Designers Corner with Prof. Mike Smith  Part 1  Part 2

Producing documentation

I kept a record of the datasheets, parts, addresses of contacts, key design issues, bugs, software hints and so on as we went through the XCoNET project. At IBM we used to keep a big black loose-leaf binder that we called the “gotcha” file. Any weird software behavior, hints on how to get Spice to converge (it was called ASTAP at IBM) would go into the gotcha file, which became a valuable design aide. It was interesting that IBM would force us to keep our own notebooks, which had to be signed and witnessed, but not to share information with others in the gotcha book. “XCoNET the website” became an electronic gotcha book.

For example, you can see the BOM (bill of materials, or list of parts) at http://ilima.eng.hawaii.edu/XCoNET/Demonstation_parts.htm#BOM

The schematics for our first prototype, the Demonstation, are at http://ilima.eng.hawaii.edu/XCoNET/Demonstation.htm

The Demonstation, our first prototype; it should have been the Demonstration board, but we made a spelling mistake on the silkscreen. The FPGA, a Xilinx Virtex part in an HQ240 package is under the zero-insertion force socket towards the top right. Ethernet enters via the RJ-45 socket on the upper left. To the left of the socket is the magnetics and further to the left, the Level One PHY chip. The FPGA is surrounded by rows of 0.1-inch square headers to allow fixes and changes to be made easily. The two banks of SRAM are located above right and below left of the FPGA. The play area at the bottom left of the board allows us to add any chips or other devices to fix problems or to add functions. The 32-pin socket between the play area and the SRAM is for the PROMJet or other ROM (that forms the FPGA bootstrap). The Seiko LCD panel at the lower left is used for debugging. Further to the right is the parallel port. The banana plugs for connections to the power supplies along with the power regulators are at the upper right edge of the board.

We used Sun Circuits, a local Silicon Valley rapid turnaround PCB fabricator, for the first six prototype boards, (http://www.suncircuits.com/). There are other similar companies that will fabricate boards in just a few days from Gerber files (I have used Advanced Circuits for small student projects, for example (http://www.advancedcircuits.com/).

Transfer of the design data to the PCB vendor went smoothly because Steve Carey, our layout expert, and Bill Pabst had handled many similar design before with Sun. This experience is worth a tremendous amount (in avoiding lost time and potential errors). The prototype boards came back within a week. The first thing to do with a new board is the same as a new chip: measure the resistance between power supplies. In the past, it used to be wise to “Ohm out” a new board, measuring the resistance between each connection and checking it off on the schematic. Modern multilayer boards with surface mount components are making it more and more difficult to check all the connections on a PCB. It is still wise, though, to make a few basic checks, especially around the power supplies. At this point we were reasonably sure we hadn’t made any gross errors and there weren’t any serious fabrication errors such as missing conductor planes for example (which sometimes happens).

We were lucky to have a skilled technician at Xilinx, Lito Bonet, who could assemble the boards for us, including the surface mount components. This is not a trivial part of prototype assembly. Later, when we were assembling a dozen of our second prototype boards, we had to put together a kit to send to an outside assembly house. It was a major effort to make sure all the parts were ready and the assembly of each and every part was properly documented. Lito was able to figure out what we had forgotten, find components we had missed, substitute for some parts, correct a few mistakes we had made, and put everything together with minimum help. Lito’s skills, which came from years of experience handling similar projects, saved us weeks of effort.

The Demonstation (underneath). The nonvolative Xilinx CPLD is in the center of the FPGA footprint (four sections of pads each with three rows). The table of text at the bottom left is on the silkscreen and provides a handy reminder as to the name and function of each of the 240 FPGA pins.

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