ECE497 Notes on Creating a Beaglebone Cape
So you are thinking about building a Beaglebone cape? Awesome! There are a lot of reasons to make a cape other than the fact that you will be able to implement almost every functionality you can think of all on one PCB. In 2012, beagleboard.org and circuitco held a Beagle Cape competition and gave the top three design entries $1000 dollars plus lifetime royalties on sales of the cape on circuitco. You could also try to get your cape licensed and sold by Adafruit, Sparkfun, or other DIY websites separately. Good Luck!
The first issue that must be tackled is what exactly you want on your cape. There is a Google Spreadsheet that lists most Beaglebone capes that have been completed or are in the process of being completed. If the cape you are building does not have components on it that are in the repository, it is probably time to ask yourself whether the beaglebone supports the hardware you have selected.
Also, if your cape idea is really similar to one in the repository, I wouldn't get too down about it. Paypal is really similar to Google Checkout, Wirecard, and Moneybookers, but continues to be the standout internet money handling service for various reasons. Basically, I would suggest continuing to work on your idea. Especially if it offers something unique that the other solutions in the repository do not include.
When choosing parts that you want on your cape, realize that some parts are easier to put on PCBs than others. Try looking up libraries for the parts you want to use by googling "msp430 allegro library" or "msp430 Eagle library." If there is not library, you will likely have to create a pad and footprint for the part.
Eagle CAD is an open source schematic layout and PCB editor environment that is used by most of the PCB open source community. You can find the latest version of Eagle here.
Eagle CAD is a fully featured CAD program that can be had for the low price of $0 as long as you are okay with these limitations:
- The useable board area is limited to 100 x 80 mm (4 x 3.2 inches).
- Only two signal layers can be used (Top and Bottom).
- The schematic editor can only create one sheet.
Cadence Allegro is licensed to the Rose-Hulman, so it is free to Rose students. Allegro is a little bit harder overall to make PCB's in, but it is supported by the ECE department. Ask Dr. Simoni for help on the PCB. He has a website with video tutorials and other walk-throughs to help you with the process. Rose-Hulman licenses it free for use for any current students. It can be downloaded from the DFS folder. The folder also has instructions on how to install it.
Allegro PCB is licensed by the school and used by many professional organizations throughout the world. See Dr. Simoni for advanced help.
First Step: Prototyping
No Beaglebone cape is known to work until all of the suggested components are prototyped together with various development boards and a Beaglebone. Texas Instruments has many development kits listed on their website product pages. For example, if you navigate to the page for a common MSP430 Processor you will find on the right side of the page links to evaluation modules, development tools, and other information.
The other place to look for prototyping information is on the product page itself at vendors like Digikey, Mouser, and Allied Electronics. The MSP430 that we just looked at on the TI website can also be found on the Mouser website where you can buy it for $2.00. See also that the Mouser website has information such as a datasheet and schematic footprints for the specific product.
After the evaluation modules are ordered, grab your breadboard such as:
and begin placing components and wiring everything together.
Realize that if you need any help with the power supplies or other aspect of wiring, refer to the datasheets of the parts you are using. For example, the datasheet for the MSP430 that I mentioned before indicates that it needs a power supply voltage between 2.2 and 3.6. This means that the 3.3V VCC coming out of the Beaglebone would work perfectly in powering the MSP430.
Building a Schematic
After prototyping, you know where each of the wires is to be routed. The next step is to connect all of the components together in your CAD program of choice. There are excellent circuit layout examples to follow in both OrCAD and Eagle. Whichever program you use, a trick you can use is to find capes that use components in them that you are using in your design. Find the capes at the links below and download their respective OrCAD or Eagle files and find pads and footprints for your design. If you do this, make sure to reference the design that you used. Otherwise, pads and footprints can also be found by googling the respective part: "msp430 footprint"
Their are advantages to using OrCAD over Eagle. There are more examples and you can run simulations in PSpice before laying out the board in OrCAD. Rose-Hulman also continues to buy licenses for the complete OrCAD suite.
There are fewer Eagle examples, but a few excellent examples include:
- QiLiCape Credit goes to http://ioclk.com/beagebone-qilicape-a-wireless-power-charger-cape-for-beaglebone/
- The Adafruit prototype board https://github.com/adafruit/Adafruit-BeagleBone-ProtoBoard
Excellent Cadence Examples include:
- CircuitCo Examples Most of the capes on this page where designed in OrCAD.
The Physical Layout
Laying out a PCB can be incredibly difficult. Understanding whether you want 1, 2, or 3 layer boards, how thick should the traces be, how close should the components be placed together, and more are often incredibly overwhelming at first. I cannot stress enough how helpful it would be to attend a PCB workshop that Dr. Simoni holds. He usually has at least one a year. If you cannot make it to a workshop, there are still plenty of resources that you can use to quickly design a PCB.
- Ask Dr. Simoni for his PCB videos. They are not operational currently on the internet, but I believe he still has copies of the videos on his computer
- http://www.expresspcb.com/expresspcbhtm/tips.htm has tips on PCB layout. Definitely a must read for any PCB newbie.
- http://www.alternatezone.com/electronics/files/PCBDesignTutorialRevA.pdf *****Excellent Tutorial*****
Really the best advice from one Rose student to another is to start asking questions. The lab techs at Rose-Hulman are incredibly helpful if you are respectful. Mark Crosby is the man to ask if you need a PCB printed. After you believe that your circuit is correct, print as many PCBs as needed until it works correctly. Each board that Mark prints is roughly $20. At Rose, you are only able to print a maximum of 2 layers on a board. If you want to print more than that, you will have to send your PCB to a company to get it printed. The cheapest printing that I found in my time at Rose was at 4pcb.com. They can do almost anything and can get the board back to you in as little as 3 days if you pay for the expensive shipping.