There’s making Hardware, and there’s making it interact with you!

Background

For this week’s assignment we started our own board design. The goal was to replicate the Echo Hello World Board: A simple board with an ATtiny, an FTDI interface, capacitor for GND, and a pull-up resistor for ResetN. We were asked to add some components such as LEDs and/or Push buttons.

Eagle, Board Design

I’ve always designed and simulated with Spice, so Eagle was new to me. As a first step I created the schematics for the board, and added a few features:

• A button with a pull-up resistor (default input value for the pin will be 1, when pushed will be 0)

• An LED connected to an output pin.

Here’s a sketch of the extra parts added:

And here’s a screenshot of the Eagle schematic:

As one can see, this is a bit messy! Wires on top of each other is not something that will work for us while using the Modela. So moving on to the Board design itself:

• Added a ruler. Why? No real reason. Because it’s cool.
• Shifted the components around to try to get it all into a 2D layout. This was the best the AutoRouter could come up with (as we can see there are still some unrouted traces):

(There’s a mistake here! We’ll get to it later)

In order to wire the traces that are unrouted, I realized I would have to either reposition everything, or use some 0-Ohm resistors as bridges. I chose the latter option. Eagle is pretty annoying here - putting a zero ohm resistor, even though it is only a Board consideration, requires going back to the schematics. So I gave up on Eagle, exported what I had so far to an image, and added the 0-Ohm resistors in my favorite Image editor. Here’s the result:

(There’s still a mistake here! We’ll get to it later)

Here’s a link to the outline png.

Millin’!

Here are a few takeaways from the Modela process:

• People always leave the Modela with the outline bit on. Make sure to change before you run the traces!
• Modela likes white on black, not black on white (think of it this way: Modela etches what will not be conductive). So make sure you invert the image before running!
• Some delicate areas (see with a black circle below) were not milled properly because of the resolution. That’s not a problem though - a small cut with an exacto-knife to separate the pieces does the trick. Just remember to check with a voltmeter that the two separate parts are indeed not short-circuited.

Soldering

No real issues here, pretty straight forward. Just remember to check with a voltmeter that the two separate parts are indeed not short-circuited (this is especially important when using 0-Ohm resistors as bridges). I applied a generous layer of hot glue (insulating material) to make the FTDI interface stronger.

The biggest mistake I’ve made yet!

Just as I was about to connect my FabISP to the Hello World chip, I noticed a HUGE mistake. I connected the wires on the 2-by-3 in a mirrored way. Meaning, if we look at this original schematic we see that VCC can be either on the bottom right, or, when ROTATED (not mirrored), on the top left. I accidently mirrored the wires and had it on the top right. Fixing this means redoing everything, and I wasn’t in the mood for that, so I made a female-to-male cable that mirrors the pins.

The soldering work here wasn’t perfect, and I also applied a generous layer of hot glue to protect it. At some point one of the pins broke, but I managed to replace just this one single pin from another connector. Ugly - but functional!

Here’s how the final setup looks like for programming:

Programming

I basically followed This tutorial which is fairly simple. Just have to remember to connect the cables in the right direction.

• Installing Pyserial on mac is just once command: sudo pip install pyserial. (This may require some extra installations that I already have on my computer though):

• Making the fuses:

• Programming:

IT’S ALIVE, ALIVE!!!

After programming, we can disconnect the FabISP from the computer and work with the new board standalone. Here’s a video: