Last Edit July 22, 2001
From the mid-1960s, there are small-scale integration (SSI) gates: NAND,
NOR, EXOR, and NOT or INVERT. SSI can be defined to be about 2-10 gates
on a single chip. Anything can be built from SSI, but the design time,
power, and size make this approach obsolete for designs that must be built
quickly and in quantity. Custom design at the transistor and resistor
level is reserved for special projects.
From the early 1970s there are larger blocks, medium-scale integration
(MSI): registers, decoders, multiplexors, counters, adders, comparators,
etc. MSI is loosely defined as approximately 20-100 gates. MSI allows
more modular designs, speeding the design process when the blocks could
In the late 1970s arithmetic logic units (ALUs) with on-board registers,
microprogrammable sequencers and interrupt controllers in a bit-slice
format became available. Memory chips (ROM, PROM, RAM) in increasing sizes
became readily available. Large-scale integration (LSI) culminated in
the one-chip microprocessors.
LSI is loosely defined as approximately 200-1000+ gates. Very large scale
integration (VLSI) has reached 20,000 gates and higher. LSI and VLSI further
increase the modular block size, reducing design time, space, and power
considerations and increasing reliability as connections are moved inside
the components. Many LSI and VLSI blocks are designed by their manufacturers
and referred to as fixed-instruction-set modules.
For any given design, if the architecture of the fixed LSI and VLSI blocks
suit the application then the design time is considerably shortened. When
a one-chip microprocessor is not quite suitable, microprogrammable architectures
can often provide sufficient customization.
Microprogrammable architectures, such as bit-slice, allow a closer control
over the architecture but not total control. The basic building blocks
are still designed by the chip manufacturer for generic applications.
Bit-slice architectures include interruptable sequencers and 32-bit ALUs.
The customization of the bit-slice modules to an application is done
through customer-designed module interconnection, the implemented commands
and their sequences. The commands or instruction set is called the micro-program
for the design.
The 1980s saw the acceptance of ASICs ( application specific integrated
circuits), VLSI devices large enough to allow designers to implement architectures
that were suited to solving the design problem rather than forcing one
architecture to solve everything. It was the natural extension to the
bit-slice architectures, where some control of architecture was possible
through microprogramming but where the basic building blocks were fixed
designs. The application-specific customization of the design solution
allows the designer to have the creative power of a gate-level breadboard
design while keeping the production advantages of VLSI.
Not far behind the ASIC and ASIC developments, multimedia and design
integration saw a need to incorporate analog functions into digital systems.
For years the trend had been away from analog design as a chosen career
and now there was a shortage of design engineers. First came massive re-training
of internal staff as companies struggled to cope. Then came the creation
of Electrically Programmable Analog Circuit (EPAC) and related devices.
Now designers are coping with 8-12 inch wafers, 1 million gate chips,
a deep submicron technologies with a shrinking design time window. For
example, the next-generation Pentium chips are mandated to be first-time
silicon success. The first took four tapeouts to achieve success.
Table 2 Integration Sizing Terminology
|| small scale integration where a few gates were lumped together
as a means of improving the design and the design process,
|| medium scale integration when more gates were packed together in
a single chip for the same reasons,
|| large scale integration when functional blocks could be contained
on a chip,
|| very large scale integration and its various offshoots (VHLSI,
etc.) where larger functional blocks and their related circuitry could
be brought together in lower power, faster chips.
|| application-specific integrated circuit
|| application specific standard product
|| Electrically Programmable Analog Circuit
|| arithmetic-logic unit
|| central processor unit
||Very Deep SubMicron
||Intellectual Property - precoded functional block for design re-use