Advances in semiconductor process technology have driven CPLD costs down to a point where they now offer a highly compelling discrete logic replacement alternative. This article discusses why it makes sense today - both economically and strategically - to migrate from discrete logic devices to CPLDs.
Over four decades ago, digital logic was built from discrete resistors and transistors. Integrated circuit TTL logic technology gained almost universal acceptance after Texas Instruments introduced its TTL 74XX family of integrated circuits in 1962 to support NASA's lunar landing and space exploration programs.
TTL variations came rapidly during the 1970s, but along with the rampant proliferation of multiple 7400 series came complexity. By the early '80s, a veritable alphabet soup of logic family variants had been released - TTL, S, LS, AS, F, ALS, CD4000, HC, HCT, BCT, AC, ACT, FCT, ABT, LVT-A, AHC, AHCT - forcing designers to require a scorecard to keep track of which family best fit each application.
While there were many 7400 devices to choose from - perhaps far too many - a revolutionary trend began to appear in the form of programmable logic devices (PLDs). Delivering user programmability, the first generations of these chips replaced five to 10 logic devices. As logic integration improved, PLDs were able to replace more and more standard logic functions.
The same factors that drove down TTL costs are continuously driving down CPLD costs. As Figure 1 shows, the cost of a logic circuit implemented using discrete 7400 devices today is higher than one using CPLDs.
Closing the cost/performance gap
When comparing costs between technology alternatives, it is important to consider the total system costs, that is, the real cost of the solution. Accordingly, our analysis provides a detailed breakdown of all costs, starting with production costs, and then comparing the costs associated with total PC board area, power consumption, and the number of board layers.
Other advantages of using CPLDs
Reduced production costs: the total cost for one Xilinx CPLD is much less than one-half of the cost of the same circuit using 7400 series devices. Besides lower unit production costs, additional savings for CPLD-based designs are realised through reduced component count, including interfacing resistors, decoupling capacitors, additional PCB size, and assembly costs.
CPLDs gain an additional cost advantage through:
* Use of one CPLD family for multiple applications, reducing the number of individual part numbers traced and handled.
* Lower availability risks and low minimum order quantities (MOQ).
* Reduced expediting costs and production delays incurred by parts shortages.
* Avoiding lost revenues because of lack of component availability or down production lines.
Board area savings: When compared with discrete logic, CPLDs require fewer components and thus less board area and fewer layers. Further, the fewer devices leads to lower power consumption and improved reliability. In addition, lower heat dissipation avoids the need for cooling fans and heatsinks.
Time-to-market benefits: The proliferation of electronic wireless, industrial and communication devices continues to put pressure on companies to get designs from concept to production as soon as possible. According to McKinsey & Co, even if within budget, products that are six months late earn 33% less profit over five years. Unfortunately, designs that employ discrete 7400 devices are at the mercy of several barriers that not only prolong time to market, but add complexity and costs, and reduce reliability. These barriers include:
* A long manufacturing, assembly, test, and debug cycle that is susceptible to delays and multiple design decisions that can directly impact board layout.
* The lack of easy-to-use design tools makes debugging and maintenance tedious chores.
* The high number of TTL components required in discrete designs introduces availability risk. In many cases, components might be out of stock or even obsolete.
CPLDs, on the other hand, provide designers with numerous advantages that enable designers to react to last-minute design changes and compress time to market. These include component inventory reduction, faster production cycles, efficient device and board testability, and the ability to modify designs during all phases of design and production.
The real advantage - programmability: while time to market is a major benefit, the ability to program and reprogram devices is equally important. Unlike CPLDs, once a PCB using discrete TTL devices is laid out and goes into production, typically it cannot be altered or upgraded without ripping out components and doing board re-spins. Another advantage of CPLDs is quick system upgrades and bug fixes in the field. As a result, designers can easily integrate exactly the logic functionality needed without adding 'cuts and jumps' on the PCB. For example, imagine a scenario where automobile manufacturers can add new functions to in-car systems by allowing consumers to dial up and purchase upgrades over the phone to reconfigure the system. In addition, CPLDs help avoid board redesigns and scrapped parts when specific programs are cancelled.
The re-configurable nature of CPLDs also enables a single product footprint to implement multiple product personalities. That means manufacturers can standardise on a single PCB footprint and package, and thus quickly leverage economies of scale through reduced inventory overhead. Further, reprogrammability reduces the deployment of design resources for maintaining old designs, which allows engineers to focus on introducing new products and features.
Reliability: 7400-based systems introduce higher complexity with more components, interconnects, layers, handling, and thus, lower overall reliability. In addition, because discretes are typically larger and require more board real estate, and have a significant number of external chip-to-chip interconnects, they increase power consumption and EMI, which further heightens failure risks. In contrast, CPLD-based systems require fewer components and layers. This reduces PCB layout density, lowers heat dissipation, reduces EMI levels, and thus greatly decreases failures in time (FIT).
Reduced EMI: given the large number of components, traces, and board layers, TTL-based designs are more susceptible to high EMI. By contrast, CPLDs enable reduced EMI through fewer components plus features including: programmable I/O slew rate; programmable ground, programmable I/O signalling; phase-locked loops.
Increasing design security: CPLDs offer several unique advantages that safeguard system designers against code theft. Unlike discrete logic devices, which are extremely susceptible to reverse engineering, CPLDs inherently require a user-defined bit stream which easily prevents customer read-back. More elaborate security schemes that exploit reprogrammable capabilities of CPLDs to keep attackers at bay are also possible.
Summary
CPLDs offer a clear, cost-effective alternative to 7400-based logic systems. CPLDs pack more functionality into ever-shrinking die, offering compelling benefits of low-cost on-the-spot re-programmability, short lead times, higher performance, expanded densities, and unmatched flexibility.
Xilinx CPLDs consume less power and offer re-programmable flexibility with superior design security - without a price premium. The result is a scalable technology that delivers a superior solution for a wide range of high volume applications.
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