Deconstructing the AR.drone: Part 5

In this article I will describe some of the tools I've been using in my quest to understand the AR.drone. Some of them I had to build myself; that's part of the fun.

When it's engines aren't rotating, the 11.1 volt AR.drone battery lasts considerably longer than the nine to twelve minutes of flight time you typically get. But I wanted to spend as much time as necessary poking around in the drone's software without having to worry about losing power. Unable to find the battery connector locally, I bought the cheapest used battery I could find off EBay, snipped the connector off of it, and attached it to a 12V power supply I got from Radio Shack.

Important safety tip: I probably voided the warranty on my drone as soon as I hooked it up. But it seemed to work fine and it permitted me to work at a much more leisurely pace. If I'd had an expensive lab bench-quality power supply, I probably would have used that instead.

The AR.drone has an eight-pin (one pin missing) diagnostic connector accessible from its underside, covered with a rubber plug with a USB symbol on it. This shows my drone lying upside down with the plug removed and a logic analyzer hooked up to three of the pins.

Important safety tip: the AR.drone really doesn't like sitting at much of an angle and its software will complain bitterly and eventually refuse to do useful things. But placing it upside down was relevant to my interests.

This is Parrot's diagram describing the USB cable you can build that allows you to flash new software onto your drone without relying on the wireless network connection. What this diagram doesn't tell you is that pins 4 and 6, shown unconnected here, are receive data (a.k.a. RX, RD, RXD) and transmit data (a.k.a. TX, TD, TXD) respectively for a TTL-level serial port right off the drone's microprocessor. Pin 7 is ground just as with the USB cable. This serial port is configured to be a console terminal for the system.

Important safety tip: the serial port pins are transistor-transistor-logic (TTL) level, which on the drone's ARM-based Parrot P6 processor means something like zero to 3.3 volts. RS-232 peak signal levels vary widely, but logic values can range from -15 to 15 volts, and the standard requires that hardware tolerate voltages as high as 25 volts. Standard compliant RS-232 ports will not recognize the input signal levels of the drone's serial port, and peak RS-232 output voltages may damage the microprocessor in the drone. More on this below.

This is the USB flash cable I fabricated using the Parrot cable diagram above. I just bought a USB cable at Radio Shack, hunted down another cable that had the right eight-pin Molex connector on it, snipped the ends off both cables and did a bit of soldering. I've successfully used this cable, and some software tools developed by others, to flash my drone with the latest drone software from Parrot. I'll describe that process in a later article.

This is a DB9 null-modem serial cable that I fabricated using the same approach as the USB cable, using the transmit and receive pins and the same ground pin as the USB cable. After using these cables for a bit it occurred to me that an even better approach would have been to build a single Y-cable with a the eight-pin Molex connector on one end and both a DB9 and a USB cable on the other end. That would have given me console access while reflashing my drone.

This is a commercially available USB to 3.3 volt TTL DB9 adaptor that uses an IC from Future Technology Devices International, Limited (FTDI). It automatically does the conversion from the TTL signal level to USB serial. Otherwise it works just like any other USB serial adaptor.

This is not my first experience with FTDI products, and it won't be the last; I'm an FTDI fanboy. This cable uses one of their ICs embedded in the DB9 hood. This same or similar FTDI chip can be surface mounted in a embedded application. This is a huge win. First, you can use one of the mini or micro USB B connectors, which have a tiny footprint compared to other serial connector alternatives, as a console port for your application. Second, USB cables with the right connectors are easier to come by these days than serial cables, and it is impossible to hook them up wrong. Third and most importantly, the FTDI chip is powered not from your embedded application, but over the USB cable from your laptop or desktop; this means it doesn't draw power when it's not being used, and it's active and running long before your microprocessor comes out of reset and the boot loader configures and starts using the serial port. (A big Thank You to my hardware colleagues for introducing me to this.)

Important safety tip: this is the third such device I've tried on the AR.drone, and is the only one that has worked, which it did flawlessly. The others, made by companies I'd never heard of, were crap. (This is in fact the reason for the long pause between articles on this project.)

It may sound like overkill, but I used a logic analyzer to make sure I had the pins right on the serial console port. But at around US$150 for an eight channel logic analyzer, that you can fit in your laptop bag, with a 24MHz sampling rate and serial decoders, doing so was kind of a no-brainer. I used a Saleae Logic, one of many USB-connected logic analyzer pods and software packages used in conjunction with a laptop. This is the Logic pod hooked up to my little HP 110 Mini netbook running the Logic software.

Here is the nice zippered case (complete with cat hair, I now notice) that the Logic comes in, and its contents, along with my little key ring pocket knife just for scale. The Logic pod is about one inch or so square and is shown here with a company property sticker on it that almost covers the entire unit.

This is a screen snapshot of the actual hexadecimal serial decode when the Logic was hooked up to the AR.drone while the drone was booting. It captured part of a "BMI Write Memory" message.

When you're building cables it pays to check voltages and continuity before committing yourself to your soldering. I use a Radio Shack digital multimeter. It's probably the slowest reading digital multimeter I've ever used. But it's also a fraction of the price of the nice lab bench-quality units I've used in the past, and otherwise seems to work fine.

If you're going to solder, particularly if you're going to solder digital components (as I did when adding an expansion connector to my Beagle Board), a quality soldering iron is an absolute must. And it'll make cables too. This is a Weller WESD51 Digital Soldering Station with digital temperature control. I really like the fact that I can just dial in the recommended soldering temperature right from the manufacturer's data sheet, and the unit heats up in seconds.

This is a Weller 6966C 250 Watt Industrial Heat Gun. It's overkill for use on shrink wrap when making cables. But it's ESD safe and gets hot enough to melt solder, so it is not one of Alton Brown's dreaded uni-taskers. Alton would also use it to brown the top of Creme Brulee.

There are also some tools in my AR.drone arsenal that I haven't quite needed yet, or which are more in the experimental stages.

Parrot sells a small tool kit with the specialized parts necessary to take the AR.drone apart and to replace parts like the propellers likely to be damaged during normal use. I'll be using these when I follow in other's footsteps and open the drone up to take a closer look at its digital components.

For sure when I open the AR.drone up I'll be using a grounded Electro-Static Discharge (ESD) or anti-static mat and wrist strap. I have a permanent one wired up on the computer desk where I do most of my poking around (you can see it in some of my other articles). But I like to keep this fold-up field service model in my big tool kit.

In one of my more speculative side-projects, I'm experimenting with how the AR.drone uses its ventral camera to stabilize itself during hovering. Consensus opinion on the web is that it is using edge detection, so I am using a vinyl checker board as a high contrast take off pad. Others have suggested that sometimes the drone has problems seeing the take off pad in dim light, so I'm trying to illuminate it with a tiny button cell-powered flashlight whose LED emits in the infrared (IR). Finally, I'm using an IR night vision viewer to see the LED myself; remarkably, this viewer can be found in your local toy store. You may also be surprised to find, as I did, how many LEDs in objects around the house emit in the IR portion of the spectrum.

Next up: I describe how I flashed new software into my AR.drone over its diagnostic port via the USB cable by depending upon the kindness of strangers.