Computing stuff tied to the physical world

Archive for June 2013

From PWM to voltage

In Hardware on Jun 30, 2013 at 00:01

The Arduino library has always had an “analogWrite()” function, even though the ATmega doesn’t have any way to generate a varying voltage. So why the name?

Well, what most microcontrollers can do is generate a pulse-width modulated signal, also known as PWM. That’s just a fancy way of saying that the microcontroller periodically generates a pulse, of which the width can be varied under software control.

Here’s a diagram from the Arduino PWM tutorial page:

pwm

It’s very easy to generate an approximate voltage between 0 and VCC by simply adding a resistor and capacitor to act as low-pass filter:

CRLowPass1

That image came from an excellent article by Scott Daniels about this same topic on the ProvideYourOwn.com website. Check it out for much more background information.

So how do these two little components turn PWM into a voltage?

To see what’s going on, we can tie the resistor to a PWM pin, call “analogWrite(N,128)” to set a 50% duty cycle, i.e. halfway, and then watch the result on the oscilloscope:

SCR79

(the above was generated with a 10 kΩ resistor and 0.1 µF capacitor – see this calculator)

What you’d want to see, is a 1.65 V output. As you can see, the real thing is indeed centered around that value, but with a lot of “ripple” (yellow = PWM, blue = Vout).

The reason for this is that the PWM is charging and discharging the capacitor. The average value will be the ratio of on-to-off time against the digital voltage, i.e. 3.3V, so a 50% duty cycle will lead to half VCC, i.e. 1.65V. But it’s still charging & discharging.

Can we get a cleaner signal out, without that up-and-down wavering? Yes: by increasing the repetition rate, i.e. the PWM frequency (while keeping the on-off ratio the same). There’s a page on the Arduino site about this. Here’s the result, with a 64x higher rate:

SCR80

The purple line is a third probe on the same output signal, but set to AC coupling and much higher sensitivity, to show that the ripple is still there, but much much lower.

Now I can get to the point of this post: my intention was to see if I could generate a clean sawtooth wave, to use as supply voltage (after amplification) for testing circuits at various VCC levels – as used a few times in recent posts. Here’s the code I came up with:

void setup () {}

void loop () {
  for (int i = 0; i < 256; ++i) {
    analogWrite(5, i);
    delay(10);
  }
}

The result is this:

SCR81

And now there’s indeed a sawtooth, but as you can see, sudden changes like the down edge take some time to pass through the RC filter, somewhat messing up the signal.

No problem, if we increase the delay time in the above loop by a factor of 50:

SCR84

Voilá – things are fine now. A pretty smooth voltage ramp from 0 to VCC every 2 seconds, all from a single digital output pin on the ATmega – we’ve created a DAC!

FanBot Registration System

In Linux, Software on Jun 29, 2013 at 00:01

Nearly 400 FanBots have already been built – see this page and the KekBot weblog.

To finish this little series, I wanted to describe the little setup I built and how it was done. Nothing spectacular, more an example of how you can create a software cocktail / mashup to get a one-off short-lived project like this going fast.

First the basic task at hand: allow kids to register their FanBot and print out a “passport” with a picture of their creation and some information about when and how to get their FanBot back after the RoboCup 2013 event. At the same time, it would be nice to have a monitor display the current count and show images of the last few FanBot pictures taken.

passport

I decided to use 3 Raspberry Pi’s as “slave”, each with a webcam, an LCD screen (thanks David, for the laser-cut frame!), and Ethernet + power. The code running on them is on github. It would be nice if each slave could continue to accept registrations even with the network connection temporarily down, so I went for the following approach:

  • collect registrations locally, using rsync to push new ones to the master
  • everything is based on a few simple shell scripts for quick adjustment
  • two C/C++ programs were needed: to read the FanBot data and to drive the LCD
  • do a git pull on power-up and re-compile as needed before starting operation
  • periodic webcam readout and periodic rsync, using loops and sleep calls

For the master, I thought about using the Odroid/U2, but decided against it as there would be too much setup involved in the last minute. Next idea was to use an Acer Inspire One Linux netbook, but it turned out to be way too slow. For some reason, Node.js was consuming 100% CPU (I suspect a bad nodemon disk change polling error on the slow SSD). No time to figure that out, so I ended up using my second Mac laptop instead.

The master logic started out as follows:

  • an rsync server was set up to collect files from all the slaves in one spot
  • the display is presented via a browser in full-screen mode and Node.js
  • the original plan was to also allow outside access, but we never got to that
  • for live updating, I decided to use SocketStream (same as in HouseMon)
  • a PDF was generated for each image by convert, which is part of ImageMagick
  • an AppleScript then prints each PDF to the USB-attached laser-printer

These choices turned out to be very convenient: by having the rsync folder inside the Node.js application area, and using nodemon, refresh became automatic (nodemon + socketstream will force a refresh on each client whenever a file system change is detected). So all I had to do was write a static Node.js app to display the count and the latest images.

So far so good. A couple of glitches, such as an erratic printer (new firmware fixed it), an overheating RPi (fixed by clocking at 700 MHz again), and all the slave rsyncs fighting when the same registration was present on more than one (fixed by adding the “-u” flag).

During installation, we found out that there was no wired ethernet w/ DHCP, as I had assumed. Oops. So the Mac laptop was turned into a WiFi -> wired bridge, since WiFi was available most of the time. Whoops: “most” is not good enough, the RPi’s have no clock, they really need an NTP server – especially since the whole system was based on rsync and file modification times! We hacked a fixed DNS – but internet still has to be there.

And then the fun started – various little wishes came up after the fact:

  • can we get images + passports off-site during the event? sure: rsync them to Dropbox
  • could we have a page with all images? sure: on my server at JeeLabs, I made an rsync loop from one Dropbox area to another, and made a little photo-album using Dropbox
  • could we show a live count, even though the monitor website was not accessible? sure: I simply added another shell loop on my server again, to generate an image with the count in it, and made it show up as first one in the photo album.

Here is the shell script for that last feature:

cd ~/Dropbox/FanBots
while :
do
  N=$(echo `ls *.jpg | wc -l`)
  if [ "$N" != "$P" ]
  then
    echo $N - `date`
    convert -background black -fill red -size 480x640 \
      -gravity center "label:$N\n\nFanBots" 0-teller.png
    P=$N
  fi
  sleep 10
done

This can run anywhere, since Dropbox is taking care of all updates in both directions!

The point of all this is not which languages I used (your choice will definitely be superior), nor what hardware was selected for this (it changed several times). The point is that:

  1. it all got done in a very limited amount of time
  2. the basic rsync approach made things resilient and self-recovering
  3. github pulls made it possible to apply last-minute tweaks
  4. some nice changes were possible very late in the game
  5. dropbox integration made it possible to add features remotely

Happy ending: all the kids are having a good time … including me :)

PS. Tomorrow (Sunday) is the RoboCup 2013 finals. See you there?

Status of the RFM12B

In Hardware on Jun 28, 2013 at 00:01

The RFM12B wireless radio module has been around for quite some time. When I found out about it at the time, I really liked the mix of features it provided – far more capable than the morse-code like OOK modules in cheap sensors, very low power, and available at a considerably lower cost than the XBee and other ZigBee solutions out there:

rfm12b

There was little software for it at the time, and writing an interrupt-driven driver for it was quite a challenge when this all started, but nowadays that’s no longer an issue – the RF12 driver which is now part of JeeLib has turned out to work quite well.

There have been some rumours recently, and understandably also some worries, that the RFM12B might be phased out, but if I may paraphrase Mark Twain on this: the reports of the RFM12B’s death have been greatly exaggerated…

After some email exchanges between HopeRF and Martyn Judd, I’m now happy to report that we have received a formal statement from HopeRF to clarify the issue:

“The popular RFM12B S1/S2 modules remain in production with a large volume of long standing and long term orders. There is NO plan to discontinue the series and NO plan to end production in the foreseeable future. It is correct that we no longer recommend this series to customers developing new projects, since we have developed more recent designs, for example the RFM6X series. These incorporate functionality improvements that can provide a better price/performance for specific applications.”

– Derek Zhu, Marketing Manager, HopeRF

As I see it, this means that the RFM12B will be around for quite some time and that we don’t have to worry about supplies for keeping all current networks based on these modules working. I definitely intend to keep using RFM12B’s at JeeLabs and in products designed and produced by JeeLabs.

Having said that, I’m also evaluating alternatives and looking for a convenient option to act as next step forward. My focus is on low-power consumption and good range for use within the house, both of which I hope to take even further. I’d also like to make sure that over-the-air flashing with JeeBoot will work well with the current as well as future choices.

In my opinion the RFM12B continues to be a simple & excellent low-cost and low-power foundation, so I’m taking my time to very carefully review any new options out there.

Meet the FanBot

In Hardware, Musings on Jun 27, 2013 at 00:01

The FanBot is a very simple robot based on a small PCB with microcontroller by Peter Brier, some LEDs as eyes and mouth, and a servo to allow the robot to wave its arms:

DSC_4497

Over a thousand boards have been produced, along with accessories to let children create a little cardboard robot with their own name and a little program to store a personalised sequence of LED blinks and servo movements. The µC is an ARM LPC11U24 chip, donated by NXP – which has plenty of power, but more importantly: can be programmed by powering it up as a USB memory stick.

_MG_1606-2

Wednesday was the kick-off / trial day, with 120 kids dropping by and creating their own FanBot. The FanBots will all be used for the main event to cheer on the main RoboCup contestants. Here’s a quick impression of the first 80 “fans” (it’s a huge task to get them all up there, checked, stapled, and connected – not to mention the power/network setup!):

_MG_1704

It’s a really wonderful project, IMO: lots of young kids get exposed to new technology, learning about robots by building their own, and creating a huge collection of truly individual and personal robots, all cheering together!

For more info, see Peter and Marieke’s KekBot weblog – there’s also a progress page.

The RoboCup championship itself uses more sophisticated robots, such as these:

BvOF RoboCup2013

Many more pictures of this event can already be found on the RoboCup 2013 website and on their Flickr group. The event has only just started so if you’re in the neighbourhood: it’s free, and bound to be oodles of fun for kids of any age!

Myra and I had a wonderful time, and I even had a chance to see Asimo in action, live!

And JeeLabs ended up getting a spot on the sponsor page – not bad, eh?

Update – Forgot to mention that one of the requirements of RoboCup is that everything must be made open source after the event. This means that any advances made can be used by anyone else in the next year. What a fantastic way to stimulate shared progress!

Packaged but not virtualised

In Hardware, Linux, Software on Jun 26, 2013 at 00:01

Since VMware started virtualising x86 hardware many years ago, many uses have been found for this approach of creating fully self-contained software environments. I’ve used VMware many years, and was pleased when Apple switched the Macs to “Intel inside”.

Nowadays, I can run numerous variants of Windows as well as Linux on a single laptop. I switched to Parallels as VM core, but the principle remains the same: a large file is used as virtual hard disk, with an operating system installed, and from then on the whole environment simply “runs” inside the Mac host (in my case).

Nowadays, we all take this for granted, and use virtual environments as ultra-low cost web servers without even giving it any thought. I have a number of virtual machines running on a Mac Mini for all the JeeLabs sites in fact (and some more). One piece of hardware, able to suspend and resume N completely independent systems at the click of a mouse.

But there’s a drawback: you end up duplicating a lot of the operating system and main services in each VM, and as a result this stuff not only consumes disk space but also leads to a lot of redundant backup data. I keep the VM’s very lean and mean here, but they still all eat up between 3 and 10 gigbytes of disk space. Doesn’t sound like much, but I’m pretty sure my own data uses less than 10% of that space…

Both Parallels and VMware (and probably also VirtualBox) support snapshotting, which means that in principle you could set up a full Linux environment with a LAMP stack, and then use snapshots to only store what’s different for each VM. But that’s not useful in the long run, because as soon as you get system updates, they’ll end up being duplicated again.

It’s a bit like git “forks”: set up a state, create a fork to create a variant, and then someone decides to add some features (or fix some bugs) to the original which you’d also like to adopt. For git, the solution is to merge the original changes into your fork, and use “fast-forwarding” (if I understand it correctly – it’s all still pretty confusing stuff).

So all in all, I think VM’s are really brilliant… but not always convenient. Before you know it, you end up maintaining and upgrading N virtual machines every once in a while.

I’ve been using TurnkeyLinux as basis of all my VM’s because they not only maintain a terrific set of ready-to-run VM images, but because they also make them self-updating out of the box, and because they’ve added a very convenient incremental daily backup mechanism which supports restarting from the back-up. You can even restart in the cloud, using Amazon’s EC2 VM “instances”.

And then someone recently told me about Docker – “The Linux container engine”.

This is a very interesting new development. Instead of full virtualisation, it uses “chroot” and overlay file systems to completely isolate code from the rest of the 64-bit Linux O/S it is running on. This lets you create a “box” which keeps every change inside.

My view on this, from what I understand so far, is that Docker lets you play git at the operating system level: you take a system state, make changes to it, and then commit those changes. Not once, but repeatedly, and nested to any degree.

The key is the “making changes” bit: you could take a standard Linux setup as starting point (Docker provides several base boxes), install the LAMP stack (if someone hasn’t done this already), commit it, and then launch that “box” as web server. Whereby launching could include copying some custom settings in, running a few commands, and then starting all the services.

The way to talk to these Docker boxes is via TCP/IP connections. There’s also a mechanism to hook into stdin/stdout, so that you can see logs and such, or talk to an interactive shell inside the box, for example.

The difference with a VM, is that these boxes are very lightweight. You can start and stop them as quickly as any other process. Lots of ‘em could be running at the same time, all within a single Linux O/S environment (which may well be a VM, by the way…).

Another example would be to set up a cross compiler for AVR’s/Arduino’s (or ARMs) as a box in Docker. Then a compile would be: start the box, feed it the source files, get the results out, and then throw the box away. Sounds a bit drastic, but it’s really the same as quitting an application – which throws all the state in RAM away. The point is that now you’re also throwing away all the uninteresting side-effects accumulated inside the box!

I haven’t tried out much so far. Docker is very new, and I need to wrap my mind around it to really understand the implications, but my hunch is that it’s going to be a game changer. With the same impact on computation and deployment as VMware had at the time.

As with VM’s, Docker boxes can be saved, shared, moved around, and launched on different machines and even different environments.

For more info, see this page which draws an analogy with freight containers, and how one standard container design has revolutionised the world of shipping and freight / cargo handling. I have this feeling that the analogy is a good one… a fascinating concept!

Update – Here’s a nice example of running a Node.js app in Docker.

FanBots at RoboCup 2013

In ARM, Hardware, Linux, Software on Jun 25, 2013 at 00:01

There’s an event coming up in a few days in Eindhoven, called the RoboCup 2013:

teun

This is the world championship of robotic football. For all ages, and with various levels of sophistication. Some robots are tiny cars, trying to work together to play a match, some robots are little human-like machines consisting of lots of servos to make them walk on two feet, and some robots are very sophisticated little “towers” on three wheels, zipping along at impressive speeds and able to catch, hold, and kick a standard-size football.

I’m not directly involved in this, but I saw some of the preparations and it really promises to be a fantastic event. Lots of geeks with laptops, soldering irons, and mechanical stuff. The kick-off is next Wednesday and the event lasts for five days, with the finals on Sunday.

On the side, there is a gorgeous (not-so) little project going on, whereby kids of ages 8 to 12 can visit a workshop to build their own FanBot (for free). The FanBot can blink its eyes and mouth, and wave its arms – part of the exercise is to create a little “program” of things it does at the press of a button, so this will also be a first encounter with programming!

There’s a Dutch page about the project and a general page in English mentioning FanBots.

But the really exciting goal will be to create a huge stand for all the FanBots and make them cheer the football-playing robots in the main event hall. The aim is to get up to 1000 of them, all blinking and waving – and all orchestrated and controlled from a central system. It’s all being designed and built by Peter Brier and by Marieke Peelen of KekkeTek.

One task in the project is to register all the FanBots and print out a little “passport” as last step – which the kids can then use to pick up their own FanBot once the RoboCup 2013 event is over. This is the part I volunteered to help with.

I’ll describe the setup in more detail soon, but that’s where the Raspberry Pi’s w/ LCD screen and webcams will be used. It’ll be fun to see whether we can get 1000 kids to build and sign up their little robots during three days – there’s quite a bit of logistics involved!

Admission is free, so if you’re in the neighbourhood: don’t miss it – it’s going to be fun!

Using eMMC as root disk

In Hardware, Linux on Jun 24, 2013 at 00:01

The Odroid/U2 mentioned yesterday also has the option to add an eMMC card, right next to the µSD card slot – I went for a whopping 64 GB “SSD” in this case:

DSC_4489

The reason to use this, is that it’s indeed nearly three times as fast:

# hdparm -tT /dev/mmcblk0

/dev/mmcblk0:
 Timing cached reads:   1604 MB in  2.00 seconds = 802.41 MB/sec
 Timing buffered disk reads: 146 MB in  3.02 seconds =  48.40 MB/sec

That’s faster than the maximum transfer rate of USB 2.0, by the way.

But how do you set these things up? The bottom view shows yet another connector, yuck:

DSC_4490

It turns out that you can get an adapter from the Odroid/U2 shop:

DSC_4491

That turns it into a µSD card, but unfortunately it wasn’t recognised in my Mac laptop. Luckily, this concoction did seem to do the trick:

DSC_4492

Through this, I was able to burn the Debian disk image onto it, and through some clever logic in the bootstrap loader, the eMMC card simply takes precedence when present and starts up fine with the same device name as the initial µSD card (which now ends up on /dev/mmcblk1 if also present).

One last step is needed to resize the root partition to take full advantage of the entire eMMC card. Google found this article which explains the whole (nasty) procedure.

Final result:

# free
             total       used       free     shared    buffers     cached
Mem:       2031632      77660    1953972          0       9540      37420
-/+ buffers/cache:      30700    2000932
Swap:            0          0          0

And… tada!

# df -H
Filesystem      Size  Used Avail Use% Mounted on
rootfs           62G  1.2G   58G   2% /
udev            1.1G     0  1.1G   0% /dev
tmpfs           209M  222k  208M   1% /run
/dev/mmcblk0p2   62G  1.2G   58G   2% /
tmpfs           5.3M     0  5.3M   0% /run/lock
tmpfs           417M     0  417M   0% /run/shm
/dev/mmcblk0p1  133M  8.5M  124M   7% /boot

Neat, neat, neat – although the total system cost is close to $200 at this point, due to the expensive eMMC memory card – five times the cost of a Raspberry Pi, but with four times the RAM, a comfy 60 GB of fast SSD storage, and probably 5..10 times the performance.

It’s not for everyone and it has no GPIO pins to hook anything up, but I like it!

More about the Odroid/U2

In ARM, Hardware, Linux on Jun 23, 2013 at 00:01

As mentioned recently, there are many alternatives to the Raspberry Pi. I’m looking mostly at ARM-based systems these days, because Node.js support for MIPS is not really there (yet?), and you really want a JIT-type system to get the most out of a Node.js based setup.

Well, having received the Odroid/U2, first thing I had to do is get a micro-HDMI to HDMI adapter (always some new connector/cable type, yawn…). Also had to get a 16 GB µSD card, because the Debian image I wanted to try was for cards of 8 GB or more (yawn!).

But now that everything is in, I’m finally able to take that little black cube for a spin:

article_img

First impression: it’s fast, much snappier than a Raspberry Pi. Try launching aptitude, for example – which takes a long time on RPi’s and other low-end ARM & MIPS systems.

Both are running class-10 SD cards, as far as I can tell. The RPi is running at 700 MHz:

# cat /proc/cpuinfo 
Processor   : ARMv6-compatible processor rev 7 (v6l)
BogoMIPS    : 697.95
Features    : swp half thumb fastmult vfp edsp java tls 
[...]
Hardware    : BCM2708

The OU2 has four cores running at 1.7 GHz:

# cat /proc/cpuinfo 
Processor   : ARMv7 Processor rev 0 (v7l)
processor   : 0
BogoMIPS    : 1992.29
processor   : 1
BogoMIPS    : 1992.29
processor   : 2
BogoMIPS    : 1992.29
processor   : 3
BogoMIPS    : 1992.29
Features    : swp half thumb fastmult vfp edsp thumbee neon vfpv3 tls 
[...]
Hardware    : ODROIDU2

Disk speeds are interesting, first RPi then OU2:

# hdparm -tT /dev/mmcblk0
/dev/mmcblk0:
 Timing cached reads:   340 MB in  2.00 seconds = 169.91 MB/sec
 Timing buffered disk reads:  62 MB in  3.00 seconds =  20.66 MB/sec

# hdparm -tT /dev/mmcblk0
/dev/mmcblk0:
 Timing cached reads:   1616 MB in  2.00 seconds = 808.23 MB/sec
 Timing buffered disk reads:  52 MB in  3.05 seconds =  17.03 MB/sec

I haven’t set up the eMMC card yet, which is said to be several times faster than µSD.

Power consumption of the OU2 is 2.9 W on the AC mains side of a little 5V adapter I’m using. Not bad. I’m tempted to try and set this up as my only always-on server here at JeeLabs, including all the web server stuff even. There’s 2 GB of RAM in this thing, should be enough to run some serious stuff. Perhaps even the (Java-based) CrashPlan backup system I’ve been using for almost a year now.

I don’t really care that much about file storage – a 64 GB eMMC disk would be plenty for everything that needs to be online here at all times, perhaps with an external 2.5″ USB hard disk for secondary / archival backup.

Storage – let’s do the math

In Musings on Jun 22, 2013 at 00:01

Doesn’t it ever end? Storage density, I mean:

DSC_4487

That’s a 16 GB µSD card. There are also 64 GB cards, but they are a bit more expensive.

Ok, let’s see – that’s 0.7 x 11 x 15 mm = 115.5 mm3 in volume.

Now let’s take a 2.5″ hard disk. The dimensions are 9.5 x 70 x 100 mm = 66,500 mm3.

So an already-small 2.5″ disk drive (or cell phone, if you prefer) takes as much space as 575 µSD cards, if you don’t count the card slot or other mounting options.

Let’s go for the 64 GB cards, that’s a whopping 36.8 terabytes of solid-state data. At €50 each, that’d require a €28K investment for about €0.80 per gigabyte. I’ll pass for now :)

But how much is 36 TB? Well, you’d be able to store over 5 Kb of data for each living person on this planet, for example. And carry it in your pocket.

Hello sir. One millisec… let me check your name, travel history, family tree, and CV.

What a terrifying thought.

Cheap webcam

In Hardware on Jun 21, 2013 at 00:01

For a project I’m involved in (yeah, yeah, it’ll all be posted in due time), I needed a couple of webcams to take simple snapshots. Started out with a cheap unit from eBay, but that one really wasn’t up to its task. Then I found this limited-stock offer at Conrad:

DSC_4486

It’s €10 and it even includes a headset which I don’t need. Go figure…

Can’t be much, eh? Well, how about 1280×1024 with auto-focus and auto-white balance?

snap

(you can click-through to see the full-size image)

Amazing stuff. Amazing bargains. Amazing internet. Amazing mail-order. Wow…

The Knapsack problem

In Musings on Jun 20, 2013 at 00:01

This is related to the power consumption puzzle I ran into the other day.

The Knapsack problem is a mathematical problem, which is perhaps most easily described using this image from Wikipedia:

500px-Knapsack.svg

Given a known sum, but without information about which items were used to reach that sum (weight in this example), can we deduce what went in, or at least reason about it and perhaps exclude certain items?

This is a one-dimensional (constraint) knapsack problem, i.e. we’re only dealing with one dimension (weight). Now you might be wondering what this has to do with anything on this weblog… did I lose my marbles?

The relevance here, is that reasoning about this is very much like reasoning about which power consumers are involved when you measure the total house consumption: which devices and appliances are involved at any point in time, given a power consumption graph such as this one?

Screen Shot 2013-06-17 at 09.35.49

By now, I know for example that the blip around 7:00 is the kitchen fridge:

Screen Shot 2013-06-17 at 09.35.49

A sharp start pulse as the compressor motor powers up, followed by a relatively constant period of about 80 W power consumption. Knowing this means I could subtract the pattern from the total, leaving me with a cleaned up graph which is hopefully easier to reason about w.r.t. other power consumers. Such as that ≈ 100 W power segment from 6:35 to 7:45. A lamp? Unlikely, since it’s already light outside this time of year.

Figuring out which power consumers are active would be very useful. It would let us put a price on each appliance, and it would let us track potential degradation over time, or things like a fridge or freezer needing to be cleaned due to accumulated ice, or how its average power consumption relates to room temperature, for example. It would also let me figure out which lamps should be replaced – not all lamps are energy efficient around here, but I don’t want to discard lamps which are only used a few minutes a day anyway.

Obviously, putting an energy meter next to each appliance would solve it all. But that’s not very economical and also a bit wasteful (those meters are going to draw some current too, you know). Besides, not all consumers are easily isolated from the rest of the house.

My thinking was that perhaps we could use other sensors as circumstantial evidence. If a lamp goes on, the light level must go up as well, right? And if a fridge or washing machine turns on, it’ll start humming and generating heat in the back.

The other helpful bit of information, is that some devices have clear usage patterns. I can recognise our dishwasher from its very distinctive double power usage pulse. And the kitchen boiler from it’s known 3-minute 2000 W power drain. Over time, one could accumulate the variance and shape of several of these bigger consumers. Even normal lamps have a plain rectangular shape with fairly precise power consumption pattern.

Maybe prolonged data collection plus a few well thought-out sensors around the house can give just enough information to help solve the knapsack problem?

What if the signal lines are very long?

In Hardware on Jun 19, 2013 at 00:01

Welcome to the weekly What-If series, also available via the Café wiki.

This question came from figuring out how to drive an SPI chip over a few meters distance. Normally, you’re not supposed to do that: I2C and SPI in particular are designed for use on a PCB, i.e. distances in the order of dozens of centimeters at best. Fortunately, in this case speed was not an issue, so what if we simply take the clock signal way down, to say 10 KHz (50 µs on + 50 µs off). Can we create a solid SPI setup across a few meters of wiring?

To try this out, I decided to take a reel with some 70 to 80 meters of multi-core wire:

DSC_4494

Being untwisted and unshielded, this is probably as bad as it gets for signalling purposes. Then I started with a clean 10 KHz square wave of 0..5V from the signal generator:

SCR67

Here’s what happens to the signal on the signal generator side (i.e. same probe placement), when simply connecting the open wires of the above cable to signal and ground:

SCR68

This is caused by the load and the reflection of the signal on the end of that long cable. Think of it as an echo of that voltage change, propagating back and forth and colliding with the signal – causing this fairly substantial change in signal rise time and shape..

But to really see the effects, we have to also look at the signal at the other end, where it’s going to be picked up and used. I made the following setup, with three different probes:

JC's Grid, page 81

(sorry for the confusion: “B” and “M” were mixed up in this drawing)

  • YELLOW probe on the incoming signal generator end
  • MAGENTA probe on the outgoing signal
  • BLUE probe on the outgoing ground connection
  • RED is the MAGENTA minus the BLUE level (as math function)

All the vertical scales are set to the same 1V per division from now on:

SCR69

Note that there are some really strange effects in there: the magenta output signal is “ringing”, as is to be expected with such a long coil of wire and lots of stray capacitance, inductance, and resistance. But the blue “ground” level is also ringing, almost as much in fact! And the yellow input is funny, too. Let’s zoom in a bit on that rising edge:

SCR70

What does this mean for signal propagation? Well, as you can see, everything rattles and shakes in situations like these. It really makes a difference for example as to how the end is connected to power. If you use the (blue) output ground as reference, then the signal will appear as the difference (i.e. the red line), which has less extreme ringing than if you use the (magenta) output referenced to the incoming ground pin.

None of these effects are problematic at this 10 KHz signalling rate. If you need to pass signals like these, it definitely pays to A) keep the signalling rate low, B) reduce the steepness of the edges, and C) add a fairly substantial load resistance at the end of the wire (between signal and ground, say 330..1000 Ω). The reasons and details of all this will have to wait for some future posts.

What I do want to mention, is that you can actually see the speed of light in action here. Let’s zoom even further into that rising edge (sorry, blue and magenta are reversed now):

SCR78

It takes over 400 ns for the yellow rising edge to reach the other end. In vacuum, light travels some 120 meter in that amount of time – not so very different from the 80 meter or so of cable the electricity has to traverse. Pretty amazing what you can do these days, with nothing more than a signal generator, a modern oscilloscope, and a coil of wire!

Again?

In Hardware on Jun 18, 2013 at 00:01

After yesterday’s puzzle, I thought that would be the end of it. But sensitised to this, I did check again this morning. And guess what… the problem came back!

Screen Shot 2013-06-17 at 08.32.41

This time, I was determined to find the culprit. Starting around my office with lots of powered devices, I narrowed it down to the power adapter of an Acer Inpire One netbook:

Screen Shot 2013-06-17 at 09.03.52

Seems conclusive, right? Extra evidence: I took that netbook apart a while ago to install more RAM. All went well, except that the battery charge circuit never seems to have recovered – it’s pretty tiny stuff in there, I must have damaged or disconnected something. Luckily, I was using this netbook only in the lab, i.e. plugged in, so no big deal for me.

Except… this netbook was not plugged in before yesterday. This is a different puzzle!

And digging a little deeper, the pattern is not even exactly tied to when it’s plugged in.

Oh well – this gives me an excuse to talk about knapsacks… stay tuned.

Power consumption puzzle

In Musings on Jun 17, 2013 at 00:01

Recently I had to go abroad for a few days, and given a new access path I just added to my HouseMon setup, it let me track things while away from home. Is that good? Eh, sort of…

The nice thing is that I can now check the solar production in the evening while away from home. And obviously I can then also check on the energy consumption – here’s what I saw:

Screen Shot 2013-06-15 at 19.03.26

Very odd. Something was turning on and off all the time! Here’s a close-up:

Screen Shot 2013-06-15 at 19.00.26

As you can see, I did figure it out in the end – but only after this had been going on for two days, on the phone with Liesbeth to try and identify the source of this behaviour.

It was a bit worrying – that’s a 300..400 watt power draw, every 45 seconds. Going on for days. This is enough energy to cause serious heat problems and my worry was that it was some sort of serious malfunction, with a thermal safety constantly kicking in.

Yikes. Especially when you’re away, knowing there is no one at home during the day…

The answer to this riddle isn’t 100% conclusive, because I did find a way to stop the pulsing, but turning power on again did not cause the pulsing to resume:

Screen Shot 2013-06-15 at 19.29.32

This is crazy. The only explanation I found, was the laser printer: I had printed a few pages before going away (and as far as I can remember, I did turn the printer off after use). The printer was behaving erratically, and I had to turn it on and off a few times.

So my conclusion is: the laser printer (which has a toner heater built-in that can indeed easily draw a few hundred watts) got stuck in some weird mode. And kept me worried for a few days – all just because of me being a geek and checking my graphs!

I don’t know what lesson to take home from all this: stop monitoring? stop checking? stop traveling? – or perhaps just stop worrying when there’s nothing you can do about it ;)

Oscilloscope sampling rate

In Hardware, Linux on Jun 16, 2013 at 00:01

Just to finish the series on Raspberry Pi I/O – here’s proof of its GPIO pulsing speed:

SCR57

The trouble is the irregularity caused by Linux’s multi-tasking properties, and I wanted to show you some examples of that. After a few dozen captures, I came up with this one:

SCR58

Looks pretty bad, right? Tons of hickups in what was meant to be a 5.2 MHz pulse train.

Not so fast…

The sample rate used by the oscilloscope for this capture was 10 MHz, so it’s sampling the signal at 100 ns intervals. That’s extremely risky, because when the signal is also pulsing at this rate, you might end up missing each peak for a substantial amount of time. This would make it appear as if the signal was low or high all the time, despite its rapid changes.

This effect is called “aliasing”, and the solution is simple:

Do Not Sample At A Rate Close To The Frequencies In Your Signal!

I’m inclined to discard the above snapshot – it’s probably a measurement artefact.

Here’s another screen, this time at 50 MHz sample rate, i.e. 20 ns between captures:

SCR59

At that rate, it’s impossible to miss pulses which are all in the range of 100 ns or longer. As you can see, there are still glitches (almost on every snapshot there is one), but it’s not as bad as the previous screen shot was suggesting.

Anyway. Enjoy your RPi and all the Linux high-level stuff it can do (like, ehm, run the Arduino IDE and cross-compile stuff for ATmega’s), but beware of treating it like a CPU which is exclusively at your service.

A Raspberry Pi is fast most of the time, but it’s (occasionally) a bit occupied with itself…

Fast I/O on a Raspberry Pi

In Hardware, Linux, Software on Jun 15, 2013 at 00:01

To follow up on yesterday’s post about WiringPi and how it was used to display an image on a colour LCD screen attached to the RPi via its GPIO pins.

A quick calculation indicated that the WiringPi library is able to toggle the GPIO pins millions of times per second. Now this might not seem so special if you are used to an ATmega328 on a JeeNode or other Arduino-like board, but actually it is

Keep in mind that the RPi code is running under Linux, a multi-tasking 32-bit operating system which not only does all sorts of things at (nearly) the same time, but which also has very solid memory and application protection mechanisms in place:

  • each application runs in its own address space
  • the time each application runs is controlled by Linux, not the app itself
  • applications can crash, but they cannot cause the entire system to crash
  • dozens, hundreds, even thousands of applications can run next to each other
  • when memory is low, Linux can swap some parts to disk (not recommended with SD’s)

So while the LCD code works well, and much faster than one might expect for all the GPIO toggling it’s doing, this isn’t such a trivial outcome. The simplest way to deal with the RPi’s GPIO pins, is to manipulate the files and directories under the /sys/class/gpio/ “device tree”. This is incredibly useful because you can even manipulate it via plain shell script, using nothing but the “echo”, “cat”, and “ls” commands. Part of the convenience is the fact that these manipulations can take place entirely in ASCII, e.g. writing the string “1″ or “0″ to set/reset GPIO pins.

But the convenience of the /sys/class/gpio/ virtual file system access comes at a price: it’s not very fast. There is too much involved to deal with individual GPIO pins as files!

WiringPi uses a different approach, called “memory mapped files”.

Is it fast? You bet. I timed the processing time of this snippet of C code:

int i;
for (i = 0; i < 100000000; ++i) {
  digitalWrite(1, 1);
  digitalWrite(1, 0);
}
return 0;

Here’s the result:

    real    0m19.592s
    user    0m19.490s
    sys     0m0.030s

That’s over 5 million pulses (two toggles) per second.

This magic is possible because the I/O address space (which is normally completely inaccessible to user programs) has been mapped into a section of the user program address space. This means that there is no operating system call overhead involved in toggling I/O bits. The mapping is probably virtualised, i.e. the kernel will kick in on each access, but this is an interrupt straight into protected kernel code, so overhead is minimal.

How minimal? Well, it takes less than 90 ns per call to digitalWrite(), so even when running at the RPi’s maximum 1 GHz rate, that’s less than 90 machine cycles.

Note how the RPi can almost toggle an I/O pin as fast as an ATmega running at 16 MHz. But the devil is in the details: “can” is the keyword here. Being a multi-tasking operating system, there is no guarantee whatsoever that the GPIO pin will always toggle at this rate. You may see occasional hick-ups in the order of milliseconds, in fact.

Is the RPi fast? Definitely! Is it guaranteed to be fast at all times? Nope!

What it took to generate that LCD image

In Hardware, Linux, Software on Jun 14, 2013 at 00:01

As shown yesterday, it’s relatively easy to get a bitmap onto an LCD screen connected to a few I/O pins of the Raspberry Pi.

But there were some gotcha’s:

  • the image I wanted to use was 640×480, but the LCD is only 320×240 pixels
  • the WiringPi library has code to put a bitmap on the screen, but not JPEGs
  • this is easy to fix using the ImageMagick “convert” command
  • but the result has 24-bit colour depth, whereas the LCD wants 16-bit (5-6-5)

The “convert” command makes things very easy, but first I had to install ImageMagick:

    sudo apt-get install ImageMagick

Then we can run “convert” from the command-line (long live the Unix toolkit approach!):

    convert snap.jpg -geometry 320x240 -normalize snap.rgb

This handles both the rescaling and the transformation to a file with (R,G,B) types, each colour is one byte. As expected, the resulting image file is 320 x 240 x 3 = 230,400 bytes.

I didn’t see a quick way to convert the format to the desired 5-6-5-bit coding needed for the LCD, and since the code to write to the LCD is written in C anyway (to link to WiringPi), I ended up doing it all in a straightforward loop:

#define PIXELS (320*240)
uint8_t rgbIn24 [PIXELS][3];
unsigned rgbOut15 [PIXELS];

[...]

for (x = 0; x < PIXELS; ++x) {
  uint8_t r = rgbIn24[x][0] >> 3;
  uint8_t g = rgbIn24[x][1] >> 2;
  uint8_t b = rgbIn24[x][2] >> 3;
  rgbOut15[x] = (r << 11) | (g << 5) | b;
}

Note that this is a C program compiled and running under Linux on the RPi, and that it can therefore easily allocate some huge arrays. This isn’t some tiny embedded 8-bit µC with a few kilobytes – it’s a full-blown O/S running on a 32-bit CPU with virtual memory!

The other gotcha was that the bitmap to be supplied to the LCD library on top of WiringPi was expected to store each pixel in a 32-bit int. Total waste of space, and probably the result of a rushed port from the UTFT code (written to assume 16-bit ints) to the RPi. Which is why rgbOut15 is declared as “unsigned” (int, i.e. uint32_t) array, although the image really only needs an uint16_t. Oh well, what’s an extra 150 kB, eh?

Anyway, it works. Perfection can wait.

Note that this sends out all the data pixel by pixel to the display, and that each pixel takes the bit-banging of 2 bytes (the interface to the LCD is 8-bit wide), so each pixel takes at least 20 calls to digitalWrite(): 8 to set the data bits, and 2 two to toggle a clock pin – times two for the 2 bytes per pixel. That’s at least 4.6 million GPIO pin settings in half a second!

Tomorrow, I’ll show how WiringPi can make GPIO pins change in under 100 nanoseconds.

Getting an image on the LCD

In Hardware, Linux on Jun 13, 2013 at 00:01

To follow up on yesterday’s setup, I’ve managed to drive the display from Linux, using the WiringPi library by Gordon Henderson.

It supports driving the Raspberry Pi’s GPIO pins from C using Arduino- / Wiring-like calls to digitalWrite(). That, in combination with the UTFT library by Henning Karlsen, and some work done by iTead to combine everything was enough to get this self-portrait:

DSC_4482

The display is a bit shiny, and the contrast and brightness are a bit low, unfortunately :(

Drawing the image takes about half a second, but considering that it’s all done through bit-banging of a few I/O pins from a “user space” program, that’s pretty good actually!

There are a couple of interesting (and sneaky) details, which I’ll go into in the next post.

Hurray for open source and the remixing / sharing it enables!

LCD screen for the RPi

In Hardware, Linux on Jun 12, 2013 at 00:01

For a little side-project I’m involved in (more on that in a future post), I wanted to try adding a little LCD screen with touch sensor to a Linux setup. Found this one at iTead:

DSC_4480

There’s a little ribbon cable and adapter (just re-wiring) available for it, which allows plugging this thing into a Raspberry Pi. The nice thing about this screen, apart from being 320×240 pixels and 16-bit color TFT, is its physical size – just right for a stacked setup:

DSC_4481

Stay tuned, as I explore how to hook this thing up and make it show stuff…

Accurate timing with jittery packets

In Hardware on Jun 11, 2013 at 00:01

A few days back I mentioned the SYNC message mechanism in CANopen, and how it can have about 150 µs jitter when sent out on a CAN bus running at 1 Mbit/s.

Here’s the basic idea of sending out SYNC packets at a pre-defined fixed rate:

JC's Grid, page 78

Once you have such a mechanism, you can do things like send out commands which all devices need to perform on the next sync for example. Nice for controlling the X, Y, and Z axis of of a CNC machine with the accuracy needed to make them all move exactly in sync.

The SYNC message has a very very high priority on the CAN bus, but even so, it can be delayed if it wants to send while some other transmission is taking place. So even though it will probably be first one to go out, it still has to wait for the bus to become available:

JC's Grid, page 78 copy

As a result, that SYNC pulse we wanted to arrive on time T ended up coming in at T + X. With X being under 200 µs when the bus is running at 1 MHz, but still.

Can we do better?

Yes, and no. We can’t avoid the jitter in that SYNC packet. But while the master sending out that SYNC packet cannot time it more precisely, it does know when the packet actually went out. So it does in fact know what T + X ended up being for that transmission.

And here’s the clever bit: the master can now send out a second (slightly lower priority) packet with a time stamp in it, i.e. containing that “T + X” value, down to the microsecond in fact. The arrival time of that time stamp packet is not so important. What matters is that each slave tracks exactly when the sync came in, and then uses the time stamp to adjust for any difference or drift that may be taking place.

This won’t help make the SYNC pulses come in more accurately, but it will let each slave know exactly what the time of that pulse was, and hence how accurately their own clocks are running. With this mechanism, all clocks can safely be assumed to run within 1..2 µs of each other, and therefore all future commands can now be tied to specific times which are known to be matched up between all slaves with two orders of magnitude better accuracy.

Ingenious, that CAN bus, eh?

Another new kid on the block

In ARM, Linux on Jun 10, 2013 at 00:01

Besides the Odroid U2 quad-core Linux board mentioned a few days ago, there’s another option in the same price range as the Raspberry Pi – the Beaglebone Black, a.k.a. “BBB”:

Screen Shot 2013-06-06 at 23.15.47

Yeah, ok, it’s slightly more expensive, but it comes with 2 GB of speedy eMMC memory built-in, whereas you need to add an SD card to make the RPi start up.

This is a bigger deal than you might think, because the BBB comes with Angström Linux pre-installed, and you can get going by simply plugging the BBB into a USB port:

  • the moment you do, a memory disk appears, and it gets auto-mounted
  • there’s a “start.htm” file you can double-click to launch your web browser
  • then just follow instructions and you’ll be up and running

This is very similar to the way the MBED ARM module works, and it shows that streamlining the first encounter really can help people to get started – fast!

From a general point of view, the Beaglebone black is actually quite different from the RPi, despite their obvious similarities of both being low-cost, low-power, yet fairly performant little Linux stand-alone boards with with keyboard-monitor-and-mouse capability and built-in Ethernet. Here’s where they differ, at least as I see it:

  • The RPi is aimed at the educational market, i.e. for learning about Linux and doing cool stuff with it once you’ve mastered that first step. Get a board, hook into a huge community, get your hands dirty, and start learning and hacking!

  • The BBB is aimed more at the embedded Linux market, i.e. for building all sorts of advanced projects with Linux and a powerful computing platform inside. Plug it in, put your Linux and physical computing skills to work, and make neat things happen!

The difference? The RPi is the most affordable solution out there, and does really well at being a complete Linux system with fairly good video and audio capabilities, but somewhat limited I/O expandability (even just thinking about how to hook up and size an expansion board on top is a puzzle). The BBB is packed with faster and more powerful hardware (but less focused on video and audio) and is loaded with expansion connectors and pins, ready to add a board, or even a whole stack of them. Even without boards on top (called capes), you have tons of I/O pins to play with – whether you need digital, analog, PWM, or all sorts of bussed interfaces, it’s often all there.

Some details about the BBB running at 300 MHz idle (it scales to 1000 MHz on demand):

# cat /proc/cpuinfo 
processor       : 0
model name      : ARMv7 Processor rev 2 (v7l)
BogoMIPS        : 297.40
Features        : swp half thumb fastmult vfp edsp thumbee neon vfpv3 tls 
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x3
CPU part        : 0xc08
CPU revision    : 2

Hardware        : Generic AM33XX (Flattened Device Tree)
Revision        : 0000
Serial          : 0000000000000000

For a nice 20-min story about a project where RPi’s and BBB’s were used, see this video.

There’s a project called BoneScript, which aims to make the Beaglebone’s I/O capabilities as easy to use from JavaScript (i.e. Node.js) as what the Arduino IDE did for C/C++ and the ATmega’s I/O pins. Sounds like an excellent step in the right direction to me.

Either way, I see a bright future ahead for Node.js running on these little boards!

What’s inside that chip?

In AVR, Hardware on Jun 9, 2013 at 00:01

Came across an interesting post the other day, from a Russian site which looks “under the hood” of some sophisticated chips – some well-known in the West, some less so.

First, the technique of using sulfuric and nitric acid is described and illustrated:

SONY DSC

But the really bit interesting bit is what comes out. Here’s the ATmega8:

atmega8

(both pictures were copied from that site, i.e. zeptobars.ru – I hope that’s ok)

The size of this chip is 2855 x 2795 µm, i.e less than 3 x 3 mm.

Fascinating! I wonder what all the different bits and pieces are…

Trying out CANopen

In ARM, Hardware on Jun 8, 2013 at 00:01

Time for some electronics doodling… here are two LPCxpresso boards, with LPC11C24 ARM chips on each. One of them has been cut in half and hacked, as you can see:

DSC_4477

The reason for using this chip is that they have CANopen drivers built into them, in ROM. That makes it a lot easier to get started, with so many things being unclear anyway – from the compiler toolchain (I ended up with a mix of ARM’s Linaro toolchain and NXP’s LPCxpresso IDE), to figuring out how to get a basic CAN packet on the bus and off again.

Anyway, couple of days of hardware + firmware tinkering, and here’s the result:

SCR56

I’m getting a lot of mileage out of the Hameg HMO2024 scope as you can see, including its hardware CAN bus decoding (a paid option, but free at the time I got the intro bundle).

You’re looking at 4 packets, i.e. 2 request/response pairs going over the bus.

Note how the voltages are jumping up and down a bit. For CAN, this is irrelevant, because it works based on the difference between the CAN-L and CAN-H levels. It’s caused by the master node running its CAN interface at a slightly lower voltage (USB + diode drop). Then again, it’s rather useful for debugging because you can see who sends what! As is clearly visible above, the ACK bit comes from the other node in each exchange.

In terms of speed, there’s one packet every 145 µs, which translates to 55 Kbyte/sec for the 8-byte payload, i.e. 440 Kbit/s out of 1000 Kbit/s actual throughput (more, since there is also an address, the ACK bit, etc). It might not seem like much, but keep in mind that this is also the worst-case timing. The CAN bus does not start thrashing or losing packets when pushed to its limit – all that can happen on a maximally loaded-bus is that the lowest-priority messages will hardly ever get a chance to pass through.

On the software side, a CAN bus is really quite simple to use, since the built-in hardware does all the hard work of acquiring and releasing the bus, resending on packet damage (a noise spike, for example), adding and checking CRCs, buffering a few incoming packets, and even filtered reception – so that only specific ranges of message IDs get picked up.

Not much of this is available for Atmel chips, alas. And that ARM world sure is a lot less physical computing friendly w.r.t. open-source and platform-agnostic toolchains.

All those ARM Cortex chip manufacturers still seem to be living in the dark ages…

CANopen sits on top of CAN

In Hardware on Jun 7, 2013 at 00:01

The CANopen protocol sits on top of the low-level CAN bus protocol. Due to the way CAN bus packets are tagged with a message ID, but not specifically routed from or to a specific device (i.e. node), and due to the maximum packet payload size of 8 bytes, you really need something like CANopen to make it usable. For reference, here’s a 2-node CAN bus setup:

can-term

The interesting bit about CANopen, is that it’s all about data acquisition and process control (that’s what you do when you put this stuff in a car, after all). And what intrigues me about that, is the similarity with monitoring and controlling stuff around the house.

I’ve been reading myself silly lately, to wrap my head around all the CANopen terminology and to try and distill the essence of… ehm, CANopen, life, and a few other things :)

But instead of just describing CANopen as it is, and pouring lots of its concepts and new terminology on you, let me try to describe this from an outsider’s perspective:

  • CANopen works in terms of accessing and saving objects. It’s not necessarily 100% centralised, but that’s how it often gets used: a central CANopen node to control, inquire, and manage a bunch of CANopen slave nodes, all on the same CAN bus.

  • There are a lot of conventions w.r.t. these objects, because the goal of CANopen is to allow devices from different manufacturers to inter-operate (that’s probably why it’s called “open”). So there’s a standard way to find out which devices are present and what their basic capabilties are, to enable/disable each one of them, to configure their communication settings, to start and stop data acquisition streams – all with lots of room for manufacturer- and device-specific details to be managed as well.

  • The central concept here is the “Object Dictionary” in each device, which is indexed by a 16-bit number, plus a numeric 8-bit subindex for entries which are more structured. Objects can be 1..4 bytes, in which case they can be exchanged in one packet, or larger, in which case CANopen takes care of all the packing and unpacking.

  • There is a simple yet effective heartbeat mechanism, so that each device (and of course also the master) can figure out which devices are currently present, which are active, and – with timeouts – which devices seem to have dropped out for some reason.

  • Each device announces itself when powering up, so auto-discovery is possible, even for devices which the master has never seen before – although it’ll be more effective when the master has a detailed description of each type of device, so that it can optimally manage and control it.

  • There are mechanisms for urgent error reporting, dropping out gracefully, and throttling to avoid bus congestion.

  • Everything is prioritised (the CAN bus does that), with top priority (msg ID 000) given to essential (but infrequent) master messages, such as starting and stopping device activity on the bus. Note that, due to the top priority, these will always get through.

  • Next in priority is a time synchronisation message, which can be used to make all device clocks “sing in sync”. With a 1 Mbit/s bus rate, these sync messages will never have more than about 150 µs jitter (and there’s a clever optional scheme to increase the time resolution in all slaves to 1 µs).

There is much more which I can’t all describe in this post, but the one design idea I’d like to highlight is the distinction between managing a large set of objects (via SDO’s, i.e. Service Data Objects) and maximally efficient data streams (via PDO’s, i.e. Process Data Objects). Where PDO’s can be set up to be sent across in a regular time-synchronised manner or in a more ad-hoc / event-based / asynchronous fashion.

I really like this distinction: there is a large heterogenous mix of data objects, which can be accessed and changed via individual SDO requests, and there are streams of PDO data which don’t waste time and bandwidth on describing themselves – just the raw bytes. The heterogenous data objects can fully describe those streams, their exact byte (even bit) structure, the conditions under which data gets sent, their frequency, and their priority.

Lots of interesting connections with the way the RF12 driver sends packets, the announcer packet idea, and the way HouseMon distinguishes between incoming data and the meta-data inside each driver describing these incoming packets.

A thousand days!

In AVR, Hardware on Jun 6, 2013 at 00:01

It’s getting a bit repetitive, but I just noticed this:

Screen Shot 2013-06-05 at 17.27.15

That’s over 1000 days of sending out one packet a minute on this thing:

dsc_1822

… and over 100 days on the recent JeeNode Micro v3:

DSC_2858.jpg

The battery boost version runs off a single Eneloop AA battery, and is doing fine too:

DSC_4401

Onwards!

What if you’re out of wireless range?

In Hardware on Jun 5, 2013 at 00:01

Welcome to the weekly What-If series, also available via the Café wiki.

Ok, so you’ve got some JeeNodes up and running, all talking to each other or to a central node via the wireless RFM12B module. Or… maybe not: the signal is too weak! Now what?

There are several approaches you can try to improve wireless range:

  • optimise your existing antenna(s)
  • lower the data rate and reduce the bandwidth
  • use a more advanced type of antenna
  • use a directional antenna
  • install a repeater of some kind

Let’s go through each of these in turn.

First thing to try is to optimise the little wire “whip” antenna’s that come standard with a JeeNode. Make sure the antenna wire is 82 mm long (that’s for 868 MHz), is sticking up (or sideways) perpendicular to the board, and check that both antenna’s are pointing more or less in the same direction (but not in the direction of the other node: the RF field is circular around the wire, not on top or below).

One thing to keep in mind with these weak signals, is that salty bags of water (us people, that is) tend to absorb RF energy, so these radios work better with us out of the way. Be sure to take a step back while tweaking and hunting for the best orientation!

If that doesn’t help enough, you can do one more thing without messing with electronics or hardware: reduce the datarate of the transmitter and receiver (they have to match). See the RFM12B Command Calculator for settings you can change. To reduce the data rate by two thirds, call rf12_control(0xC614) after the call to rf12_initialize(), for example. The bad news is that you have to do this in all the nodes which communicate with each other – all the data rates have to match!

This in itself won’t extend the range by that much, but with lower data rates you can also reduce the bandwidth in the receiver (with rf12_control(0x94C2)). You can think of this approach as: speaking more slowly and listening more closely. The effects should be quite noticeable. Radio amateurs have been using this technique to get halfway around the world on mere milliwats, using a system called QRSS.

If that doesn’t give you the desired range – here are a few more tricks, but they all require extra hardware: improve the antenna, use “directional” antennas, or use a repeater.

Here’s an example of an improved omni-directional antenna design, as seen on eBay:

omni-antenna

And here’s a directional “Yagi” antenna, which needs to be aimed fairly accurately:

yagi

I haven’t tried either of these (you can build them yourself), but the omni-directional one was mentioned and described in Frank Benschop’s presentation on JeeDay. He reported getting quite good results, once all the antenna + cabling quirks were resolved.

If neither of these are an option, then the last trick you can try is to add a relay / repeater node to your network, as described in this weblog post some time ago. This will double the range if you place that node in the middle of the two nodes which can’t reach each other, but it adds some complexity to the packet addressing mechanism.

Move over, raspberry

In Hardware, Linux on Jun 4, 2013 at 00:01

The Raspberry_Pi is a great little board at an amazing little price. But as mentioned yesterday, it’s not very fast as a server. I suspect that a lot of it has to do with the SD card interface and the SD card itself, so I started looking around for alternatives.

And boy, there sure are lots of ‘em – still well below €100. I’ll single out one system, the Odroid U2 – knowing full well that there must be over a dozen others out there:

article_img

It’s smaller than a Raspberry Pi, but it comes mounted on a “big” (6x6x6 cm) heat sink.

The specs are pretty impressive:

Screen Shot 2013-06-01 at 11.45.13

And the board is neat – the result of a huge mobile phone market driving size down:

201301251344193656

201301251345520979

Could this be used as central web / file / home automation server?

Speedy Raspberry

In Hardware, Linux on Jun 3, 2013 at 00:01

The Raspberry Pi is an amazing little board, with an amazing amount of power and functionality at an incredible price – it’s probably fair to say that our technology geek’s world will never be the same again, now that we have the RPi.

But it’s no speed monster…

Sometimes during development, it really gets in the way – especially when you’re used to working on a modern fast laptop, which is some 20 times faster (and that’s per-core).

So I decided to overclock one of my RPi’s, and see what it does. Overclocking in Raspbian is trivial with the standard raspi-config utility:

Screen Shot 2013-05-28 at 16.52.58

Just go to the Overclock section, and pick one:

Screen Shot 2013-05-28 at 16.46.48

I went for the fastest there is, but decided to run a 2-hour test to make sure it really works. Turns out that someone has already done all the work, and even created such a stress test:

root@raspberrypi:~# ./overclock-test.sh 
Testing overclock stability...
reading: 1
945+1 records in
945+1 records out
3965190144 bytes (4.0 GB) copied, 352.455 s, 11.3 MB/s
reading: 2
[...]
reading: 10
945+1 records in
945+1 records out
3965190144 bytes (4.0 GB) copied, 358.522 s, 11.1 MB/s
writing: 1
512+0 records in
512+0 records out
536870912 bytes (537 MB) copied, 95.2065 s, 5.6 MB/s
writing: 2
[...]
writing: 10
512+0 records in
512+0 records out
536870912 bytes (537 MB) copied, 83.4848 s, 6.4 MB/s
./overclock-test.sh: line 18:  2414 Terminated
              nice yes > /dev/null
CPU freq: 1000000
CPU temp: 58376
[    5.217175] Registered led device: led0
[    8.943589] EXT4-fs (mmcblk0p2): re-mounted. Opts: (null)
[    9.418819] EXT4-fs (mmcblk0p2): re-mounted. Opts: (null)
[   19.181046] smsc95xx 1-1.1:1.0: eth0: link up, 100Mbps, full-duplex, lpa 0x45E1
[   19.431053] bcm2835-cpufreq: switching to governor ondemand
[   19.431082] bcm2835-cpufreq: switching to governor ondemand
[   21.736829] Adding 102396k swap on /var/swap.  Priority:-1 extents:1 across:102396k SS
[   22.134893] ip_tables: (C) 2000-2006 Netfilter Core Team
[   22.174573] nf_conntrack version 0.5.0 (7774 buckets, 31096 max)
[  750.951153] smsc95xx 1-1.1:1.0: eth0: kevent 2 may have been dropped
Not crashed yet, probably stable.

Ya gotta love that last message, nicely semi re-assuring ;)

I did fit a set of cooling fins from Watterott (some nice IR heat pictures there):

DSC_4475

As checked with vcgencmd measure_temp, the temperature never rose above 58.4°C.

Onwards!

Solar PV production

In Hardware on Jun 2, 2013 at 00:01

This is the sort of graph I’ve been waiting for – now that there is more sun, at last:

Screen Shot 2013-05-29 at 00.28.30

(the steep increases and decreases are probably due to shadows from the other roofs)

All values were obtained from the SB5000TL inverter as 5-minute readouts over Bluetooth:

  • total delivered power on AC mains – 29 kWh for the entire day
  • total incoming power on the east-facing roof (12 panels)
  • total incoming power on the west-facing roof (10 panels)

The east-facing panels are actually slight to the north, and the west-facing ones slightly to the south. Which might explain the similar peak production levels despite the different number of panels, i.e. different “insolation” angles.

The peak values are not the highest I’ve seen so far. A cold day with light clouds recently pushed the maximum to over 4700 watts. But that was a rare event. These values are fine with me – I find such levels of solar PV output in a location well up north on this planet (52°N, to be precise) pretty amazing, in fact.

It takes two to CAN

In ARM, Hardware on Jun 1, 2013 at 00:01

The CAN bus described yesterday keeps drawing my interest. It feels right on many levels…

Anyway, for a real CAN bus test, I’ve set up two nodes, to be able to see a full handshake:

SCR39

That’s one node sending out a 0×101 packet, and another node responding to it with a 0×202 packet with the same payload. There are lots of interesting things going on here:

  • The CAN protocol requires any receiving node to send out an ACK bit the moment it has received a valid packet – this doesn’t tell the sender who got it – only that some node was listening and got it. The only thing you can conclude at this stage really, is that there is actual communication – it’s up to higher level layers to verify things.

  • The sender is a node connected to a 3.3V-based CAN transceiver, so the voltages it puts out on the bus are lower than the receiving / echoing node (which is an ARM-based LPCxpresso 11C24 board with a 5V transceiver, running a standard CAN example in the LPCxpresso IDE). As you can see, voltages are hopping a bit, but all that matters is the voltage difference between the two lines, so everything is fine.

  • There is no one listening to the reply, so the LPC11C24 doesn’t get a good ACK on that, and keeps re-sending the packet. It does this a number of times and will then give up.

  • The Hameg HMO2024 scope with its added serial decoding options is very effective in this setup – I made it trigger on a 0×202 packet, and it shows a nice list of the data it has captured. Since I’m using the analog channels, the levels are also clearly visible.

  • The bus is running at 500 KHz, so the basic bit timing is in multiples of 2 µs. Zooming in, I saw that the gap between each packet is 22 µs, which matches exactly the CAN bus specs requiring nodes to wait for at least 11 idle bits before they start. Speaking of which: the CAN names for “0″ and 1″ are “dominant” and “recessive”, respectively – because a dominant bit always wins and forces the bus to a “0″ (voltage applied).

  • The packet times here are ≈ 250 µs, so there’s some 10% of overhead for the gaps between packets, and the total “overhead” for getting 8 bytes of payload across (i.e. 128 µs) is about 100% – but that includes things like bit-stuffing, a message ID, a CRC check, and an ACK. Half the theoretical bit rate to be able to send real-time data with little latency, good reliability, and precisely tagged is actually very good.

Note that the CAN bus requires special transceiver chips. The level conversions are minimal (from “0″ and “1″ to pulling the two lines apart, in terms of voltage), but what they also add is a substantial amount of protection against common-mode signals, which is when both signals move up or down by the same voltage. This is extremely important in a car, where large currents can lead to huge spikes and voltage changes, but it really doesn’t hurt in other applications either: the CAN bus is a very nice way to tie pieces together – and when there is an issue with ground levels, the whole design even allows for opto-couplers to be used for complete isolation between nodes.

The “standard” way to hook up CAN nodes is through three pins (CAN-H, CAN-L, and GND), but it’s not uncommon to include a +5V supply pin, making this a neat 4-wire bus.