Circuit Design on Your Linux Box Using gEDA
A lot of attention—and hype—has focused on bringing traditional office-productivity programs, such as the OpenOffice.org suite, to Linux. However, another important—and far less-hyped—area where Linux's desktop abilities come to the fore is in engineering software, and in particular, CAD (computer-aided design).
Non-engineers tend to think of the term CAD as referring to mechanical design software, and they are partially right. We are used to seeing complicated drawings of mechanical assemblies appearing on computer screens in advertising and television. However, CAD doesn't mean only mechanical design. Electronics designers also long have used computer-based design tools to help them perform their design tasks.
The area of CAD software used for electronics design is typically referred to as EDA, short for electronic design automation. Happily for Linux users, many EDA applications—both proprietary and open source—are now available for Linux. Perhaps the largest and oldest open-source project is the gEDA Project. The gEDA Project is an active community of hackers who have developed high-quality EDA applications for many different electronic design tasks. In this article, we examine a set of gEDA programs you can use to design your own circuit boards from scratch.
Designing circuits using EDA tools is analogous to creating software using traditional software engineering tools. That is, rather than using only one program to accomplish a job, electrical engineers will use a wide variety of tools, each tailored to accomplish its particular task efficiently. They use each tool in a sequence as they progress from start to finish through their design work, a concept known as design flow. This concept should be familiar to software engineers who might first use a flowcharting tool, then an editor, followed by a compiler, then a debugger, as they create applications ready for distribution.
Different types of circuit design use different flows. For example, if you work on digital electronics, you might code your design using a hardware description language such as Verilog, and then use a Verilog compiler followed by a waveform analyzer to simulate and analyze it. On the other hand, if you are doing analog circuit design, you might draw your circuit using a schematic capture program, and subsequently use a circuit simulation program, such as SPICE, to verify its correctness before prededing to create a PC board. Other, more-complicated designs might require additional steps in the design flow.
In this article, our interest is to create a garden-variety printed circuit board (PCB). To create a simple circuit board using any EDA toolset—whether proprietary or open source—the design flow looks like this:
Gather information about your design's requirements, as well as find data sheets for available parts.
Create a high-level block diagram of your design, using either paper and pencil or a general-purpose drawing program such as Dia.
Enter your detailed design into the computer, and draw its schematic using a special-purpose schematic capture program.
Netlist your design's schematic, meaning that you export a so-called netlist that captures all your design's components and connections in a special-purpose file format.
Read the netlist into a layout editor, which is a special-purpose drawing program for creating PCBs. Use the layout editor to create a drawing of your PC board's physical layout.
Export Gerber files from your layout program. The Gerber files are industry-standard files that describe your PCB to your PCB manufacturer.
Send the Gerber files to a PCB fabricator.
Receive your freshly manufactured PCBs, populate them with components and test them out.
At any point in this flow, you might have to go back and repeat a step if you find an error in a proceeding step. Again, this is similar to software engineering, where you have to go back to edit and recompile your program if you find that it has a runtime error. The only difference is that if you take a PCB design to its completion by ordering boards, and then you find a design error, you're stuck with a batch of bad boards—you can't simply delete your work and recompile.
The gEDA Project was originated by Ales Hvezda in 1998. After graduating from the University of New Mexico, he became interested in robotics. Ales wanted to design circuits for robots using his Linux box, but found that no suitable applications running on Linux existed. Consequently, like so many open-source developers active in other areas, he felt the itch to write his own EDA suite. Ales scratched the itch by writing a schematic capture program and a rudimentary netlister. He also created a Web site to share his efforts, which he named the gEDA Project, because he wanted to release his EDA creations under the GPL.
Very quickly, a small team of developers joined him and started patching and adding to the gEDA tools. Others started contributing tools of their own, some of them modest design utilities and others major software projects in their own right. In the latter category fall the ngspice Project, which aims to develop an open-source implementation of the popular analog simulator SPICE; Gnucap, an analog/digital circuit simulator; and Icarus Verilog, a tool for digital logic generation and simulation. Finally, a very important allied program is pcb—the printed circuit board layout editor.
Because his idea was met with such enthusiasm, and so many tools allied themselves with the gEDA Project, Ales decided to rename his original suite of tools gEDA/gaf, an acronym for gschem and friends. Meanwhile, the term gEDA Suite has come to designate the entire ensemble of open-source EDA tools that have allied themselves with the gEDA Project. A main distinction between gEDA/gaf and the other tools is that the gaf tools tend to be used at the front-end stages of a design, while the other tools are used in later stages.
Since he started it in 1998, gEDA/gaf has grown to a full suite of front-end EDA applications. Besides the core programs gschem and gnetlist, there are almost two-dozen contributed utility programs, including an attribute manager, a symbol checker, an refdes generator (or component numberer), symbol generators and file format conversion utilities. Meanwhile, the various back-end tools, such as pcb, ngspice and Gnucap have also evolved and matured, enabling a variety of different flows adapted to different design types. Currently, the design tools in the open-source gEDA Suite will support almost any kind of low- to mid-level electronic design task, offering the same power as tools costing thousands of dollars.
When discussing the future of EDA on Linux with my friends and colleagues, I like to point out that Sun Microsystems became the corporate giant it is today by offering a great platform for engineering design. Linux can follow the same path to greatness because it, too, offers a powerful platform for engineering applications. Engineers are a natural customer base for Linux as they are smart, technology-savvy and actually want to use UNIX-native power tools such as TCL, Perl and Python to automate their work. Commercial EDA vendors have just begun to realize the customer leverage they can get from the Linux platform, and they are moving fast to serve that market. However, open-source developers recognized the importance of Linux to electronic design many years ago, and the maturity of the gEDA Suite is proof of their early insight.
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