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Beginner’s Luck (KiCad Part 2)

A project log for RAIN Mark II Supercomputer Trainer

High-Performance Computing for Everyone

jason.gullicksonjason.gullickson 04/23/2018 at 18:120 Comments

In the last log I left-off with a 2-D printed version of a printed circuit board I designed in KiCad which was enough to confirm that the board I was designing would fit into the board I’m designing it for. This felt like a lot of progress!

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However there was still a lot to do, primarily consisting of making the actual electrical connections between the components on the board. For some dumb reason I thought this would be easy, but I ran into problems getting the traces to connect, or prevent them from overlapping, etc.

Part of the problem is that I have no real experience in the more abstract (i.e., not related to learning the software) aspects of PCB design. Luckily Bob was willing to help me out and gave me a list of steps to use as a starting point when laying out a board:

  1. Lay out the connectors where they have to go
  2. Logically place groups of components where they need to go (power section should be in one area, microcontroller + decoupling caps in another area, etc.)
  3. Refine component placement to have as few overlapping airwires as possible to ease routing
  4. Route length sensitive traces first
  5. Route other traces
  6. Route power traces, but do a ground pour to make it easier
  7. Tidy up
  8. Lay out silkscreen and be as descriptive as possible

In addition to this, Bob said traces could be 6 mil wide minimum, but to aim for 10 mil in general and 12 mil for power.

This advice helped a lot and between this an finding a “mode” for laying traces that worked for me, I was able to connect all the parts and get the board to pass the Design Rules Check (DRC). Bob eyeballed my layout, gave me some tips on improving it and said it looked like it would work.

panel_driver_pcbnew

It was around this time that I noticed an error in the pin assignment where the panel driver board connects to the headers on the Clusterboard. Since the driver board will connect using a right-angle connector, but I designed it with a straight connector in mind, pin 1 would end up in the wrong place. This was easily fixed by rotating the connector on the board, but since this connector has traces leading to almost every other component, the layout became a complete mess. Instead of fighting with this, I took the opportunity to redo the board from scratch and apply everything I’ve learned so far.

The second time around went much faster and I think it looks nicer as well.

panel_driver_v1.4.0

After reviewing the design (for what felt like the millionth time), I was reasonably sure I didn’t make another mistake like the connector and decided it was ready to upload to OSH Park for production.

panel_driver_osh_upload1.png

This process went very smooth. I don’t have any personal experience to compare it to (being the first board I’ve designed to be produced this way) but the website was easy to use, the visualizations and “checklist” guides were very helpful and I felt like I had a  clear idea of what the final product would look like when they were done.

Now it’s just a matter of waiting.

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The boards showed up a few days early and I was lucky enough to have time to try one out (the minimum order was 3 boards).  I had only enough parts on-hand to complete one, but this was intentional because I assumed that I would have made some mistakes and that I could order more parts after I fixed the design and re-ordered a second batch of boards.

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After assembly, I tried to quell my excitement and properly check-out the board in steps (inspired by Bob’s article) to reduce the chances of burning-up the new panel driver board or worse, the precious Clusterboard.

First, I tested the driver board alone to make sure power was flowing to the right places.  Then, I attached it to the Clusterboard and checked the i2c bus to see if it was showing-up correctly. Finally. I wired-up the LED’s and toggle switch and ran the python script.

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Much to my surprise, the LED’s lit up and the switch correctly toggled between display modes!

Something was not quite right in that the LED’s don’t illuminate in exactly the right order.  It’s not clear yet if that’s a problem with the LED wiring or a design error on the board. The good news is that I might be able to correct this with a software change, so at least the boards themselves may be salvageable. Even if I have to make changes to the board to fix this, there’s a number of improvements I’d like to implement anyway so this won’t be the final version of the panel driver board regardless.

It’s hard for me to express how exciting it was for me to complete this process. Learning to design circuits using an EDA is something I’ve put-off for a long time, but now that I’m getting the hang of it I can see so many ways in which learning to use these tools will expand and accelerate my ability to make things. There are many ideas I’ve had in the past that I didn’t pursue because I knew they would involve working with components that I couldn’t use with the old construction techniques, or that wouldn’t be practical to produce in any volume using those techniques.

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I have a feeling that learning these skills will be like building my 3d printer. Building the printer taught me a lot, but also opened my mind creatively by making it faster and easier to make both things I was already making but also things I never considered because I didn’t have the tools/skills/materials necessary.

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