FTDI USB driver woes on Debian LiveCD

Just a quick info that might be useful for some of you Xbee users. I wrote this little software to visualize and plot Xbee accelerometer data a while back.

Now, my brother tried to use it, and since he is not into Linux at the moment, I prepared a LiveCD for him using the Debian Live project. The reason why I did that is because the python program needs wxpython which is not included in the default Debian LiveCD.

The problem is that the kernel currently used (at the time of build: started Mon, 05 Mar 2012 20:21:01 -0600 and ended Mon, 05 Mar 2012 20:52:50 -0600) includes a buggy driver for the FTDI USB kernel driver. Details about the bug can be found here :
http://comments.gmane.org/gmane.linux.usb.general/44150

and changes needed to any python software (including my own xviz) that wants to work around this bug, so that you don't need to replace the kernel or rebuild the liveCD, are to look for a line like this:

 self._port = serial.Serial(self._comport, 9600)

and add a rtscts= parameter like this:

 self._port = serial.Serial(self._comport, 9600, rtscts=1)

This tells the driver to open the serial port using RTS/CTS flow control, which actually disables it, because the driver inverts both states, duh.

Realtime visualization for Xbee+Accelerometer data

Here is the long awaited follow-up to the Xbee experiments that my little brother outsourced to me.

Before I get to the description of the software itself, if you want to try this out, be sure to follow the guidelines in the previous post on the same topic. You are then ready to proceed.

And here is what you get with this software :

• Realtime visualization of all three axis
• Can be adapted to many different accelerometers by tuning some global variables
• Recording of samples to file, with timestamps and X, Y and Z values
• Automatically generated plots for each axis and a combined 3-axis plot
• Exported files named according to experiment date and time (for easier sorting)
• All the above, in pure opensource python goodness
• English and French localization
• Theoretically works from within a LiveCD

Now let me briefly describe what the software does. As you can see in the screenshot on the side, the user interface is quite easy to understand. The three rectangle-shaped black boxes are showing whatever comes from the Xbee chip hooked up to the USB port, in realtime.

There are also three clickable buttons. The leftmost button labelled 'start' triggers the beginning of the recording. You will usually push this just before your experiment. The middle button named 'stop' obviously halts the recording. It also takes care of dumping all the samples into a CSV and generating the four plots : a combined plot with all 3 axis and three separate plots, one per axis. Of course the last button closes the USB port and quits the program. Beware that it does not check if you have a running experiment and will hence lose some data unless you push the stop button first.

Enough talking now, here is the python archive, all you have to do is extract the contents somewhere and run the software by launching the xviz.py script file.

I also uploaded this to the github social coding site, feel free to fork this, submit patches or whatever !

https://github.com/jean-/xviz

Accelerometer + Xbee = geekish fun

Hi there, I hope you are all having a nice holiday season.

As for me, I am keeping quite busy despite the loneliness and am desperately trying to do my job, some volunteer help for the Tohoku area, turning the website for our PTA upside-down, transforming a two storeys (stories?) open room type house in a 3+LDK, and helping out my lil brother on his science assignment. And there's today's story.

So introduce my brother, who has been playing with water rockets since he was in high school. Now for his first year at university he managed to get himself a science assignment dealing with those

very things. I won't explain the principle at hand here because I already have lots of things to say, so look up wikipedia (I suggest reading the discussion page there too, it is abysmal in epic proportions) or check out this NASA picture.

Now you can imagine that you can do lots of fun stuff with your own rocket, especially when you learn that the record height achieved is around 600 meters... Well, a 2L soda bottle won't go that high, but it is still a very interesting way to conduct extreme experiments around embedded/autonomous systems.

When I was around the parents' home back in France, he tells me about his latest experiments

and how they thought they wanted to get telemetry readings for the system. That sounded cool, and I was thinking about my co-worker here who does all those neat things with arduinos, xbees, sensors and so on...

Actually, my brother and his friends' main skills are neither geekygadgetry nor computer science, so the final picture is not as complex as I imagined, but still beyond their reach. Based on a survey around his friends and professors, he gathered that Xbees are cool and can work in an autonomous way without any microcontroller, which is good when you don't have a clue. They are small chips (see pic on the right) that do 802.15.4, present a serial emulation to the user, feature 5 analog inputs and an associated ADC, and have a decent (depending on your own definition of decent) sampling frequency.

Well, to be honest he actually told me about this all on the phone before I came back, so that I could do a small shopping trip around akiba and bring an accelerometer. There it is : KXM52-1050, straight from Akizukidenki, behind the now defunct Yamagiwa/Livina (just saw they were doing some construction work there, something happening soon I guess). So this little chip eats 3.3V which is all great because most of the boards that can host the Xbee provide a 3.3v regulator to feed the sensors.

Now here's his plan : one Xbee on the rocket, with some form of power supply, hosted on a regulated board, where to hook the accelerometer sensor as well. On the receiving side, another Xbee (using just one doesn't make sense, does it ?) on a USB board plugged directly into a computer, and voila. It sounded so EASY.

Now that I look back on the whole process, it was. But for some reasons, the Xbee manufacturers went through great pain to write a very detailed yet completely useless documentation. All the information needed is inside, but after reading the whole 68 pages, I felt like I did not know what to do. Also there has been several versions of the chip, things changed, and there is no clear history of what went and what didn't. One important thing I learned at my expense is that the ADC used to do the conversion on the fixed 0~1.2V range only. But, in the latest document there is no mention about the voltage range. There is a two-line section about ADC that states that "Xbee supports ADC and here is how to enable it". Period. After lots of digging I found out that the "Pin signals" table stated that Pin14 is for "Voltage reference for A/D input". If the 68 pages were a haystack, this line would be the needle.

And then, the plan worked ! That is, until I saw the "software" that some friend gave to him in order to do the Xbee monitoring. If you can call a VB 20-liner a software, that is. Which is way I made another one for him that does .csv export and much more. More on it in another article but meanwhile, enjoy the output. Guess which line's the Z ?

PS: I know I am getting behind schedule with the follow-up to the pachube article. It is still in the pipeline...