical Design
of VLSI Circuits
2
Course Outline by Topics
• Circuit Partitioning
• Floorplanning
• Placement
• Global / Detailed Routing
• Technology Mapping in FPGA
• Interconnect Optimization
3
VLSI Design Cycle
System Specification
Circuit Design
Architectural Design
Physical Design
Functional Design
Fabrication
Logic Design
Packaging
4
VLSI Design Cycle
Netlist
System Specification
Physical
Design
Architectural
Design
Architectural
Specification
Functional
Design
Timing & relationship
between functional units
Logic
Design
RTL in HDL
Layout
Circuit Design
or
Logic Synthesis
Fabrication
Chips
Packaging
Packaged and
tested chips
5
VLSI Design Cycle
System Specification – Specification of the size, speed,
power and functionality of the VLSI system.
Architectural Design – Decisions on the architecture,
e.g., RISC/CISC, # of ALU’s, pipeline structure, cache
size, etc. Such decisions can provide an accurate
estimation of the system performance, die size, power
consumption, etc.
6
VLSI Design Cycle
Functional Design – Identify main functional units and
their interconnections. No details of implementation.
7
VLSI Design Cycle
Logic Design – Design the logic, e.g., boolean
expressions, control flow, word width, register allocation,
etc. The outcome is called an RTL (Register Transfer
Level) description. RTL is expressed in a HDL (Hardware
Description Language), e.g., VHDL and Verilog.
X = (AB+CD)(E+F)
Y= (A(B+C) + Z + D)
8
VLSI Design Cycle
Circuit Design – Design the circuit including gates,
transistors, interconnections, etc. The outcome is called
a netlist.
9
VLSI Design Cycle
Physical Design – Convert the netlist into a geometric
representation. The outcome is called a layout.
10
VLSI Design Cycle
Fabrication – Process includes lithography, polishing,
deposition, diffusion, etc. to produce a chip.
Packaging – Put together the chips on a PCB (Printed
Circuit Board) or an MCM (Multi-Chip Module)
11
Physical Design Cycle
Circuit Partitioning
Floorplanning & Placement
Routing
Layout Compaction
Extraction and Verification
12
Physical Design Cycle
Circuit Partitioning – Partition a large circuit into subcircuits (called blocks). Factors like #blocks, block sizes,
interconnection between blocks, etc. are considered.
1
3
2
13
Physical Design Cycle
Floorplanning – Set up a plan for a good layout. Place
the modules (modules can be IP blocks, functional units,
etc.) at an early stage when details like shape, area, I/O
pin positions of the modules, …, are not yet fixed.
Deadspace
14
Physical Design Cycle
Placement – Exact placement of the modules (modules
can be gates, standard cells, etc.) when details of the
module design are known. The goal is to minimize delay,
total area and interconnect cost.
Feedthrough
v
Standard cell type 1
Standard cell type 2
15
Physical Design Cycle
Routing – Complete the interconnections between
modules. Factors like critical path, clock skew, wire
spacing, etc. are considered. Include global routing and
detailed routing.
Feedthrough
v
Type 1 standard cel1
Type 2 standard cell
16
Physical Design Cycle
Compaction – Compress the layout from all directions to
minimize the total chip area.
Verification – Check the correctness of the layout. Include
DRC (Design Rule Checking), circuit extraction (generate a
circuit from the layout to compare with the original netlist),
performance verification (extract geometric information to
compute resistance, capacitance, delay, etc.)
17
Design Styles
• Full-Custom Design
• Standard Cell Design
• Gate Array Design
• Field Programmable Gate Array Design (FPGA)
… or mixtures of the above
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Full-Custom Design
• No rigid restrictions on layout.
• More compact design.
• Longer design time.
• Hierarchical: chip  clusters  units 
functional units.
19
Full Custom Design
20
Standard Cell Design
• Rectangular cells of the same height.
• Cell library (has 500 - 1200 cells).
• Cells placed in rows and space between rolls are called
channels for routing.
• Feedthroughs
21
Standard Cell Design
22
Gate Array Design
• Each chip is prefabricated with an array of identical gates
or cells.
• The chip is “customized” by fabricating routing layers on
top.
23
An Uncommitted Gate Array
24
A Committed Gate Array
25
Field Programmable Gate Array
• Chips are prefabricated with logic blocks and
interconnects.
• Logic and interconnects can be programmed (erased and
re-programmed) by users. No fabrication is needed.
• Interconnects are predefined wire segments of fixed
lengths with switches in between.
26
Field Programmable Gate Array
27
Trends in VLSI
• Transistor
• Smaller, faster, use less power
• Interconnect
• Less resistive, faster, longer (denser
design)
• Yield
• Smaller die size, higher yield
28
Chip Area
micron
29
Processor Performance
MIPS
1000,000
100,000
100,000 MIPS
10,000
1,000
100
10
1
0.1
Pentium Pro Processor
Pentium Processor
80486 Processor
80386 Processor
80286
8086
75 80 85
90 95
00 05
10
15
Source: Intel
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Transistor Count
K
1,000,000
100,000
10,000
1,000
100
10
1
1975 1980 1985 1990 1995 2000 2005 2010 2015
Projection
Source: Intel
31
Average Transistor Price
100
$
10
1
0.1
0.01
0.001
0.0001
0.00001
0.000001
68 70 72 74 76 78 80 82 84 86 88 90 92 94 96
Source: Intel
32
Technology Characteristics
Year
Technology
(m)
Density
(# transistors / cm2)
Chip size
(cm2)
Power
(W)
# Routing
Layers
1999
2001
2003
2006
2009
2012
0.18
0.15
0.13
0.1
0.07
0.05
6.2M 10M
18M
39M 84M 180M
3.40
4.30
5.20
3.85
1250 1500
6-7
7
6.20
7.50
2100 3500 6000 10000
7
7-8
8-9
9
33
Scaling
• The process of shrinking the layout in which every
dimension is reduced by a factor is called Scaling.
• Transistors become smaller, less resistive, faster,
conducting more electricity and using less power.
• Designs have smaller die sizes, higher yield and
increased performance.
34
Can Scaling Continue?
• Scaling work well in the past:
Year
1989
Technology
0.65
(m)
1992
1995
1997
1999
2001
0.5
0.35
0.25
0.18
0.15
• In order to keep scaling work in the future, many technical
problems need to be solved.
35
Can Scaling Continue?
• Some characteristics of transistors do not scale
uniformly, e.g., leakage current, threshold voltage, etc.
• Mismatch in scaling of transistors and interconnects.
Interconnect delay has increased from 5-10% of the
overall delay to 50-70%.
36
Roadmap
• International Technology Roadmap for Semi-conductors (ITRS)
• Projection of future technology requirements for the next 15
years.
Edition
1st
2nd
3rd
4th
5th
6th
Year of Publication
1992
1994
1997
1999
2001
2003
http://public.itrs.net
37
These trends have brought many
changes and new challenges to circuit
design.
38
Complicated Design
Too many transistors and no way to handle them
manually.
Solutions:
• CAD
• Hierarchical design
• Design re-use
39
Power and Noise
Huge power consumption and heat dissipation
becomes a problem
Noise and cross talk.
Solutions:
• Better physical design
40
Interconnect Area
Too many interconnects
Solutions:
• More interconnect layers (made possible by ChemicalMechanical Polishing)
• CAD tools for 3-D routing
41
Metal Layers
42
Interconnect Delay
Interconnect delay becomes a dominating factor
in circuit performance
Solutions:
• Use copper wire
• Interconnect optimization in physical design, e.g., wire
sizing, buffer insertion, buffer sizing.
43
Interconnect Delay
40
35
Gate delay
Interconnect delay
30
25
20
15
10
5
0
0.65
1989
0.5
1992
0.35
1995
0.25
1998
0.18
2001
0.13
2004
0.1
2007
Source: SIA Roadmap 1997