Saturday, March 12, 2011

Making Development Boards That Don't Suck

Lately I've had to use a few microcontroller development kits from manufacturers that, well, suck. From unlabeled connectors/components to not bringing out the unused pins, I'm convinced that many designers of development boards never actually use them to design anything.

When I'm trying out a new design, I frequently make my own development board. By "development board" I mean a schematic and layout that is the same circuit as the end product but optimized for firmware development and debugging. This allows a few things:
  • Firmware development can start sooner - this is probably the most important since FW typically takes an order of magnitude longer than HW design
  • Verify PCB footprints for components - you can spend a month checking and double checking a PCB footprint, or just build it and verify it on a real PCB in a week.
  • Power Measurement
  • Fast layout. Since the point of a development board is to optimize for development not layout, you can make things a little larger and not worry about squeezing everything in.
  • Related to the previous, a development board helps you get a feel for how small you can make the design once everything is working.

Here are the a few rules for how to design development boards that don't suck:
  • Expose all the pins of the components on test points or at least SMT pads. Soldering wires to fine pitch QFNs is a pain in the ass, and forget trying to do anything with BGAs. I recently helped out a major IC manufacturer and soldered some of their dev kits together because they didn't expose the unused pins.
  • Include a serial port (RS-232 is my preference).
  • Related to the above, bring out all signals that you can on easy 0.100" headers. This makes development much easier since you can attach logic analyzers, scopes, etc. Label them all with their signal name (more on this below).
  • Label liberally. Silkscreen is free, so on a development board label all components, as well as the signals on 0.100 headers, etc. Taking 5 minutes to label the signals on the PCB is faster than taking 5 minutes every time to look it up in documentation.
  • If it is a RF design with an onboard antenna, make an option for a connector. Can be SMA, U.FL or whatever, but something that you can connect to directly. I like to do this with a component in the signal chain that can be populated one way for the onboard antenna, or rotated 90 degrees to go to the connector instead.
  • Include a power connector and LDO to allow powering from a wall-wart. Makes it much easier to hand over to the firmware engineer and reduces unnecessary thought.
  • Enable power measurement: add a 0.100" header between LDO output and power input. This will have a shunt that can be removed and an ammeter attached instead. For extra credit, do this for the major power subsystems of the circuit.
  • Use full-sized JTAG connectors, and used shrouded (polarized) headers to prevent having to think about it.

Also, if this will be a commercially available development board (not just an intermediate step towards a final design) then there are a few more recommendations:
  • Use larger packages to make it easy to assemble and modify. I like to use 0805s for all the non-RF passives, and use the largest package I can get for the micro. And label everything!
  • If the board has onboard peripherals (LEDs, buttons, etc.) then make them able to be disabled (disableable?). My favorite way to do this is with "Cuttable Jumpers", basically an 0805 footprint with a trace connecting the two pads. That way a user can cut the trace to disable the peripheral but can also re-enable it by soldering on a zero-ohm resistor across the pads.
  • Be sure that you include firmware drivers for all the peripherals. You have to develop them to test the board anyway, might as well provide them.
The best development boards I've seen (besides the ones that I've designed of course) are the Stellaris development boards from Texas Instruments. Sadly, that company also makes some of the worst development boards too.

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