Since I got the prototype working, I now need to convert it to something real. In the past, I’ve typically done this by hand on perfboard. This takes a lot of time to do, and the results are functional but not that great.
This time I’ve decided to go direct to a PC board, so I fired up Eagle. I think this is my third design with it, so I’m reasonably decent at it, but I’ve forgotten how strange CAD user interfaces always are; they were developed before much was known about how to build good graphical user interfaces, they are all different, and they don’t follow UI guidelines. Oh, and they tend to have some weird metaphors as well, and there is no equivalent of github or stackexchange for them.
I installed the Adafruit library by downloading it, unzipping it, copying it to a directory and then adding that directory to the library path (nuget, anybody?) so that I could start with their arduino shield outline. It has all the pins on it. I then searched for the 74HC589 shift registers. No luck, so I stated to design my own, went back, did some more browsing, and found the 74xx589 SOIC-16 design, so I could use that. I need a 34-pix (2×17) connector header; I found a 2×20 and modified it. Then, I built a new version with named pins to make it easier to wire things up.
At that point, I started the wiring. A bunch of work, an “invoke” to get the Vcc and Gnd to show on the shift registers (once again, CAD weirdness), and I ended up with the following schematic.
Basically, we have the 8 data lines and two strobe lines going from the connector to the shift registers, the two strobes heading to Arduino (pins 2 and 3 because they are the ones that support interrupts), and then a bunch of lines between the Arduino and the shift registers. Prettiness isn’t important here, though it needs to be neat enough to know whether it’s correct or not.
I also threw on two test point headers; one for ground and one connected to pin 4. I often use a pin to write debug information for an oscilliscope, so the headers give me an easy way to hook that up. I run the schematic rules engine, fix a few things, and the schematic is done.
Once you have the schematic in good shape, you can create a board to implement it. I should have taken a snapshot at the beginning because it was quite the mess; there are 8 data lines that need to make it to both shift registers, 3 shared lines from the arduino to teach shift register, plus Vcc & Gnd. Lots of overlap potential.
I like doing board layout; it starts out looking pretty hopeless, and you play a little, it gets better, and then finally it sort of jells and then you are doing cleanup. Here’s the board layout I ended up with:
Lots of left-right rights to route all the signals that go to both chips. I’m pretty happy with this layout; it’s reasonably ordered, and you can tell where things go. I run one line right under the pins of the shift registers; something that I couldn’t do if they were through-hole chips.
Two things I could have done; I could have switched pins 2 and 3, and I could have pulled one of the signals off of pin 11 and used pin 7 instead. I didn’t because I would need to redo the software; a small thing, but the current layout works fine.
Once the layout is done, I run through the design rules. It’s really easy to end up with weird stuff; multiple vias slightly overlapping, traces that don’t quite go to pins, that sort of thing. I’m planning on fabbing the board at OSHPark.com, and I remember that they have their own design rules, so I download them and run them.
Then the last task is to shrink the design down from full shield size to something smaller to save a bit of money. Oh, and I add a label or two. Upload it to OSHPark, and they tell me $20 for 3 boards. Pretty good.