Experiments with 1-Wire

Jan 24, 2017 / 0 comments

As previously mentioned, at the end of last year I got involved with a project involving the use of 1-Wire. In particular a DS28E15 device, intended to be used as a royalty tracker for a licensed piece of hardware IP. I’d no previous experience with 1-Wire (other than knowing it’s commonly used for driving temperature sensors), so I took it as an opportunity to learn a bit more about it.

The primary goal was to program a suitable shared key into the DS28E15 device that would also be present in the corresponding hardware device. A Maxim programmer had been ordered, but wasn’t available in stock so had to be back ordered. Of course I turned to my trusty Bus Pirate, which claimed 1-Wire support. However it failed to recognise the presence of the device at all. After much head scratching I finally listened to a co-worker who had suggested it was a clock speed issue - the absence of any option to select the 1-Wire speed in the Bus Pirate or any mention of different speeds in the documentation I had read had made me doubt it was an issue. Turns out that the Bus Pirate was talking “standard” 1-Wire and the DS28E15 only talks “overdrive” 1-Wire, to the extent that it won’t even announce its presence if the reset pulse conforms to the standard, rather than overdrive, reset time period. Lesson learned: listen to your co-workers sooner.

A brief period of yak shaving led to adding support to the Bus Pirate for the overdrive mode (since landed in upstream), and resulted in a search request via the BP interface correctly finding the device and displaying its ROM ID. This allowed exploration of the various commands the authenticator supports, to verify that the programming sequence operated as expected. These allow for setting the shared secret, performing a SHA256 MAC against this secret and a suitable nonce, and retrieving the result.

Next problem: the retrieved SHA256 MAC did not match the locally computed value. Initially endianness issues were suspected, but trying the relevant permutations did not help. Some searching found an implementation of SHA256 for the DS28E15 that showed differences between a standard SHA256 computation and what the authenticator performs. In particular SHA256 normally adds the current working state (a-g) to the current hash value (h0-h7) at the end of every block. The authenticator does this for all but the final block, where instead the hash value is set to the working state. I haven’t been able to find any documentation from Maxim that this is how things are calculated, nor have I seen any generic implementation of SHA256 which supports this mode. However rolling my own C implementation based on the code I found and using it to compare the results retrieved from the device confirms that this is what’s happening.

So at this point we’re done, right? Wait for the proper programming hardware to turn up, write the key to the devices, profit? Well, no. There was a bit of a saga involving the programmer (actually programmers, one with at least some documentation that allowed the creation of a Python tool to allow setting the key and reading + recording the ROM ID for tracking, and one with no programming documentation that came with a fancy GUI for manually doing the programming), but more importantly it was necessary to confirm that the programmed device interacted with the hardware correctly.

Initial testing with the hardware was unsuccessful. Again endianness issues were considered and permutations tried, but without success. A simple key constructed to avoid such issues was tried, and things worked fine. There was a hardware simulation of both components available, so it was decided to run that and obtain a capture of the traffic between them. As the secret key was known this would then allow the random nonce to be captured, and the corresponding (correct) hash value. Tests could then be performed in software to determine what the issue was & how to generate the same hash for verification.

Two sets of analyzer software were tried, OpenBench LogicSniffer (OLS) and sigrok. As it happened both failed to correctly decode the bitstream detected as 1-Wire, but were able to show the captured data graphically, allowing for decoding by eye. A slight patch to OLS to relax the timing constraints allowed it to successfully decode the full capture and provided the appropriate data for software reproduction. The end issue? A 256 bit number (as defined in VHDL) is not the same as 32 element byte array… Obvious when you know what the issue is!

So? What did I learn, other than a lot about 1-Wire? Firstly, don’t offhandedly discount suggestions that you don’t think make sense. Secondly, having a tool (in this case the Bus Pirate) that lets you easily play with a protocol via a simple interface is invaluable in understanding it. Thirdly, don’t trust manufacturers to be doing something in a normal fashion when they claim to be using a well defined technology. Fourthly, be conscious about all of the different ways bitstreams can be actually processed in memory. It’s not just endianness. Finally, spending the time to actually understand what’s going on up front can really help when things don’t work as you’d expect later on - without the yak shaving to support Overdrive on the BP I wouldn’t have been able to so quickly use the simulation capture to help diagnose the issue.

Cloning a USB LED device

Jan 14, 2017 / 0 comments

A month or so ago I got involved in a discussion on IRC about notification methods for a headless NAS. One of the options considered was some sort of USB attached LED. DealExtreme had a cheap “Webmail notifier”, which was already supported by mainline kernels as a “Riso Kagaku” device but it had been sold out for some time.

This seemed like a fun problem to solve with a tinyAVR and V-USB. I had my USB relay board so I figured I could use that to at least get some code to the point that the kernel detected it as the right device, and the relay output could be configured as one of the colours to ensure it was being driven in roughly the right manner. The lack of a full lsusb dump (at least when I started out) made things a bit harder, plus the fact that the Riso uses an output report unlike the relay code, which uses a control message. However I had the kernel source for the driver and with a little bit of experimentation had something which would cause the driver to be loaded and the appropriate files in /sys/class/leds/ to be created. The relay was then successfully activated when the red LED was supposed to be on.

hid-led 0003:1294:1320.0001: hidraw0: USB HID v1.01 Device [MAIL  MAIL ] on usb-0000:00:14.0-6.2/input0
hid-led 0003:1294:1320.0001: Riso Kagaku Webmail Notifier initialized

I subsequently ordered some Digispark clones and modified the code to reflect the pins there (my relay board used pins 1+2 for USB, the Digispark uses pins 3+4). I then soldered a tricolour LED to the board, plugged it in and had a clone of the Riso Kaguku device for about £1.50 in parts (no doubt much cheaper in bulk). Very chuffed.

In case it’s useful to someone, the code is released under GPLv3+ and is available at https://the.earth.li/gitweb/?p=riso-kagaku-clone.git;a=summary or on GitHub at https://github.com/u1f35c/riso-kagaku-clone. I’m seeing occasional issues on an older Dell machine that only does USB2 with enumeration, but it generally is fine once it gets over that.

(FWIW, Jon, who started the original discussion, ended up with a BlinkStick Nano which is a neater device with 2 LEDs but still based on an Tiny85.)

2016 in 50 Words

Jan 6, 2017 / 0 comments

Idea via Roger. Roughly chronological order. Some things were obvious inclusions but it was interesting to go back and look at the year to get to the full 50 words.

Speaking at BelFOSS. Earthlings birthday. ATtiny hacking. Speaking at ISCTSJ. Dublin Anomaly. Co-habiting. DebConf. Peak Lion. Laura’s wedding. Christmas + picnic. Engagement. Car accident. Car write off. Tennent’s Vital. Dissertation. OMGWTFBBQ. BSides. New job. Rachel’s wedding. Digital Privacy talk. Graduation. All The Christmas Dinners. IMDB Top 250. Shay leaving drinks.

(This also serves as a test to see if I’ve correctly updated Planet Debian to use https and my new Hackergotchi that at least looks a bit more like I currently do.)

IMDB Top 250: Complete. Sort of.

Dec 31, 2016 / 0 comments

Back in 2010, inspired by Juliet, I set about doing 101 things in 1001 days. I had various levels of success, but one of the things I did complete was the aim of watching half of the IMDB Top 250. I didn’t stop at that point, but continued to work through it at a much slower pace until I realised that through the Queen’s library I had access to quite a few DVDs of things I was missing, and that it was perfectly possible to complete the list by the end of 2016. So I did.

I should point out that I didn’t set out to watch the list because I’m some massive film buff. It was more a mixture of watching things that I wouldn’t otherwise choose to, and also watching things I knew were providing cultural underpinnings to films I had already watched and enjoyed. That said, people have asked for some sort of write up when I was done. So here are some random observations, which are almost certainly not what they were looking for.

My favourite film is not in the Top 250

First question anyone asks is “What’s your favourite film?”. That depends a lot on what I’m in the mood for really, but fairly consistently my answer is The Hunt for Red October. This has never been in the Top 250 that I’ve noticed. Which either says a lot about my taste in films, or the Top 250, or both. Das Boot was in the list and I would highly recommend it (but then I like all submarine movies it seems).

The Shawshank Redemption is overrated

I can’t recall a time when The Shawshank Redemption was not top of the list. It’s a good film, and I’ve watched it many times, but I don’t think it’s good enough to justify its seemingly unbroken run. I don’t have a suggestion for a replacement, however.

The list is constantly changing

I say I’ve completed the Top 250, but that’s working from a snapshot I took back in 2010. Today the site is telling me I’ve watched 215 of the current list. Last night it was 214 and I haven’t watched anything in between. Some of those are films released since 2010 (in particular new releases often enter high and then fall out of the list over a month or two), but the current list has films as old as 1928 (The Passion of Joan of Arc) that weren’t there back in 2010. So keeping up to date is not simply a matter of watching new releases.

The best way to watch the list is terrestrial TV

There were various methods I used to watch the list. Some I’d seen in the cinema when they came out (or was able to catch that way anyway - the QFT showed Duck Soup, for example). Netflix and Amazon Video had some films, but overall a very disappointing percentage. The QUB Library, as previously mentioned, had a good number of DVDs on the list (especially the older things). I ended up buying a few (Dial M for Murder on 3D Bluray was well worth it; it’s beautifully shot and unobtrusively 3D), borrowed a few from friends and ended up finishing off the list by a Lovefilm one month free trial. The single best source, however, was UK terrestrial TV. Over the past 6 years Freeview (the free-to-air service here) had the highest percentage of the list available. Of course this requires some degree of organisation to make sure you don’t miss things.

Films I enjoyed

Not necessarily my favourite, but things I wouldn’t have necessarily watched and was pleasantly surprised by. No particular order, and I’m leaving out a lot of films I really enjoyed but would have got around to watching anyway.

  • Clint Eastwood films - Gran Torino and Million Dollar Baby were both excellent but neither would have appealed to me at first glance. I hated Unforgiven though.
  • Jimmy Stewart. I’m not a fan of It’s a Wonderful Life (which I’d already watched because it’s Lister’s favourite film), but Harvey is obviously the basis of lots of imaginary friend movies and Rear Window explained a Simpsons episode (there were a lot of Simpsons episodes explained by watching the list).
  • Spaghetti Westerns. I wouldn’t have thought they were my thing, but I really enjoyed the Sergio Leone films (A Fistful of Dollars etc.). You can see where Tarantino gets a lot of his inspiration.
  • Foreign language films. I wouldn’t normally seek these out. And in general it seems I cannot get on with Italian films (except Life is Beautiful), but Amores Perros, Amelie and Ikiru were all better than expected.
  • Kind Hearts and Coronets. For some reason I didn’t watch this until almost the end; I think the title always put me off. Turned out to be very enjoyable.

Films I didn’t enjoy

I’m sure these mark me out as not being a film buff, but there are various things I would have turned off if I’d caught them by accident rather than setting out to watch them.

I’ve kept the full list available, if you’re curious.

The terrible PIC ecosystem

Dec 25, 2016 / 0 comments

I recently had call to play with some 1-Wire devices at work (more of which in a future post). It was taking a while for the appropriate programmer to turn up, so of course I pulled out my trusty BusPirate. It turned out the devices in question would only talk in overdrive mode, while the Bus Pirate could only offer standard mode. So I set about trying to figure out how to add the appropriate support.

This is is a huge endorsement for test equipment with Free Software firmware. Rather than giving up I was able to go and grab the current firmware, which has been adopted by the community since Dangerous Prototypes have discontinued development. What let me down was the ecosystem around the PIC24FJ64GA002.

My previous recent experience with microcontrollers has been with the ATTiny range and the STM32. Getting up and running with both of these was fairly easy - the tool chains necessary were already present in Debian, so all it took was a simple apt invocation to install everything I needed to compile code and program it to the devices.

Not so with the PIC series, which surprised me. There seems to be some basic support for the earlier PIC16 range, but for later chips there’s nothing that works out of the box with Debian. Investigation revealed that this was because there’s nothing maintained that enabled Free development for the PIC range. The accepted solution is the closed MPLAB X. Now, in one sense fair play to Microchip for making this available. But in another, shame on you. I can’t imagine ever choosing to build something based on a chip that only had a closed source tool chain available. I want things I can use in Makefiles and properly script, that are available in my distro of choice and that generally work in the same fashion as the tool chains I’m used to. I understand there might be some benefit in a closed compiler in terms of performance (and have HPC friends who would never trust a benchmark provided using GCC), but in general that’s not the space I move in. Nor does it seem to be the sort of attitude you should be taking if you are trying to attract the hobbyist and small production run market.

Any yet this seems common amongst hardware manufacturers. People whose core business is selling physical items, where the software is only relevant in terms of being able to use those items, seem to consider the software to be precious. Instead of opening up programming specifications and allowing a more widespread use of the hardware, increasing sales. I understand there are some cases where this isn’t practical, but the default attitude is definitely one of being closed rather than open, which is a terrible shame.

Anyway. I do have some Bus Pirate 1-Wire overdrive support now working (pending some testing to ensure standard mode still works), but I am glad I never spent a lot of time getting involved with PICs now.

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