May 8, 1997

Design reuse: from a wish to a reality

by Wolfram Blume

Having easy access to five years' worth of your designs would greatly improve your productivity.

Design reuse tops the list of important issues to design engineers in numerous surveys. Yet, few engineers are reusing designs. Engineers are saying, "Maybe we should reuse circuits." Then they say, "Nah!"

Why put design reuse on a wish list? Recovering even a bit of the expensive engineering time that goes into designing a circuit yields significant gains in time to market and savings in engineering costs.

Engineers have taken few steps to turn design reuse into a reality, though. The rapid change in available components and application requirements is one reason. What worked two years ago may be impractical today. Today's speed, noise, and power-consumption requirements are usually more stringent, for example. New components are also available to meet those requirements. The 5V circuit you de-signed two years ago may not work in today's 3.3V design.

The truth is that changes to operating voltage or key components require changes throughout an entire circuit. As a result, you can't reuse most designs. You can, however, reuse concepts--the ideas that went into making the older circuit work well.

Capturing the thinking that created the old circuit is the most essential element for design reuse. You must leave a trail of thinking--a data log of concepts and a system of organizing those concepts.

One problem remains, however. Under pressure to create more designs in less time, design engineers often can't afford the time to develop and follow a systematic approach to recording their ideas as they work. Yet without a journal, their ideas vanish. A journal's organization might be strange or idiosyncratic, but the owner's ideas, thoughts, and intent are in there somewhere.

Imagine having easy access to five years' worth of your designs. That information alone would greatly improve your productivity. Having access to five years' worth of 10 engineers' designs could increase your productivity yet again.

A design engineer's keeping a journal is similar to a software engineer's commenting source code. In software development, code must have comments so that others can understand it when the software needs revisions. Also, programmers design software code in modules having simple connections to other modules. Simple, well-defined, and well-structured code makes modules reusable in various places. Both of these techniques--documenting concepts during development and designing the product in modules to be easily reused--are accepted disciplines in software development. Neither technique has taken hold in electronic design, however.

Design engineers aiming to create circuits that maximize reusability need to use simpler, more standard components and connections. This approach usually sacrifices some performance. And that is the problem: Electronic circuits invariably must trade performance for standardization. By squeezing the last bit of performance from a circuit, you make it more difficult to reuse that circuit later. By dropping the requirement for performance, you have to live with standard components and connections, but you reduce time to market and create a more reusable circuit.

Even if you design highly customized products, you still benefit by designing to reuse key concepts. The thinking that goes into customized designs may prove useful in shortening the design cycle for future designs. In contrast, if you neither document a trail of your design concepts nor aim for reuse, then you have to start over in future designs.

One useful area of development in EDA is tools that support design reuse. EDA tools can help by offering capabilities that enable engineers to easily document their thinking during development.

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