The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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Page 25 of 115

26 Chapter 4 Capturing the Schematic I n our story, Ian's schematic seemed to "capture itself" because he was using preset values of which he did not understand. However, before we delve into how to avoid a situation like what Ian had ex- perienced, it is important to understand some key schematic design fundamentals, starting with what a schematic is and its main purpose. A schematic is created by the electronic engineering stakeholder of a project. It serves the purpose of recording and communicating infor- mation about a PCB assembly's parts, connectivity, and functionality on the front-end of the creative process. The front-end of any electronics design project involves quick iteration and adjustment before any parts are purchased or copper tracks are laid down. A schematic diagram uses simple, stick-figure symbols and lines to reflect actual electronic parts and circuit connectivity. Creating a schematic diagram is a way for an engineer to quickly document the elements of a complex circuit in a way that is easy to read and under- stand. A properly captured schematic helps the PCB design process by hierarchically organizing the electrical areas of the evolving PCB design. A complex schematic drawing package, or design database, usually starts with a system-block diagram showing all the parts of the design on the first sheet. In a hierarchical schematic, users can click onto each of the system blocks on the cover sheet to be transported to a subsequent sheet of the schematic where the system block is expanded and drawn schematically. A completed schematic should look simple and basic to any viewing stakeholder such as a field service tech or test engineer. However, stakeholders such as those involved in design and layout leverage the vast amounts of data embedded within the ECAD schematic to complete the design layout. Throughout the years, engineers have been using some creative methods to capture schematics. Capture software has very few input requirements and can be used by anyone to make connections between almost anything. For instance, ECAD tools today are so easy to use even a surfer with no electrical knowledge could create a schematic symbol for a beach hut, a surf shop, a surfboard, and a wave and tie them together with connections to show other surfers what he would be doing throughout the day: Examining this simple schematic, it is easy to see the surfer is going to leave his hut, go to the surf shop, come out with a surfboard, and head for the ocean to surf a wave. Now, ask three different EEs to draw a schematic depicting a surfer's day and you will come up with some very different-looking symbols and schematics. As you can imagine, each EE will have his or her own take on what the beach hut looks like: does it have a back door or a basement; is it solar powered or off grid? How an EE would depict a surf shop or surfboard schematic symbol is anyone's guess. When it comes to depicting a wave, well, let's just say EEs can be very knowledgeable about waves observed on an oscilloscope, but schematically depicting a wave viewed from a beach will again yield a wide variety of interpretations; leading to confusion amongst those trying to decipher these simple schematic drawings.

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