How Schematic Design Reuse Can Help Your Next PCB Schematic Layout

Created: November 21, 2017
Updated: October 21, 2020

Picture of someone working at a sewing machine

In the town where I live there is this lady who is a magician with a sewing machine. She is in charge of the wardrobes for the high school musicals and is a whiz at taking costumes from old shows, and reusing them for new productions. Sometimes she alters the older costumes, and sometimes she simply uses them the way they already are. In the show this year, the evil queen is wearing a flamboyant dress that is actually an altered version of a colorful bird costume from four years ago.

Reusing existing work is a key factor in reducing the amount of time and effort required for any project. Without the ability to reuse those old costumes, the high school would never have the time or the budget to put on a new show each year. This same principle also applies to creating a schematic. If you are creating a new design that requires existing functionality, it can take a lot of time and effort to recreate that circuitry. Fortunately, with design reuse techniques, you can add existing circuitry to your new design without having to recreate it.

If you haven’t worked with design reuse before, you’re going to like what I’ve got to tell you. By understanding the way that design reuse works, you enable a greater ability to creatively engineer smarter, more effective uses of your time. So sit back, relax and be prepared to be amazed—it’s show time!

Isn't Copy and Paste the Same Thing as Schematic Design Reuse?

Schematic reuse is using existing circuitry in a new design through the same schematic editor. Although copying and pasting circuitry has been used for this, it is not true design reuse. Here are some potential problems that can happen when you copy and paste circuitry:

Copying too much or too little. You need to carefully select exactly what circuitry you want in order to get the copy that you need. It is really easy to miss important details of circuitry due to a mistake in selecting, or if it resides on another sheet. It is also possible to select circuitry that doesn’t belong to the circuit diagram that you are copying. I’ve seen unexpected circuitry show up in PCB layout because of an incorrect copy and paste in the schematic capture.
Copying circuitry will also copy unwanted attributes. When copying and pasting circuitry into a new schematic, there may be unintentional attributes that come along as well. These can include old reference designators, or design specific attributes such as unique values, design variants, or even physical circuit board parameters.

True schematic reuse imports an existing design into the design flow that you are working on. Schematic tools have different ways to do this, such as reading individual sheets or an entire schematic design into the master sustainable design. The best method, however, is to use hierarchical block functionality.

Picture of red key on computer keyboard that says “Copy / Paste” — *Copy and paste is not necessarily the best way to reuse schematic capture*

Schematic Design Reuse and Hierarchical Schematics

A hierarchical schematic is where different areas of functionality are represented on the top sheet of the design project by block symbols. Each block symbol is linked to a separate design that is incorporated into the master design. By selecting and opening a block symbol, you will open up its linked design. The schematic tool will manage these different designs, including their nets and reference designators, so that there aren’t any data conflicts. The end result is that all of the linked designs are merged into the master design.

This functionality lends itself to design block reuse very well. Take, for instance, a power supply that was created as a hierarchical schematic for an existing design. When a new design requires that same power supply, it can simply be added as a referenced schematic. In the new design a block symbol is created that links to the existing power supply schematic. Once the block symbol is placed, the new design will pull in an instance of the power supply schematic and synchronize the nets and references designators. All that's left is to connect the desired nets to the power supply block symbol ports.

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