Most of the students of Electronics Engineering are exposed to Integrated Circuits
(IC's) at a very basic level, involving SSI (small scale integration)
circuits like logic gates or MSI (medium scale integration) circuits like
multiplexers, parity encoders etc. But there is a lot bigger world out
there involving miniaturisation at levels so great, that a micrometer and a
microsecond are literally considered huge! This is the world of VLSI - Very
Large Scale Integration. The article aims at trying to introduce
Electronics Engineering students to the possibilities and the work involved
in this field.
What is VLSI?
VLSI stands for "Very Large Scale Integration". This is the field which
involves packing more and more logic devices into smaller and smaller
areas.Thanks to VLSI, circuits that would have taken boardfuls of space can now
be put into a small space few millimeters across! This has opened up a big
opportunity to do things that were not possible before. VLSI circuits are
everywhere ... your computer, your car, your brand new state-of-the-art digital
camera, the cell-phones, and what have you. All this involves a lot of
expertise on many fronts within the same field, which we will look at in later
VLSI has been around for a long time, there is nothing new about it ... but as
a side effect of advances in the world of computers, there has been a dramatic
proliferation of tools that can be used to design VLSI circuits. Alongside,
obeying Moore's law, the capability of an IC has increased exponentially over
the years, in terms of computation power, utilisation of available area, yield.
The combined effect of these two advances is that people can now put diverse
functionality into the IC's, opening up new frontiers. Examples are embedded
systems, where intelligent devices are put inside everyday objects, and
ubiquitous computing where small computing devices proliferate to such an
extent that even the shoes you wear may actually do something useful like
monitoring your heartbeats! These two fields are kinda related, and getting
into their description can easily lead to another article.
DEALING WITH VLSI CIRCUITS
Digital VLSI circuits are predominantly CMOS based. The way normal blocks like
latches and gates are implemented is different from what students have seen so
far, but the behaviour remains the same. All the miniaturisation involves new
things to consider. A lot of thought has to go into actual implementations as
well as design. Let us look at some of the factors involved ...
1. Circuit Delays. Large complicated circuits running at very high frequencies
have one big problem to tackle - the problem of delays in propagation of
signals through gates and wires ... even for areas a few micrometers
across! The operation speed is so large that as the delays add up, they can
actually become comparable to the clock speeds.
2. Power. Another effect of high operation frequencies is increased
consumption of power. This has two-fold effect - devices consume batteries
faster, and heat dissipation increases. Coupled with the fact that surface
areas have decreased, heat poses a major threat to the stability of the
3. Layout. Laying out the circuit components is task common to all branches of
electronics. Whats so special in our case is that there are many possible
ways to do this; there can be multiple layers of different materials on the
same silicon, there can be different arrangements of the smaller parts for
the same component and so on.
The power dissipation and speed in a circuit present a trade-off; if we try to
optimise on one, the other is affected. The choice between the two is
determined by the way we chose the layout the circuit components. Layout can
also affect the fabrication of VLSI chips, making it either easy or difficult
to implement the components on the silicon.
THE VLSI DESIGN PROCESS
A typical digital design flow is as follows:
Tape Out to Foundry to get end product….a wafer with repeated number of identical Ics.
All modern digital designs start with a designer writing a hardware description of the IC (using HDL or Hardware Description Language) in Verilog/VHDL. A Verilog or VHDL program essentially describes the hardware (logic gates, Flip-Flops, counters etc) and the interconnect of the circuit blocks and the functionality. Various CAD tools are available to synthesize a circuit based on the HDL. The most widely used synthesis tools come from two CAD companies. Synposys and Cadence.
Without going into details, we can say that the VHDL, can be called as the "C"
of the VLSI industry. VHDL stands for "VHSIC Hardware Definition Language",
where VHSIC stands for "Very High Speed Integrated Circuit". This languages is
used to design the circuits at a high-level, in two ways. It can either be a
behavioural description, which describes what the circuit is supposed to do, or
a structural description, which describes what the circuit is made of. There
are other languages for describing circuits, such as Verilog, which work in a
Both forms of description are then used to generate a very low-level
description that actually spells out how all this is to be fabricated on the
silicon chips. This will result in the manufacture of the intended IC.
A typical analog design flow is as follows:
In case of analog design, the flow changes somewhat.
Parametric Extraction / Back Annotation
Tape Out to foundry.
While digital design is highly automated now, very small portion of analog design can be automated. There is a hardware description language called AHDL but is not widely used as it does not accurately give us the behavioral model of the circuit because of the complexity of the effects of parasitic on the analog behavior of the circuit. Many analog chips are what are termed as “flat” or non-hierarchical designs. This is true for small transistor count chips such as an operational amplifier, or a filter or a power management chip. For more complex analog chips such as data converters, the design is done at a transistor level, building up to a cell level, then a block level and then integrated at a chip level. Not many CAD tools are available for analog design even today and thus analog design remains a difficult art. SPICE remains the most useful simulation tool for analog as well as digital design.
MOST OF TODAY’S VLSI DESIGNS ARE CLASSIFIED INTO THREE CATEGORIES:
Small transistor count precision circuits such as Amplifiers, Data converters, filters, Phase Locked Loops, Sensors etc.
2. ASICS or Application Specific Integrated Circuits:
Progress in the fabrication of IC's has enabled us to create fast and powerful
circuits in smaller and smaller devices. This also means that we can pack a lot
more of functionality into the same area. The biggest application of this
ability is found in the design of ASIC's. These are IC's that are created for
specific purposes - each device is created to do a particular job, and do it
well. The most common application area for this is DSP - signal filters, image
compression, etc. To go to extremes, consider the fact that the digital
wristwatch normally consists of a single IC doing all the time-keeping jobs as
well as extra features like games, calendar, etc.
3. SoC or Systems on a chip:
These are highly complex mixed signal circuits (digital and analog all on the same chip).
A network processor chip or a wireless radio chip is an example of an SoC.