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In the world of digital electronic systems, there are three
basic kinds of devices: memory, microprocessors, and logic. Memory
devices store random information such as the contents of a
spreadsheet or database. Microprocessors execute software
instructions to perform a wide variety of tasks such as running a
word processing program or video game. Logic devices provide
specific functions, including device-to-device interfacing, data
communication, signal processing, data display, timing and control
operations, and almost every other function a system must
perform.
Fixed Logic Versus Programmable Logic
Logic devices can be classified into two broad categories -
fixed and programmable. As the name suggests, the circuits in a
fixed logic device are permanent, they perform one function or set
of functions - once manufactured, they cannot be changed. On the
other hand, programmable logic devices (PLDs) are standard,
off-the-shelf parts that offer customers a wide range of logic
capacity, features, speed, and voltage characteristics - and these
devices can be changed at any time to perform any number of
functions.
With fixed logic devices, the time required to go from design,
to prototypes, to a final manufacturing run can take from several
months to more than a year, depending on the complexity of the
device. And, if the device does not work properly, or if the
requirements change, a new design must be developed. The up-front
work of designing and verifying fixed logic devices involves
substantial "non-recurring engineering" costs, or NRE. NRE
represents all the costs customers incur before the final fixed
logic device emerges from a silicon foundry, including engineering
resources, expensive software design tools, expensive
photolithography mask sets for manufacturing the various metal
layers of the chip, and the cost of initial prototype devices.
These NRE costs can run from a few hundred thousand to several
million dollars.
With programmable logic devices, designers use inexpensive
software tools to quickly develop, simulate, and test their
designs. Then, a design can be quickly programmed into a device,
and immediately tested in a live circuit. The PLD that is used for
this prototyping is the exact same PLD that will be used in the
final production of a piece of end equipment, such as a network
router, a DSL modem, a DVD player, or an automotive navigation
system. There are no NRE costs and the final design is completed
much faster than that of a custom, fixed logic device.
Another key benefit of using PLDs is that during the design
phase customers can change the circuitry as often as they want
until the design operates to their satisfaction. That's because
PLDs are based on re-writable memory technology - to change the
design, the device is simply reprogrammed. Once the design is
final, customers can go into immediate production by simply
programming as many PLDs as they need with the final software
design file.
CPLDs and FPGAs
The two major types of programmable logic devices are field
programmable gate arrays (FPGAs) and complex programmable logic
devices (CPLDs). Of the two, FPGAs offer the highest amount of
logic density, the most features, and the highest performance. The
largest FPGA now shipping, part of the Xilinx Virtex™ line of
devices, provides eight million "system gates" (the relative
density of logic). These advanced devices also offer features such
as built-in hardwired processors (such as the IBM Power PC),
substantial amounts of memory, clock management systems, and
support for many of the latest, very fast device-to-device
signaling technologies. FPGAs are used in a wide variety of
applications ranging from data processing and storage, to
instrumentation, telecommunications, and digital signal
processing.
CPLDs, by contrast, offer much smaller amounts of logic - up to
about 10,000 gates. But CPLDs offer very predictable timing
characteristics and are therefore ideal for critical control
applications. CPLDs such as the Xilinx CoolRunner™ series also
require extremely low amounts of power and are very inexpensive,
making them ideal for cost-sensitive, battery-operated, portable
applications such as mobile phones and digital handheld
assistants.
The PLD Market
Today the worldwide market for programmable logic devices is
about $3.5 billion, according the market researcher
Gartner/Dataquest. The market for fixed logic devices is about $12
billion. However, in recent years, sales of PLDs have outpaced
those of fixed logic devices built with older gate array
technology. And, high performance FPGAs are now beginning to take
market share from fixed logic devices made with the more advanced
standard cell technology.
According to the Semiconductor Industry Association,
programmable logic is now one of the fastest growing segments of
the semiconductor business, and for the last few years, sales for
PLDs have increased at a greater pace than sales for the overall
semiconductor industry.
Says EDN Magazine, a leading electronics design trade
publication: "Programmable-logic devices are the fastest growing
segment of the logic-device family for two fundamental reasons.
Their ever-increasing logic gate count per device 'gathers up'
functions that might otherwise spread over a number of
discrete-logic and memory chips, improving end-system size, power
consumption, performance, reliability, and cost. Equally important
is the fact that in a matter of seconds or minutes you can
configure and, in many cases, reconfigure these devices at your
workstation or in the system-assembly line. This capability
provides powerful flexibility to react to last-minute design
changes, to prototype ideas before implementation, and to meet
time-to-market deadlines driven by both customer need and
competitive pressures." (EDN, "Annual PLD Directory," August 17,
2000.)
The PLD Advantage
Fixed logic devices and PLDs both have their advantages. Fixed
logic devices, for example, are often more appropriate for large
volume applications because they can be mass-produced more
economically. For certain applications where the very highest
performance is required, fixed logic devices may also be the best
choice.
However, programmable logic devices offer a number of important
advantages over fixed logic devices, including:
- PLDs offer customers much more flexibility during the design
cycle because design iterations are simply a matter of changing the
programming file, and the results of design changes can be seen
immediately in working parts.
- PLDs do not require long lead times for prototypes or
production parts - the PLDs are already on a distributor's shelf
and ready for shipment.
- PLDs do not require customers to pay for large NRE costs and
purchase expensive mask sets - PLD suppliers incur those costs when
they design their programmable devices and are able to amortize
those costs over the multi-year lifespan of a given line of
PLDs.
- PLDs allow customers to order just the number of parts they
need, when they need them, allowing them to control inventory.
Customers who use fixed logic devices often end up with excess
inventory which must be scrapped, or if demand for their product
surges, they may be caught short of parts and face production
delays.
- PLDs can be reprogrammed even after a piece of equipment is
shipped to a customer. In fact, thanks to programmable logic
devices, a number of equipment manufacturers now tout the ability
to add new features or upgrade products that already are in the
field. To do this, they simply upload a new programming file to the
PLD, via the Internet, creating new hardware logic in the
system.
Over the last few years programmable logic suppliers have made
such phenomenal technical advances that PLDs are now seen as the
logic solution of choice from many designers. One reasons for this
is that PLD suppliers such as Xilinx are "fabless" companies;
instead of owning chip manufacturing foundries, Xilinx out sources
that job to partners like IBM Microelectronics and UMC, whose chief
occupation is making chips. This strategy allows Xilinx to focus on
designing new product architectures, software tools, and
intellectual property cores while having access to the most
advanced semiconductor process technologies. Advanced process
technologies help PLDs in a number of key areas: faster
performance, integration of more features, reduced power
consumption, and lower cost. Today Xilinx is producing programmable
logic devices on a state-of-the-art 0.13-micron low-k copper
process - one of the best in the industry.
Just a few years ago, for example, the largest FPGA was measured
in tens of thousands of system gates and operated at 40 MHz. Older
FPGAs also were relatively expensive, costing often more than $150
for the most advanced parts at the time. Today, however, FPGAs with
advanced features offer millions of gates of logic capacity,
operate at 300 MHz, can cost less than $10, and offer a new level
of integrated functions such as processors and memory.
Just as significant, PLDs now have a growing library of
intellectual property (IP) or cores - these are predefined and
tested software modules that customer can use to create system
functions instantly inside the PLD. Cores include everything from
complex digital signal processing algorithms and memory controllers
to bus interfaces and full-blown software-based microprocessors.
Such cores save customers a lot of time and expense --it would take
customers months to create these functions, further delaying a
product introduction.
Conclusion
The value of programmable logic has always been its ability to
shorten development cycles for electronic equipment manufacturers
and help them get their product to market faster. As PLD suppliers
continue to integrate more functions inside their devices, reduce
costs, and increase the availability of time-saving IP cores,
programmable logic is certain to expand its popularity with digital
designers.
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