建立桥路：描述并改善接口设计

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实际上，每个产品设计必须经历数字提取和真实模拟世界。设计前期的一些考虑将焦点放在接口设计上。

　　20世纪后半叶的技术创新达到空前的速度。不像以前，这个时期的许多进步很快应用到广泛的消费市场。直到那个时候，因为我们的社会趋向于从消费者利益中榨取全部价值和寿命，商人需要对新产品产生足够兴趣，引起顾客转变进一步需要：这是可任意使用经济的起源。

　　19世纪50年代开始，广告使用例如“喷气时代”、“原子时代”和“空间时代”的习语，将产品连接到显现和快速
改变现代性的社会映像。讽刺地，这些习语也许都没有剩下的一个有力量：“数字时代”。19世纪70年代开始，时代恰好表明关联事物不是简单的现代，而与早期产品有明显不同。

　　确实，数字提取改革了我们的工业、几乎每个经济部门和所有生活的方面。虽然结果相当真实，但这个提取严格的说只是概念上的。结果，事实上每个产品设计必须从数字抽取桥接到真实模拟世界。

　　需求的东西明显琐碎：IC布局考虑、IC和PCB之间接口、PCB的布版、到箱子远处点的产品接口、系统箱和系统网络，所有都必须处理本质相似的接口关系。不管这些条目，这个检查保持了非常不同的范围，跨越了多于10阶的量级。事实是，虽然接口挑战了带宽的增长，问题却通过频谱出现了。

　　早期设计阶段的一些考虑着眼于接口设计。接口琐碎特性的结果是在产品定义阶段就开始详述接口需求，在结构图、原理图和仿真阶段改进。

　　我区别反复设计改进的过程——有目的地改进设计过程和反复设计一系列修改。产品定义阶段的第一个关口应该建立对接口价格、复杂性和设计时间的粗略估计。第一阶段的输出在图表阶段作为执行定型的检查。从前两个阶段，不同价格或复杂性估计作为对三个条件中一个的警告：

        接口设计影响加工。

        第一阶段未能合理计算所有产品定义需求。

        改变产品定义浪费了费用和复杂性估计，这些估计需要更新工程管理行为的部分。

　　在后来的设计阶段，做相似的检查。

　　作为阶段一的起始点，记住每个接口的信号源、线和客户端的特性。不管信号的“数字”意义，记住接口动作是模拟和多参数的。考虑每条线带宽的需求，源阻抗、信号幅值和运行长度；连接媒介的特征；噪声源或干扰的存在，所有前后接收端电路信号的保真需求。在仿真阶段，比较最坏情况下发送和接收端波形。考虑前后信号调理技术，改进不足。如果延迟这些检查到原型估价，也许将改进设计滑到反复设计，需要更多花费。

　　英文原文：

　　Building bridges: Specifying and refining inteRFace designs

　　Virtually every product design must bridge the digital abstraction and the real analog world. A few considerations early in the design phase can focus an interface design.

　　By Joshua Israelsohn, Contributing Technical Editor -- EDN, 11/8/2007

　　Technological innovation during the second half of the 20th century came at an unprecedented pace, and, unlike earlier times, many advances during this interval moved rapidly into the broad consumer market. Because our society, until that time, tended to wring the full value and lifetime from a consumer good, marketers needed to generate sufficient excitement for new products to cause customers to switch in advance of necessity: the origin of the disposable economy.

        Starting in the 1950s, advertising copy used phrases such as “jet age,” “atomic age,” and “Space Age” to connect products to a societal image of emerging and rapidly changing modernity. Ironically, perhaps, none of these phrases had much staying power save one: the “digital age,” which, beginning in the 1970s, correctly suggested that associated goods weren't simply modern but were manifestly different from products of earlier times.

　　Indeed, the digital abstraction has revolutionized our industry, almost every sector of our economy, and all but a few aspects of our lives. Though the effects have been quite real, this abstraction is, as all are, strictly conceptual. As a
result, virtually every product design must bridge the digital abstraction and the real analog world.

　　This requirement is remarkably fractal-like: Layout considerations within an IC, the inteRFace between an IC and its PCB, the PCB's layout, the product interface to points beyond the box, systems of boxes, and networks of systems all must contend with essentially similar interface concerns. This observation holds despite these items' enormously different scales, which can span more than 10 orders of magnitude. It is also the case that, though interface challenges grow with bandwidth, the issue arises throughout the spectrum.

　　A few considerations early in the design phase can focus an interface design. A consequence of the fractal-like traits of interfaces is that you can begin specifying interface requirements at the product-definition phase and refine them at the block-diagram, schematic, and simulation phases.

　　I distinguish this process of design refinement from iterative design—design refinement being a purposeful process of design in phases, and iterative design being a sequence of design repairs. The first pass at the product-definition phase should allow you to establish a good rough estimate for an interface's cost, complexity, and design time. The first phase's output can serve as your check as an implementation takes shape at the block-diagram phase. Disparities in cost or complexity estimates from the first and second phases serve as warnings against one of three conditions:

　　The interface design is heading off into the toolies.

　　The first phase failed to account properly for all product-definition requirements.

　　Changes to the product definition have made obsolete the cost and complexity estimates, and these estimates require updating as part of your project-management activities.

　　Make similar checks at subsequent design phases.

　　As a phase-one starting point, bear in mind the signal-source, -line, and -client attributes for each interface. Despite your signal's “digital” significance, remember that interface behaviors are analog and multiparametric. Take into account each line's bandwidth requirement, source impedance, signal magnitude, and run length; the characteristics of the connecting media; and the presence of noise sources or interferers, all in the context of the receiver-side circuit's signal-fidelity requirements. In the simulation phase, compare the transmitter- and receiver-side waveforms under the worst-case combination of these measures. Consider pre- and post-signal-conditioning techniques to correct deficiencies. If you delay these observations until prototype evaluation, you may slip from design refinement into a more costly course of iterative design.

　　英文原文地址：http://www.edn.com/article/CA6495304.html

发布者：博子