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
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
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
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
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
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.
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.