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66 DESIGN007 MAGAZINE I MAY 2018 constraints that the designer is working under, in a very concise form; however retaining and using this information later is extremely diffi- cult. Adaptive Design Using these powerful constraint techniques can be a double-edged sword. While the design process is made much safer by including con- straints, it is all too easy to over-constrain the design and make it impossible to complete rout- ing and placement. Even paper design guide- lines can make products uneconomic to pro- duce unless a great deal of engineering knowl- edge is applied during the design. This means the design tool has to be adaptive—reacting to changes in the design as it progresses. Com- bining these elements is anything but trivial, but doing so allows the designer to converge on the optimum design in the first pass, rather than via multiple attempts. In PCB design, attributes (or constraints) and rules have always existed, and designers have tried to achieve their design goals, espe- cially with systems that are based around the raw attributes of the components in a design. But attributes (or properties) are essentially just data about the pins, components or track- ing, while constraints are requirements that must be obeyed. Examples include the maxi- mum delay, or an error marker, which must be flagged as functional or an operation may be at risk. These can simply be a function of the design attributes, but constraints are typically more complex nowadays and they sometimes relate to each other (e.g., in high-speed mem- ory the relationship between the byte lanes and from the data/address signals to strobe and clock signals). Constraints must exist through Figure 1: Typical PCB-related design constraints. Figure 2: Example of a constraint browser, the Zuken CR-8000.

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