MightyOhm

Let’s say you are designing a printed circuit board in Eagle, and you need to place a component that you’ve never used before.  In Eagle, before you can use a new component, you need a land pattern, a schematic symbol, and a mapping between them to fully define the part.  Often, you can search through Eagle’s included libraries and find what you need (or something close enough).  But what if that fails?

The symbol and pin definitions are usually pretty easy – just copy the datasheet.  The hard part is the land pattern: the collection of copper traces, soldermask openings, silkscreen, and other features that define the part on the PCB.

To come up with a land pattern, you usually have a few options:

1. Someone else may have done you a big favor by creating a part definition and uploading it to the Eagle library directory.  Caveat: Use it at your own risk.  Surface mount parts tend to be particularly hard to use right out of the box – often someone else’s land pattern won’t even pass your DRC.  Whose process were they using, anyway??
2. Look through the datasheet for the part to try and find a recommended land pattern. (Good luck!  Increasingly these are not included, but may be somewhere else on the manufacturer’s website.  Google is your friend!)
3. Take a guess based on the geometry of the part, assuming you have a mechanical drawing or a physical sample somewhere.
4. Skip 1-3 and use an IPC-7351 land pattern generator.

IPC-7351 is a standard for printed circuit board land pattern designs.  The standard attempts to, well, standardize land patterns to try to discourage every PCB designer from having his or her own custom library of land patterns.  IPC takes known good land patterns and combines them with accepted manufacturing tolerances to produce a land pattern that will work for most people most of the time.  Increasingly you will see references to IPC-7351 in datasheets instead of a land pattern drawing, so access to the standard is becoming more important over time.

Cool, right?  Well, the bad news is that while you can browse through the table of contents/introducton for free, downloading the standard costs big bucks.

Fortunately, PCB Matrix has a free IPC-7351 Land Pattern Calculator (direct download link here) that you can use to generate land patterns based on the standard.  You don’t need to own a copy of the standard to benefit from it.

The calculator is somewhat tricky to use but if you click the right buttons you can get something like what is shown below (click to enlarge).

PCB Matrix IPC-7351 Land Pattern Calculator Screenshot

X and Y are the dimensions of the recommended pads for an 8-lead Thin SOT-23, which happens to be the package for the LT3464.

With this information, you can return to Eagle and create a land pattern for your device.  PCB Matrix will also sell you premade Eagle libraries, but from their site it was not clear how much they cost.  Based on their other products, my guess is several hundred dollars and a yearly maintenance contract – I’ll draw my own, thanks.

Unfortunately, the calculator is Windows only, so Mac guys like me need to use VMware Fusion or similar to use it.  Can someone create a web version, please?

This Christmas I received a bunch of great books from friends and family.  My O’Reilly collection in particular doubled in size!

Here’s a list of what I’ll be reading in 2009:

• Analog Circuit Design: Art, Science and Personalities (EDN Series for Design Engineers) – Several first hand accounts of what it takes to be a good analog circuit designer by leaders in the industry.  An older book with lots of circuit examples and historical notes illustrating trade-offs, breakthroughs and the analog circuit design process.  Discussion of the looming threats of digital circuits (20+ years ago!).

• HTML & XHTML: The Definitive Guide (6th Edition) – HTML has grown a lot since I started using it in 1995.  This is my attempt to catch up.

• CSS: The Definitive Guide – So I can hopefully upgrade my web designer status from “hopeless hack” to “amateur”.

• Learning Python, 3rd Edition – You can never have too many scripting langages at your disposal (can you?).

• Zen and the Art of Motorcycle Maintenance: An Inquiry into Values (P.S.) – To give the right side of my brain something to do in between the previous four entries.

Kevin Kelleher wrote an interesting post the other day on emerging technology blog GigaOM, entitled 2009: Year of the Hacker.

Kevin’s theory is that members of the growing ranks of unemployed professionals will choose to apply their “spare cycles” towards new and innovative projects after being released from the daily grind of full time employment.

Kevin writes:

I wonder what kind of creativity could be unleashed by workers who, though deprived of a steady paycheck, are freed from such tedious tasks. Some could come up with new ideas that help vault the web to a more advanced stage. Others may make micro-contributions that are equally powerful in aggregate. Such creativity could then foster an entirely new generation of startups, which would eventually lure away some of those who had remained at steady jobs all along.

I agree with Kevin because I have seen the beginnings of this trend firsthand.   Several of my colleagues in San Francisco left or lost their jobs this year, and not a single one sat at home eating potato chips.  Why?  Kevin quotes Chris Anderson, who writes:

… I think you’ll see a boom in creativity and sharing online as people take matters into their own hands. Today, if you’re in-between jobs you can still be productive, and the reputational currency you earn may pay dividends in the form of a better job when the economy recovers.

This reputational currency is real, and it is a direct result of the ease of collaboration and the almost zero barrier to entry in software (and increasingly hardware) development.  The tools are cheap or free, high speed internet is everywhere, and somewhere out there someone is interested in helping to bring your project to life.  It is becoming easier than ever to innovate in your own garage.  Don’t have a garage?  Use someone else’s instead.

The dividends have yet to been seen, but I am optimistic that efforts today will be repaid tenfold tomorrow.

Photo credit: Chris Campbell

Better late than never – happy holidays from mightyohm.com!

Programming & Customizing PICmicro Microcontrollers, by Myke Predko, is probably the best book out there for someone who is starting out with the PIC series of microcontrollers from Microchip.  I used Myke’s book as both a tutorial and reference when I created my PIC RGB Video Display.  Since then, I have referred back to this book countless times even when working with other microcontrollers, like Atmel’s AVR family, because it contains so much useful architecture-independent technical information.  I have referred to this book for information about topics including LCD interfacing, debouncing switches, RS-232 serial interfaces, and multiplexed LED drivers.  As a technical reference it easily surpasses the majority of AVR books that are out there.

The book is starting to show it’s age by not including some of the latest PIC micros in the examples (like the PIC16F628), but the code is easily ported to newer/faster/better microcontrollers, a good learning excercise in itself.