In the spirit of Halloween (which I realize was nearly a month ago), I’ve done some unholy, Frankenstein-esque modifications to Colin’s brain!
Moving the Arduino to the Breadboard
First of all, I’ve been using an Arduino as Colin’s brain but an Arduino is actually pretty bulky, considering its capabilities. So I’ve replaced this:
with this:
It’s an ATmega328; the same chip the Arduino uses. If you flash it with an Arduino bootloader you can program and use it exactly like you would an Arduino. This page has a great tutorial for programming and using the bare chip on a breadboard. Also, if you’re willing to pay a little extra you can buy a chip that has the bootloader pre-installed. Both SparkFun and Adafruit sell pre-programmed ATmega328s.
To switch from an Arduino to an ATmega328 I just needed to rejigger my power supply a bit and rewire everything for the new pin mapping (good thing I kept detailed notes on my wiring, right?)
Next, Colin’s control libraries are getting pretty big. Right now they take up about 25% of the ATmega’s memory. When all’s said and done, the control libraries should only be a small part of his program’s total size. So we’re going to need some more space before long.
Also, writing and testing code with the Arduino is getting to be kind of a pain. First, you have to upload your program with the Arduino IDE every time you make a change or want to run a new program. This can get pretty cumbersome, especially if you want to make minor tweaks to a program or try different versions of it. Also, there’s no possibility for multi-threading or concurrent execution. So you’re pretty limited in what your programs can do.
Adding Computing Power!
The solution to this could be to use a more powerful single-board computer like a Raspberry Pi. Since a Pi just runs linux, I can run write, edit, and compile programs on the Pi itself. Also, because it has WiFi, I can leave it connected to Colin and control it via SSH. This makes programming much, much easier. However, it has a couple of significant downsides. It only has one PWM pin, so it can’t directly control two motors. Also, it can’t handle hardware interrupts so it can’t really use motor encoders either.
My solution is to keep the ATmega for low-level motor and sensor control while the Raspberry Pi handles the high-level processing. The Pi and the ATmega will communicate over a serial bus. They will use this to relay commands, position updates, sensor readings, and so on.
This means Colin now has two brains! This is what he looks like with his new braaaains!
Note that the Raspberry Pi uses 3.3V logic and all of the other hardware runs at 5V, so I need to use a level shifter to communicate between the Pi and the ATmega. Also, the Raspberry Pi runs on 5V USB power. I already have my voltage regulator supplying 5V for my ATmega, so I simply cut the end off an old micro USB cable and connected the +5V and GND wires to my breadboard’s power rail. The Pi draws a lot of current, around 1.3A under stress, and my voltage regulator can only provide 1.5 to 1.75A. So I might need to upgrade to bigger voltage regulator soon.
I’m also re-configuring Colin’s sonar sensors. I will start out with 8 sensors arranged in a ring, controlled independently by an ATtiny84. Since each sensor has a range of about 15 degrees, 24 total sensors are necessary to completely cover 360 degrees. I’m not sure I’ll need to actually use 24 sensors, but my design will allow me to add additional sensor rings if necessary. I’ll go into detail on these in a later post.
I’ll explain how to coordinate the raspberry pi and the ATmega328 using serial communication in a later post, and I’ll provide an overview of my ATtiny-based sensor controllers shortly thereafter!