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Commodity Logic: The Jellybeans Of The Digital World
T
HE VARIETY OF TRANSISTORS BORN IN THE ’50s FORESHADOWED
the logic industry of the ’60s and ’70s. As transistors entered the digital world by
forming discrete component flip-flops and gates in the mid-’50s, the necessity of
standardized building blocks became apparent. In the late ’50s, IBM
(www.ibm.com), Digital Equipment, and others created families of small plug-in
boards or modules based on discrete bipolar transistors. Each board or module contained the equivalent of a few gates, a flip-flop, a decoder, and so on. And then in
1958, Texas Instruments (www.ti.com) invented the IC, which was a logic circuit.
Integration improved, and in 1960 Fairchild Semiconductor (www.fairchildsemi.com) released its Micrologic resistor-transistor logic (RTL) family. By 1961, both
Fairchild and TI were making available off-the-shelf logic circuits. By 1964, a higher-speed logic structure, diode-transistor logic (DTL), was created to build a wide
array of logic functions.
Meanwhile, other companies developed a form of transistor-coupled logic. Better
known as transistor-transistor logic (TTL), it first commercially appeared in 1963
in the form of a logic building-block family developed by Sylvania—SUHL for Sylvania universal high-level logic. Many companies jumped on the bandwagon. Over
the next decade and a half, they created thousands of industry-standard circuits
based on TTL. For the highest-performance applications, Fairchild, Motorola
(www.motorola.com), and others also developed families of emitter-coupled logic
(ECL) circuits that reached multihundred-megahertz speeds.
While work in bipolar technology moved forward, researchers experimented with
metal-oxide structures. In the late ’50s, the MOSFET was born. With the development of MOS transistors, researchers took about five years to create proof-of-concept logic circuits. In 1962, RCA crafted a 16-transistor multipurpose logic circuit.
By 1965, companies could turn p-channel MOS technology into a production technology that could integrate about 1000 elements on a chip.
Not until the development of complementary metal-gate MOS technology in the
late ’60s did generic standard logic circuits start to appear. The RCA CD4000
series was among the first standard logic families based on CMOS. It offered
functional equivalents to many popular TTL functions at power-consumption
levels of about one-fifth to one-tenth those of the TTL parts, albeit at slightly lower operating frequencies.
But bipolar TTL still ruled the roost. Through the ’70s, TTL variations came at a
fast and furious pace. A high-noise-immunity version (HNIL) was developed,
such as TI’s 54/74 H series. Faster versions of TTL using Schottky diodes then
emerged. Called the 54/74 S series, these let the logic operate at speeds of well over
100 MHz. By the early ’80s, a veritable alphabet soup of logic family variants had
been released, and designers needed a scorecard to help determine which family best
fit each application. Operating voltage levels also started to shrink in the mid-’80s,
with a 3.3-V standard starting to build some momentum.
Despite all this diversity, a “subversive” trend began appearing in the form of programmable logic arrays. User programmable, the first generations of these chips
replaced five to 10 logic gates. As logic integration improved, these PLDs and gate
arrays were able to replace more and more standard logic functions. The logic fam-
46 ELECTRONIC DESIGN>JANUARY 7, 2002
Computer manufacturers created
standard discrete
transistor-based
logic modules for
use in mass-producing computers.
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Texas Instruments
developed the
first integrated
circuit. It opened
the door to complex logic functions on a chip.
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5
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6
2
TTL logic was created. Sylvania was
the first company
to release a commercial family of
devices. Texas
Instruments and
others rapidly
joined in.
Metal-gate CMOS
structures made
their first commercial appearance in
RCA’s CD4000 logic
family. Others like
Harris Semiconductor followed with
pin-compatible
parts.
The first commercial CMOS gate
array was introduced, signalling
a potential threat
to the commodity
logic business.
Researchers disclosed the first
metal-gate MOSFET
devices.
Fairchild Semiconductor introduced
resistor-transistor
logic circuits.
RCA unveiled a 16transistor circuit
built with MOSFETs.
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9
6
3
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5
Circuit complexity
for MOSFET-based
circuits hit 1000
elements.
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5
An explosion of
logic functions
took place as TTL
building blocks
became the workhorses of the
industry.
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5
User-programmable logic debuted
from Monolithic
Memories, and the
first 20-pin PALS
were released.
digital ICs<>STANDARD LOGIC
ilies had begun to turn into logic configuration patterns for PLDs or cell libraries
for gate arrays.
The overwhelming progression by ASICs to usurp commodity logic continued to
the point that by the mid-’90s, no new generic logic functions were being introduced. A packaging option called single-gate logic then appeared. In the TTL
days, designers typically purchased devices that actually contained multiple
instances of the desired function. For example, the 7400 was a quadruple twoinput NAND gate. Single-gate devices, housed in ultra-small surface-mount
packages, provided the perfect solution when only one of something was needed.
The shrinking feature sizes used in higher-performance processes let circuits
operate at higher clock frequencies,which in turn increased circuit power
demands. This forced IC designers to further reduce operating voltages.
Designers then faced the challenge of working with multiple signal levels in a
system—some 5-V legacy circuits, some 3.3-V logic, some 2.5-V, and now 1.8V ASICs, and soon the 1.2-V parts. The need had arisen for circuit families that
could provide level translation, bus latching, and buffering. Other support
functions for the complex microprocessors and system buses that evolved in the
late ’80s and ’90s created a need for level translators, bus drivers, latches, and so
forth. PLDs and ASICs were basically overkill for these applications.
Today, bus-interface logic is the “TTL” of the new millennium. Device propagation delays have shrunk from the 8 to 10 ns of the TTL days to the sub-3 ns
required by the 100-MHz and faster buses used in today’s systems. Future systems
will have faster buses with even lower voltages of as little as 0.9 V.This will demand
another generation of bus-interface circuits. Some of those buses, though, will be
serial rather than parallel, giving rise to another class of commodity circuit, the
serializer/deserializer (SERDES). It takes in parallel data on one end and delivers
a 2.5-Gbit/s data stream on the other end, or vice versa. Low-voltage differential
signaling (LVDS) has been employed for several years as a point-to-point connection, permitting data-transfer rates of up to 600 Mbits/s. Higher-speed and multidrop bus-oriented versions of LVDS are starting to appear.
A continued interest in single-gate packaged logic elements will extend into this
decade because these devices provide point solutions to designers in need of a gate
here, a flip-flop there, an inverter somewhere else, and so on.
A lower operating-voltage standard was proposed, but it ended up
making almost no
impact on the market
for another half-decade.
1976
1978
Gate arrays started to
take larger bites of general-purpose logic by
replacing hundreds of
chips with a single-chip
alternative.
As the need for lower-power
operation increased, companies
produce devices that operate at
3.3 V. Second generations of
CMOS commodity logic reduce
the performance penalty vs. TTL.
1979
1982
Commodity logic started
to take a backseat to
programmable logic as
PLDs grew in capability.
48 ELECTRONIC DESIGN>JANUARY 7, 2002
Operating-voltage standards added a 2.5-V
option to handle
the new generations
of VLSI ICs.
1986
1990
Commodity logic suppliers
started to refocus their product
lines, eliminating many logic
functions and concentrating on
bus-interface solutions.
Shorter propagation delays are in the wings
for bus-interface circuits. Today’s 2- to 3-ns delays will
shrink by close to 50% over the next 12 to 24
months, making possible buffers and translators that
operate at speeds exceeding 400 MHz.
Lower standby
power levels will
Higher-density
interface circuits
will pack more
buffers and latches
on a chip to reduce
package count on
boards.
become critical as
more systems go
portable. New circuits
will employ clock-gating and signal-sensing
schemes to minimize
power drain.
As operating voltages go down, signal
Expect program-
swings get smaller and
there’s a greater need for
LVDS schemes that can
handle data rates of 600
Mbits/s and higher.
mable logic devices
and ASICs to continue replacing
most general logic
functions.
Serial backplane drivers will become more popular as designers try to minimize bus widths and chip
pin counts. Today’s 2.5-Gbit/s SERDES chips will give
way to chips that can operate at 5 Gbits/s.
Expect a wider
variety of clock
buffering and distribution chips to
reduce signal skews
and improve system
performance.
More high-speed standard serial interface
choices will appear—
infiniband, hypertransport, and others to
meet the need for higher I/O bandwidth.
Look also for LVDS to work in multidrop
bus structures as opposed to point-to-point
connections.
Next-generation bus-interface logic cut propagation
delays to 4 to 5 ns. LVDS
interfaces offered highspeed serial replacements
for parallel buses.
1992
1996
Bus logic took over the
commodity logic market
with latches, buffers, translators, and other functions
that offered propagation
delays of 6 to 8 ns.
The latest generations
of bus-interface logic
trimmed propagation
delays to 2 to 3 ns.
2000
SERDES chips for
serial backplanes
pushed data rates
to 2.5 Gbits/s.
2001