Vlsi design 11

MVLSI
Design
MR. HIMANSHU DIWAKAR
Assistant Professor
JETGI
MR. HIMANSHU DIWAKAR JETGI 1

Intel processor

Think about it…
On previous slide we see a picture that is the processor of
computer and the image is from Intel lab. The zooming size is
of scale of rabies virus, that’s the technology what we are
dealing today.

Basics of
Very Large Scale Integration
(VLSI)
CMOS Digital Integrated circuits – Analysis and Design by Sung – Mo
Kang, Yusuf Leblebici, TATA McGraw-Hill Pub. Company Ltd.

Brief History and Introduction
Fig.1.1.Invention of transistor by John Bardeen, William shockely and
Walter (1947) at Bell Lab 23rd Dec

Introduction…
• MICROPROCESSORS are essential to many of products we use every day
such as TVs, cars, radios , home appliances and of course computers.
• Transistors are the main components of microprocessors.
• At their most basic level, transistors may seem simple. But their
development actually requires many years of painstaking research. Before
processors transistors computers relied on slow, inefficient vacuum tubes
and mechanical switches to process information. In 1958, engineers
managed to put two transistors onto a single crystal and create the first
integrated circuit, which subsequently led to the first microprocessors.

Moore's law
"Moore's law" is the
observation that, over the
history of computing
hardware, the number of
transistors in a dense
integrated circuit has doubled
approximately every two
years.

Evolution of logic complexity in integrated circuit
Era Date Complexity
Single transistor 1958 <1
Unit logic (one gate) 1960 1
Multifunction 1962 2-4
Complex function (SSI) 1964 5-20
Medium scale integration (MSI) 1967 20-200
Large scale integration (LSI) 1972 200-2,000
Very large scale integration (VLSI) 1978 2,000-20,000
Ultra large scale integration (ULSI) 1989 >20,000

Limitations of Moore's law
 Growth expect until
30nm gate length
(currently: 45nm)
 3D shift design needed
in fabrication process
3D shift design in VLSI

Cont’d
• Higher the growth in VLSI technology needs some structured design
methodology for economically viable VLSI products, in timely
manner.
• Logic chips (microprocessors) have more complex design compare to
memory chips. The complexity is also increased exponentially for
logic chips.
• This increase in design cycle time of chips.
• This time is strongly dependent on the efficiency of the design
methodologies as well as on design style.
• As shown in upcoming figure, two different design styles are
compared for their relative merits and demerits.

Cont’d
• Full custom design style (where the geometry and the placement
of every transistor can be optimized individually) requires a
longer time until design maturity can be reached. But the final
product have the higher level of performance.
• In semicustom design style (standard cell based or FPGA) will
allow a shorter design time until design maturity can be achieved.
• The choice is of design style depends on performance
requirements of the VLSI product.

Overview of VLSI design methodology

• Approximately every two years, a new generation of Technology is
introduced.
• This may require that the level of logic integration and chip performance
fall short of the level achievable with the current processing technology.(see
upcoming figure)
• It can be seen that the design cycle time of a successful VLSI product is
kept shorter than what would be necessary for developing an optimum
performance.
• The use of CAD tools methodologies are also essential for reducing the
design cycle time for managing the increasing design complexity.

VLSI design flow
• The Y-chart (first introduced by D. Gajski) shown in Fig. 1.4
illustrates a design flow for most logic chips, using design activities on
three different axes (domains) which resemble the letter Y.
• The Y-chart consists of three major domains, namely:
1. behavioral domain,
2. structural domain,
3. geometrical layout domain.

VLSI design flow
Figure-1.4: Typical VLSI design flow in three domains (Y-chart representation).

VLSI design flow
• The design flow starts from the algorithm that describes the behavior
of the target chip.
• The corresponding architecture of the processor is first defined.
• It is mapped onto the chip surface by floor planning.
• The next design evolution in the behavioral domain defines finite state
machines (FSMs) which are structurally implemented with functional
modules such as registers and arithmetic logic units (ALUs).
• These modules are then geometrically placed onto the chip surface
using CAD tools for automatic module placement followed by routing,
with a goal of minimizing the interconnects area and signal delays.

VLSI design flow
• Figure-5 provides a more simplified view of the VLSI design flow,
taking into account the various representations, or abstractions of
design - behavioral, logic, circuit and mask layout.
• Note that the verification of design plays a very important role in
every step during this process. The failure to properly verify a design
in its early phases typically causes significant and expensive re-design
at a later stage, which ultimately increases the time-to-market.

VLSI design flow
Figure-5: A
more simplified
view of VLSI
design flow.
Functional verification
System specifications
Functional
(Architecture) Design
Logic design
Logic verification
Circuit Design
Circuit verification
Physical Design
Layout verification
Fabrication and
testing
Behavioral
Representation
Behavioral (Gate
level) Representation
Circuit
Representation
Layout
Representation
VLSI Design
Flow

VLSI design flow
• Although the design process has been described in linear fashion for
simplicity, in reality there are many iterations back and forth,
especially between any two neighboring steps, and occasionally even
remotely separated pairs.
• In the following, we will examine design methodologies and
structured approaches which have been developed over the years to
deal with both complex hardware and software projects.
• Some of the classical techniques for reducing the complexity of IC
design are: Hierarchy, regularity, modularity and locality.

Concept of regularity, Modularity, and
Locality
• The hierarchical design approach reduces the design complexity by
dividing the large system into several sub-modules.
• Regularity means that the hierarchical decomposition of a large system
should result in not only simple, but also similar blocks, as much as
possible.
• A good example of regularity is the design of array structures
consisting of identical cells - such as a parallel multiplication array.
• Regularity can exist at all levels of abstraction: At the transistor level,
uniformly sized transistors simplify the design. At the logic level,
identical gate structures can be used etc.

Concept of regularity, Modularity, and Locality
• Figure 1.11 shows regular circuit-level designs of a 2-1 MUX
(multiplexer), an D-type edge-triggered flip flop, and a one-bit full
adder.
• Note that all of these circuits were designed by using inverters and tri-
state buffers only. If the designer has a small library of well-defined
and well-characterized basic building blocks, a number of different
functions can be constructed by using this principle.
• Regularity usually reduces the number of different modules that need
to be designed and verified, at all levels of abstraction.

Regularity
• Figure 1.11 shows regular circuit-level designs of a 2-1 MUX
(multiplexer), an D-type edge-triggered flip flop, and a one-bit full
adder.
• Note that all of these circuits were designed by using inverters and tri-
state buffers only. If the designer has a small library of well-defined
and well-characterized basic building blocks, a number of different
functions can be constructed by using this principle.
• Regularity usually reduces the number of different modules that need
to be designed and verified, at all levels of abstraction.

Regularity
Figure-
1.11: Regular
design of a 2-1
MUX, a DFF and
an adder, using
inverters and tri-
state buffers.

Modularity
• Modularity in design means that the various functional blocks which
make up the larger system must have well-defined functions and
interfaces.
• Modularity allows that each block or module can be designed
relatively independently from each other, since there is no ambiguity
about the function and the signal interface of these blocks.
• All of the blocks can be combined with ease at the end of the design
process, to form the large system.

Locality
• The concept of modularity enables the parallelization of the design
process. It also allows the use of generic modules in various designs -
the well-defined functionality and signal interface allow plug-and-play
design.
• By defining well-characterized interfaces for each module in the
system, we effectively ensure that the internals of each module
become unimportant to the exterior modules.
• Internal details remain at the local level. The concept of locality also
ensures that connections are mostly between neighboring modules,
avoiding long-distance connections as much as possible.

Locality
• This last point is extremely important for avoiding excessive
interconnect delays. Time-critical operations should be performed
locally, without the need to access distant modules or signals. If
necessary, the replication of some logic may solve this problem in
large system architectures.

Thank you

Vlsi design 11

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