Building bridges: describing and improving interface designs

       Virtually every product design must navigate the digital abstraction and the real analog world. Some of the early design considerations focus on interface design.

　　The second half of the 20th century saw an unprecedented rate of technological innovation. Unlike previous periods, many of the advances of this period were quickly applied to the broad consumer market. Until then, as our society tends to squeeze all value and longevity out of consumer products, marketers needed to generate enough interest in new products to induce customers to convert to further needs: this was the origin of the disposable economy.

　　Starting in the 1850s, advertising used idioms such as the "jet age," the "atomic age," and the "space age" to connect products to
the social image of the emergence and rapid change of modernity. Ironically, none of these idioms was as powerful as the one that remained: the "digital age." Starting in the 1970s, the era just showed that the connected things were not simply modern, but were significantly different from earlier products.

　　Indeed, digital abstraction has revolutionized our industry, nearly every economic sector, and all aspects of life. Although the results are quite real, this abstraction is strictly conceptual. As a result, virtually every product design must bridge from the digital abstraction to the real analog world.

　　The requirements are apparently trivial: IC layout considerations, interfaces between IC and PCB, layout of PCB, product interfaces to distant points in the box, system boxes and system networks, all must deal with essentially similar interface relationships. Despite these items, this check remains very diverse in scope, spanning more than 10 orders of magnitude. The fact is that while interfaces challenge bandwidth growth, the problems appear across a spectrum.

　　Some considerations in the early design stages focus on interface design. The result of the trivial nature of interfaces is to begin specifying interface requirements in the product definition stage, refining them during block diagrams, schematics and simulations.

　　I distinguish between the process of iterative design refinement - purposefully improving the design process and iterating through a series of revisions. The first gate in the product definition stage should establish a rough estimate of the interface cost, complexity and design time. The output of the first stage is used as a check for execution finalization at the diagram stage. Different cost or complexity estimates from the first two stages serve as a warning for one of three conditions:

        Interface design affects processing.

        The first stage failed to properly calculate all product definition requirements.

        Changing the product definition wastes cost and complexity estimates that require updating the part of engineering management activities.

　　In later design stages, similar checks are performed.

　　As a starting point for Phase 1, remember the characteristics of each interface's signal source, line, and client. Regardless of the "digital" meaning of the signals, remember that the interface behavior is analog and multiparametric. Consider the bandwidth requirements for each line, source impedance, signal amplitude, and run length; the characteristics of the connecting media; the presence of noise sources or interference, and the fidelity requirements of all pre- and post-receiver circuits. During the simulation phase, compare the worst-case transmit and receive waveforms. Consider pre- and post-signal conditioning techniques and improve on any deficiencies. If you delay these checks until prototype evaluation, you may slide the design improvements into more expensive iterations.