6LoWPAN part 2 – Building Contiki on Windows

Getting started

Following on from my previous post about wanting to experiment with 6LoWPAN and Contiki, I suppose I’d better do something. First things first. I needed to download Contiki and get it building. I’ve got a few options.

A Linux VM

The official guide pointed me towards using a pre-configured “Instant Contiki” Ubuntu VM and that certainly works. I needed to install and setup VMWare but that wasn’t too bad. It’s probably time I played around with Linux a bit more, so I also installed Linux Mint (easier for newbies) in a VM too.

Cygwin

There was also a guide on Sun May Sky about using Cygwin on Windows. I’m sure this works as described and I was about to give it a try, but then I though “Cygwin? Isn’t that a little outdated for Windows 10?”

Bash on Ubuntu on Windows

ubutntu-on-windows-10-logo-bannerWhen I first wrote this guide, the version of Windows was the 1607 “Anniversary Update” and the Bash shell was available but as a slightly hidden developer-only feature. I’ve updated it for the latest (at time of update) Windows 1709 “Creator’s Update”. To get things working in 1709 just install Ubuntu from the Windows store and type “ubuntu” to run.

The only thing worth mentioning is that your C drive will appear as /mnt/c rather than Cygwin’s /cygdrive/c.

YaKai Chen’s Cygwin guide mentioned installing the ARM GNU Compiler toolchain and adding the installation folder to the path. The default Windows installation path with spaces and (x86) is awkward to use in Linux and it turns out it isn’t even necessary.
I went to build a Contiki component with a make command and was told that it wasn’t installed. “sudo apt-get install make” solved that.

build-error-1-armYou’ll notice the make failed with arm-none-eabi-gcc not found, so I tried just entering arm-none-eabi-gcc. A very helpful prompt told me that apt-get install gcc-arm-none-eabi might solve my woes. And it did.

build-error-2-srecordTime to try that make again… OK, this time srec_cat was the missing culprit, but just typing srec_cat told me what needed to be installed. Simply apt-get install srecord and I was ready for attempt #3… and success.

build-error-3-successThat seemed easy! Don’t tell anyone, but I might get to like this new Bash shell. Now to get something meaningful working…

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6LowPAN (part 1) – discovering sub 1GHz mesh networking

What?

So, party inspired by my not particularly successful Tado teardown I’ve decided to play around with 6LowPAN. “What’s 6LowPAN?” you may ask. Well, it IPv6 networking over Low power Wireless Personal Area Networks. That’s what Tado uses to communicate between its components and I thought I might re-purpose their gateway for this. Unfortunately it uses a slightly out of data Stellaris microcontroller and getting this working was awkward. I got bored with it.

6lowpan_mesh

Why?

The ultimate goal would be to have drop-in smart light switches that link up in a mesh and can be controlled remotely home-automation style. Ideally I’d like something that:

  • can report its status back (unlike some commercial offerings)
  • is battery powered so that there’s no need for a neutral connection in the switch
  • doesn’t leech power through the load – that’s cheating and won’t work well with LED lighting
  • can operate as a separate manual switch in case the network is down

How far will I get with this? Who knows. Let’s see.

How?

6LoWPAN can sometimes be over a 2.4GHz network (like Zigbee) but I’m going to go with Sub-1GHz for better range through walls and hopefully as far as my garden office / workshop. TI do a nice range of Sub-1GHz ARm microcontrollers. I got myself some CC1310 launchpads and a CC1350 launchpad too for good measure.

As far as software goes, it looks like Contiki is a good place to start. It’s a RTOS designed exactly for 6LoWPAN. It looks pretty good, but despite the best efforts of the creators I must admit it doesn’t seem easy to get started with. There is some helpful stuff over at Sun May Sky which I believe is maintained by YiKai Chen who you’ll also find helping out over on the TI E2E Community.

Getting started

First things first. I needed to download Contiki and get it building. The official guide pointed me towards using a pre-configured “Instant Contiki” Ubuntu VM and that certainly works. There was also a guide on Sun May Sky about using Cygwin on Windows. I had some minor trouble with that so thought I’d try out the new Ubuntu bash shell that’s (a hidden) part of the latest Windows 10 builds. That seemed to work well, so I thought I’d add my own guide.

NFC Login 2.1

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This should be the final revision. Just a few minor tweaks. The regulator is now a TPS77533 rather than an incorrectly footprinted TPS77333. Corrected the bi-colour LED wiring. Move to a smaller 4MHz crystal for the USB side of things. Shrunk it a bit as I’m no longer expecting to find any PCB bugs. Dropped the 0.1″ pin header as the TagConnect worked fine.

The most obvious change is the small capacitive touch sensor PCB on the front. The previous version searches for a tag a couple of times a second. This works fine but I don’t like the idea of all the 13.56MHz EM noise pollution. This will poll for touch frequently and if it senses a touch it will poll for tags rapidly for a few seconds. In theory it could be more responsive too. Not had a chance to code this yet though.

There’s is also a Login NFC the way to another implantee. I’d recommend you check out Hans Peter’s embryonic.dk blog.

NFC login 2.0 finally working

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It’s taken some time, but it’s finally working! It’s hard to debug when you don’t know what’s at fault.

  • Is it the PCB design? I’ve never done anything as complicated or as high frequency as this.
  • Is the the PCB manufacture? I decided to get the board made by DirtyPCBs rather than home etching, so at least that took one potential source of screw-up out of the equation.
  • Is it my soldering? I’ve never done anything as fine as the LQFP (larger chip) and QFN (smaller one on the right). They’re both 0.5mm pitch but at least the LQFP has pins that stick out rather than just metal patches on the edges of a square block.
  • Is it the design of the antenna? This is the work of the very talented Mathieu Stephan (from here) so probably not the weakest link.
  • Is it the firmware? I’ve had the code running on a MSP430F5529 LaunchPad, but I’ve switched down to a smaller F5510 and reassigned the pins.

Well, it took three tries – as you can see from the scribble on the board above. There’s a mistake around the TPS77333 voltage regulator which truned out to be a footprint error in the Eagle-supplied libraries. It’s now a bodged-in TPS77533. There’s another by the LED which I’ve had to swap for two separate ones rather than a red/green single package. My mistake on the pinout there.

It turns out the problem was the soldering of the QFN packaged TRF7970A. Buying a microscope helped me sort it out and it’s finally there! It’s finally working! There’s more to do, of course. I need to fix the mistakes. The software could be tidier. Version 2.1 will be perfect (probably). I’ll publish the design and code when it’s tidy, but in the mean time if anyone’s interested in any of it, let me know.

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I laser cut a simple acrylic enclosure. It’s just black acrylic on the back – with some 5mm neodymium magnets press fitted into holes so it attaches firmly to my PC.
Some clear acrylic makes up the front so I can see the LEDs and the fruits of my labours. M3 nylon bolts go into threaded holes on the rear piece.

It’s been harder and slower than I expected but I’m very happy.

NFC login (version 1.0)

After my experiments with NFC readers I felt it was time to actually create something useful to work with my NFC implant. At work I need to make sure that my PC is locked whenever I leave my desk. It’s not that I work with anything really sensitive, it’s just that given half a chance my colleagues will certainly send an email on my behalf admitting to unusual sexual inclinations or offering to buy everyone biscuits.

I decided the easiest way to do this was with keyboard emulation. It doesn’t require me to have any privileged domain access, modify the PC or install anything that compromises security. Plus, it should all be possible using a MSP430F5529.

The launchpad, booster pack and antenna.

The launchpad, booster pack and antenna.

My proof of concept involved a MSP430F5529 LaunchPad and a . I made sure that I could emulate a USB keyboard, implement a USB CDC serial port and read my tag id.

The USB side of things was adapted from some of TI’s example code. The NFC stuff was a little trickier. The Booster Pack is sold bundled with either the G2 or F5529 Launchpads. However the sample code is surprisingly complicated and only supplied for the MSP430G2553. Porting it over to the MSP430F5529 should have been fairly simple – just changes to some in assignment and clocking. It somehow took me ages but I managed it in the end. I must publish my ported code to save other people the same trouble.

I combined them all together to implement the following:

  • A USB CDC serial port allows me to set the password (but not read it). It’s stored in the microcontroller’s flash so persists when powered down.
  • Pressing one button on the launchpad send Windows-L to lock my PC. (Not actually easily accessible in its current form.)
  • It scans continuously for NFC tags and if it sees mine it sends Ctrl-Alt-Del{password}Enter

The 2 part 3D printed case for my NFC login

The 2 part 3D printed case for my NFC login

Hardware wise for version 1.0 I went with the setup from my previous including a DLP coil antenna and a cheap bit of U-FL to SMA cable from eBay. I don’t intend that the final version will be using a dev board and booster pack. It’ll be a custom etched PCB, but I decided to take the same approach as with Agile software development – produce a minimum viable product first and improve later. If I don’t ever get round to a nice neat version 2.0 then at least I can actually log in with my implant.

A view showing the installed boards and antenna

A view showing the installed boards and antenna

I 3D printed a case that allows the launchpad, booster pack and antenna to slot in. It comprises two parts that clip together and a couple of magnet to hold it firmly against the PC case on my desk. The case is 3mm thick but only 1mm thick by the antenna coil so it reads fairly easily.

OK, it just looks like a plain yellow box

OK, it just looks like a plain yellow box

The final version looks a little dull. It’s a plain box that was almost done in black, but I happened to have yellow filament in the printer. All that happens when I successfully scan my tag is that a red LED shows through the case for 5s whilst scanning is temporarily disabled and my PC unlocks.

I’ve got a little bit of tidying of the code before I include it. As it’s evolved from two different lots of sample code in different styles it’s a little bit messy. I’ll also attach the STL files for the case – designed once again in my 3D modelling package of choice OpenSCAD.

Source code (still messy) and STL files (under files folder) are now available at https://bitbucket.org/fredmurphy/public/src/127c1b2f26305bd8b2b2184084927da72457f9e5/LoginNFC/?at=master

Laser improvements – coolant monitoring

One of the quickest ways to destroy a CO2 laser tube is to let it overheat. My laser has a very simple system – just a plastic breakfast cereal container of water with a small aquarium pump. It works well enough, but is easy to forget.

My first modification was to power the pump from the laser so that it’s running when the laser is switched on at the mains. That gets around the “oops, I forgot” problem, but doesn’t cover a pipe coming loose or the temperature creeping up. I really wanted to actually check cold(ish) water was flowing through the glass laser tube.

The water and temp sensor

The water and temp sensor

To check the flow, I started with a LM35DZ temperature sensor and a cheap water flow sensor from eBay. For neatness I carefully milled the flow sensor to embed the temperature sensor in it. This was then attached to the output of the laser. I intend to check that enough water is coming out and that it’s below a temperature threshold.

I etched a board with a MSP430G2533 microcontroller that counted the pulses from the flow sensor over a fixed timer period and used the onboard ADC to read the temperature. I set the threshold at 40C and 75% of the normal flow.

PCB with logic error

First PCB with logic error

My initial attempt used a MOSFET to pull the last signal down to GND if things were awry. Unfortunately this fired rather than disabled the laser! This was replaced with a 74LSxxx AND gate so I could force it to 5V and disable the active low signal. Some connectors to match the controller board meant I could drop it in with no rewiring.

The finshed PCB - with a fix for a lifted trace

The finshed PCB – with a fix for a lifted trace

So far it has been working well. (I actually finished this months ago but didn’t document it.) It’s not actually been needed to save my tube yet, but it’s nice to know it’s there. I might later and another sensor on the input and also flag if the difference between the two is too high. I could also add a cheap LCD display but that seems a little over the top.

Coolant monitor in the laser

Coolant monitor in the laser

As it’s a single sided board with some through hole pin headers, when it’s in place you only see the “boring” side of it with not tracks or components. Oh well – there’s no need for it to look pretty. Here it is in place. It’s a drop-in addition on the 6 pin cable to the controller board. It’s powered from the existing 5V line and simply forces the signal to fire the laser high (as it’s active low) when it senses trouble. From teh top you can see:

  • The connector to the sensor
  • A debugging / programming header
  • A currently unused connector for a screen
  • The connectors to patch into existing power and signals

If anyone wants a copy of the PCB layout or code, just ask.

Further PCB improvements

I’m glad to report I’m getting even better result with my PCB etching. Results do seem a bit up and down each time I get back to it but I feel I’m narrowing it down and getting more polished.

One. I’ve started using Ziplock vacuum bags to evenly press the transparency against the PCB. A really nice cheap substitute for a vacuum light box! I can thoroughly recommend them.

Etching underway

Etching underway

Two. I’m now able to get even better exposure with 3.5 minutes under the UV box rather than using the fluorescent tube. Maybe my developer solution used to be too strong. Maybe ditching the glass in favour of the Ziplock helped. Regardless, I’m getting a really good result. Here’s the partially etched board showing great definition.

Three. I got some Tin plating solution. No idea why I didn’t do this before, as it’s really easy and gives a great finish. The instruction say to carefully clean the copper with an abrasive. However, I assumed that as I’d just removed the etch resist from the copper with acetone and it had to be just about as clean as it could be. I popped it straight in the tinning solution before I got fingerprints all over it.

Etched and soldermasked board

Etched and soldermasked board

Milled, drilled and populated

Milled, drilled and populated

I had to mess something up and I removed the protective layer on the soldermask before exposing. Some of it dulled a little on contact with the transparency. Oh well. Otherwise I’m very happy. The ridiculously small negative text even came out a little. Here’s the board before drilling and populating. It a simple MSP430 based alarm for when my two year old son opens the front door. He can now just reach the handle.

When I’ve got it all sorted (and have tried the via rivets too) I think I might do a proper write-up.