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What is Programmable Logic?

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.


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.


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.