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The Methodology of Random Logic Design
The 4004 was mostly a
random logic circuit, a different type of integrated circuit than the memory
circuits which use only a few circuit types repeated over and over again. At
Intel – a memory company -- there was no expertise in random logic design.
Furthermore, the silicon gate technology required a different design and layout
strategy than aluminum-gate MOS for random logic circuits.
In 1970, the only two companies with the silicon gate process were Fairchild and Intel. Fairchild
design engineers were resisting the use of the silicon gate process on account
that they couldn’t make bootstrap load devices, and the area taken by random
logic circuits was about the same as with metal gate, despite Faggin claims to
the contrary. Faggin found that the reason for silicon gate not living up to its
potential was that the engineers were translating mindlessly the same old,
aluminum-gate circuits in silicon-gate clothes, without the necessary
rethinking. Silicon gate was different enough that old tricks didn’t work
anymore and new tricks were required. Months before leaving Fairchild, he also
found a way to make bootstrap-load devices, eliminating all objections to the
use of the silicon gate technology.
Faggin’s familiarity with silicon gate technology, computer architecture, logic and circuit design gave
him a unique vantage point to create a methodology with the best balance between
the various factors. His methodology started with minimizing the number of
translations necessary to design the chip, in order to achieve the most
effective design possible (fastest, lowest power dissipation and smallest chip
area).
In 1970, the typical design
flow consisted of making the logic design first, followed by the circuit design
and finally by the layout. At each step a translation was made; from the
language of logic gates, with its symbols, to the language of circuits made with
individual transistors and using different symbols; and then, from transistors
and their interconnections, to the geometrical patterns of the layout. People
with different skills were normally used at each step, therefore the synergy
between the different phases of the design was not fully realized. Faggin
combined the logic, circuit design and key elements of the layout topology, like
signal-line order and gate positions, into a single blueprint. He used a number
of standardized circuit types with methods to rapidly determine the transistor
dimensions based on load factors that could be determined with reasonable
precision from the blueprint itself. The layout started with a grid of silicon
and aluminum lines carrying the key signals and the circuits were “tucked”
underneath, reflecting as much as possible the spatial location in the
blueprint.
Faggin applied his methodology also to the 8008, the world’s first 8-bit
microprocessor. The 8008, whose architecture originated by Computer Terminal
Corp., was designed both at Intel, with silicon gate, and at Texas Instrument
with aluminum gate. The chip area of the TI version , which never worked, was
2.5 times larger than the area of Intel’s version, clearly demonstrating the
superior layout density of the silicon gate technology combined with Faggin’s
methodology. Faggin used the same methodology also for the design of the
8080 microprocessor.
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