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Picking the best solution for glue logic

Posted: 01 Jun 2004 ?? ?Print Version ?Bookmark and Share

Keywords:logic? pld? asic? asp? ic?

Bell Labs developed the transistor - glue logic's first breath of life - in December of 1947. From that point on, many companies have ventured into the mysterious process of creating useful equipment with discrete silicon logic. Companies such as Univac, Texas Instruments, Motorola, IBM and Fairchild manufactured and put to use logic gates to create everything from games and radios to satellites and supercomputers. Many companies had their hand in discrete logic, but Texas Instruments is most noted for the TTL 74XX family in 1962 to support NASA's lunar-landing and space exploration programs. This family included simple discrete logic gates that replaced vacuum tube technology.

In 1968, Intel introduced the first 1K RAM device. Gordon Moore predicted the exponential growth (biannual doubling) in 1965 which still holds true today. These advances in process technology helped discrete logic component fit into smaller packages and improved the number of discrete devices that could fit into a single package. This, in turn, enables designers to minimize board space and the number of components as well as providing them with the basic building blocks of early electronic design. Improved TTL variations were driven by growing needs of diverse applications in the years to follow. This was like opening the floodgates for new families of discrete logic. New applications drove manufacturers to continually introduce wider ranges of voltage, power and continuing cost reductions. The result was more discrete components that spawned a whole generation of system designers.

Through the 1970s, memory devices along with discrete logic variations came at a fast and furious pace. Proliferation of multiple 7400 series families and specialized product lines that implemented specific functionality came with advancements in semiconductor processing equipment. By the early 1980s, a multitude of logic families were introduced. With so many different selections, most logic vendors have product matrix to avoid confusing designers with respect to core voltage, I/O tolerance and speed. Also, design engineers have to be aware that some of these logic devices may become obsolete due to process technology migration and shear number of users. If a logic family does not gain acceptance, it may go away faster than planned.

Where discrete logic is used

In its infancy, discrete logic was used to build the heart of complex systems. When Zilog, Motorola and Intel introduced MCUs in the early 1970s, discrete logic was used in the periphery of complex systems. As time progressed, discrete logic became known as glue logic. The name implies its use today - logic used to interconnect different parts of a system together.

Although this may not be as glorious as its beginnings, discrete logic forms an integral part of most designs. With the diverse segments of semiconductor usage, discrete logic serves a vital part of varying business types. Telecom, datacom, wireless, portable and medical devices function as a bridge between any type of electronic logic signaling. Even in the most complex systems, there always seems to be a need for some simple logic function.

Manufacturers of discrete logic continue to implement new features in the same base logic functions to continue entice market segments. Multiple-technology sectors give discrete logic room to grow. They now include backplane logic, boundary scan, transceivers, voltage level and logic translation. There are even specialized logic devices built to suit specific functions such as frequency dividers, PLLs, comparators, ALUs and specific bus protocol accessories. But functionality is just the beginning; they can also be selected by package variation, speed, voltage levels, temperature ranges and process technology.

As with most system designs, there are multiple ways to implement logic functions. Discrete logic is just one way to solve a design task. Some designers like to use MCUs due to the sequential nature of their operation. Some prefer logic implementations due to its parallelism and high speeds. But when all things are equal, the end solution can be achieved in many different forms.

Trade-offs can include size, speed or past experience. It could be a corporate decision from an inventory perspective or even a reliability issue. Given available technology and cost issues, designers usually chose between discrete logic, PLD, ASSP or ASIC. To better understand the definitions, Gartner Inc. provides definitions as follows:

? ASIC describes IC products dedicated to a specific applications market that are customized for a single user.

? ASSP describes IC products dedicated to a specific applications market that are used by more than one user.

? PLD describes IC products programmed after assembly (memory is not included.)

Although the criteria picked are not always the leading factors for a design decision, they are usually in the equation. For instance, if time-to-market is the leading decision-maker, ASIC and ASSP would probably be eliminated. ASSP devices only exist for certain markets and are not available for all designs. For versatility and time-to-market, the best choices would be either discrete logic or a PLD. If the product had a narrow time-to-market window of opportunity, a PLD would be the best choice since the design and board layout could be done in parallel. If cost were the leading decision maker, discrete logic would be the winner. But the component cost is just one small part of an entire system, so further investigation is necessary.

Cost of manufacturing

Conversion cost is defined by the National Electronics Manufacturing Initiative (NEMI) as the cost to take a group of parts and convert them to a functioning electronic assembly. This cost includes procurement, subcontracting and test. It varies by the type of industry the board is manufactured for. Types of industries include consumer, portable, automotive/aerospace, office systems and business products. As expected, high-end business products and aerospace assemblies have the highest conversion cost due to special handling, documentation and reliability.

Most design engineers look at a bill-of-material that only includes cost of components on the PCB. Many do not understand the special handling, packaging, assembly and other equipment issues the subcontractors' face. Usually, the component cost is only a small portion of the total cost of the product. Beyond cost of materials, there are PCB layout, fabrication of the board, assembly and test costs as well as inventory cost. With more complex board design, these costs can be much higher than the actual cost of components on the board. To understand the individual cost of each component, it is necessary to look at trade-offs between cost of components and the cost of using those components.

The basics of assembly costs usually include the number of components (active and passive) the type of components (surface mount contact or through hole connections). The more discrete logic devices used, the more connections, and hence the higher the cost of assembly.

The more integration you can achieve, the less you spend on the number of components. This can be achieved through different methods. Take the example of discrete logic. Simple discrete logic gives you multiples of the same function in one package. If you do not need all the multiples, you don't use the pins they are associated with. Thus, you pay for the whole assembly cost that you do not make use of. This adds cost in the active logic device and passive components as well. It turns out the more discrete logic you need, the higher the cost of the assembly process. Factor in the additional board size and extra routing for individual components and it becomes clear that discrete logic can cost much more than a simple BoM price quote.

It appears that CPLD devices are less expensive to assemble. But considering the PCB fabrication is also a factor. For example, when laying out a PCB, fewer layers are always desirable to keep costs down. There are instances where more layers are required as in the case of high-speed system designs. If we look at the number of pins for discrete logic and PCB routing resources, the comparison leads to the conclusion that it takes far more connections to use discrete logic. These extra connections for discrete logic increase the chances of layout errors and possibly require more layers to route signals due to the increased pin count. Not only could this increase PCB layers, it could also increase the total system size and power requirements.

If you total the area for discrete logic, it calculates out to 316mm? while the area taken up by the CPLD in a 100-pin VQ package equals 196mm?. This CPLD solution takes approximately 40 percent less space and may allow fewer layers of PCB routing.

Inventory costs are another variable in the total cost of manufacturing electronic devices. Factors included in this category are shipping costs, storage, kiting and machine handling. The two of the higher-cost components of inventory control are shipping and machine handling. The more boxes you order to manufacture a product, the higher the total cost. Even if shipping comes from only a handful of suppliers, the weight, storage and time it takes to order all factors into the time involves coordinating the availability of parts to assemble a PCB.

If discrete logic is used, multiple devices must be ordered, shipped and sorted for production builds. If fewer components are used, less time is involved to order, ship, store and kit for production builds. This leads to the conclusion that fewer parts are less expensive. Even though discrete logic may first appear to be less expensive, after careful analysis, the more components used on a PCB, the higher the cost.

One consideration usually not thought of is product obsolescence. Unless you have gone through the process of trying to replace an obsolete part, the thought probably does not occur to check volume demand of a particular device. If a product family offers a wide variety of parts, it is assumed to be a good benefit. On the other hand, if a product offered by the market does not develop, chances are higher that it may not be offered for more than a few years.

In high volume consumer applications, this may not be an issue, but for other industries such as medical, industrial, telecom and datacom that expect equipment to have a lifetime of 10 years or longer, it could become a problem. It is always a good idea to ensure supply over a long period of time so last-time buys and high "gray market" vendors can be avoided. If you chose a device that can do many functions, the chances of discontinuation are low. This is due to few families being introduced in a short timeframe.

Now that there is an understanding of the hidden cost of discrete logic, what alternatives are available? As mentioned, the choices are ASIC, ASSP, PLD and if sequential operation is tolerable, any MCU or processor. For simplicity, this document will deal primarily with similar implementations and not touch on any MCU or processors. ASIC solutions are primarily for high-volume, single market applications. ASSP devices are good alternatives if they match your particular market and fit your requirements. For most systems, the best all-around solution would be a PLD. These devices continue to grow at an astounding rate while discrete logic seems to have stalled.

This trend is due to the additional features that are continually being integrated into CPLD and FPGA devices. It is also due to the fact that reprogrammable logic devices are continuing to push process technology to ever-smaller line widths. These geometry shrinks also improve die per wafer and reduce cost. There are also advancements in packaging technology. With the introduction of quad flat no lead devices, package costs are also continuing on a downward spiral.

- Steve Prokosch

Staff Marketing Manager

Xilinx Inc.

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