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Road-map fixation makes design seem oversimplified

Posted: 18 Dec 2006 ?? ?Print Version ?Bookmark and Share

Keywords:design engineering road map? optimal design? engineering complexity? optimal design and solution? mixed signal design ideas?

I bristle at our industry's frequent promotion of technology road maps. They make the entire engineering-design process seem so mundane and routine: We'll do this, then that, then this, and poof, we'll be there.

The road map obscures the inevitable, unexpected turns of realitythe problems, the trade-offs and the complexities of pushing the envelope. It presumes a linear path forward, and it assumes no disruptive developments, such as the transistors, ICs or LEDs. It makes both engineering development and scientific progress seem like glorified administrative tasks.

David Robertson, product line director for high-speed converters at Analog Devices Inc., underscored engineering's true complexity when he noted in a recent conversation that deciding how to partition the mixed-signal functions (in this case, for a communications application) between two or three distinct dice is one of the most difficult up-front design challenges. The simplistic approach would be to put all the digital functions on one chip and all the analog ones on another, so the design could use suitable processes while minimizing unwanted, mixed-signal interactions.

But this simple idea may not be the best. First, it may make more sense to put a few analog functions on the digital chip (or vice versa) to keep some feedback or control loops physically close, and to simplify or slow down some interchip interfaces. Second, there are issues of die size, modeling, testability and available process characteristics. For example, a high-speed, small-geometry CMOS process may also support implementation of some high-end analog functions on-chip, with the benefits of lower overall cost than the analog process.

Regardless of which path is chosen from among the numerous partitioning options, they all suffer from the same drawback of risks and unknowns. Market size, modeling accuracy, packaging options, device yield, first-silicon performance and many other factors are simply unknown or have large uncertainty.

The partitioning dilemma is not limited to mixed-signal ICs. Steven Brightman, marketing director of single-function digital-filter IC vendor Quickfilter Technologies, says some of its customers also confront this challenge. Should they use a more-powerful DSP and have it implement the filtering functions? Or should they instead go with a lower-cost, admittedly less-capable DSP/microprocessor, while offloading the filtering to a separate, dedicated IC? The issue, again, is whether to use two lesser ICs vs. one bigger one. The trade-offs are cost, real estate, power, uncertainty, risk and flexibility.

This is classic engineering. There is no single right answer or optimal design. There is no instructor with the correct answer key waiting at the end of the project. In fact, the concept of an optimal solution is meaningless without stating that it is optimum with respect to a specific parameter, such as cost or performance. In reality, any solution will be sub-optimum with respect to all of these factors and instead will strike a balance among all of them.

Such trade-off analysis, along with constant uncertainty, is what engineering design is aboutit is what our road-map fixation minimizes and even denies. To pretend that we can see the future unambiguously is a disservice to engineers and the true engineering process.

- Bill Schweber
EE Times

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