As part of the building blocks series, I thought I would share a modular microcontroller design that I came up with over two years ago and that I am using for the scoreboard project. The Arduino craze not yet peaked and I was not fully aware of what it was, however, I did see some value in what they were doing and couldn’t help but allow the Arduino influence my design. Basically I took the shield concept, added a passthrough bus, and threw out the rest. Arduino is beneficial to those that don’t have a Programmer or who are not familiar with C/C++ or assembly. I had both, so I saw Arduino as having way too much circuitry for my tastes. So, what did I do? I built my own. (This is a common theme with me, in case you haven’t noticed.)
What follows is an admittedly lengthy excerpt of a contribution that I made on the Society of Robots website back in 2008, where I first cut my teeth in all things electronics and mechanics. It’s interesting for me to go back and read my thoughts back then.
A Modular PCB Layout
Introduction:
This project started as a result of my frustration and consternation with etching my own PCBs. Now, if I could draw out a board to perfection in Eagle on the first try, I probably wouldn’t have thought of doing this. However, my Eagle skills are somewhat lacking, as are my soldering skills. I needed a simple way for me to avoid having to build and debug large and complex boards for each project on my list. In addition to reducing the risk of me screwing up a large board, I found that it creates a bit of a “plug and play” environment where I can add and remove modules as needed for my various projects. Changing from a robot controller with motor driver and sensors to a temperature sensor with LCD display is as simple as swapping out the modules you need and reflashing the AVR. (I know, I don’t have an LCD module yet, but I will!)
As I took a look at some of the projects I wanted to create, I realized that there was a common basis for these projects; what I will call the AVR baseboard. There was always a socketed AVR (atMega168 for me), a reset button w/ resistor, and a decoupling capacitor. I wondered if I could build a VERY simple AVR baseboard and then just create modules for the functionality I wanted to add to it. Hey! While I’m at it, I could make them all stack together in a nice neat stacking bus arrangement. At first, I thought it was going to be a bit ambitious for someone of my skill level. Well, I did it, it seems to work pretty well, and I want to share it so that I can inspire others and also (selfishly) get some good feedback from the community on improvements, errors, etc. Here’s how it goes.
Form Factor:
First, I decided on a physical form factor. If I was going to connect multiple modules together, it would be nice if they were all of a “standard” size. I picked a 2” by 3” PCB dimension for the AVR baseboard and for a large module. I picked two other sizes for smaller modules, 2”x1.5” and 1”x1.5”. (see the first attached graphic “Module Sizes.png”)
This effectively gives me a full size, half size, and quarter sized modules. It also breaks the AVR baseboard into what I like to call “quadrants”. I’ve labeled the quadrants A, B, C and D. Each of the modules that I’ve built are built specifically for one or more quadrants. You’ll see why when I go into detail on the ISP and Serial modules.
