Galois LFSR PRNG in NeoPixel goggles

I've been playing with software to run on my Adafruit Neopixel Goggles. The software gets to make pretty patterns on 32 x 24-bit RGB LEDs arranged in two rings.

One of the modes picks random colours to change each ring to. The Arduino stack supplies a random() function to do this, but it seemed to take up about 600 bytes of the limited 5kb flash program memory on the in-goggles microcontroller.

I wondered if I could make a smaller-but-less-random generator. After all, the colour pattern does not need to be cryptographically secure. I'm just trying to avoid getting the same sequence of colours every time.

Someone pointed me at Linear Feedback Shift Register PRNGs and I implemented one based around a description in Wikipedia. I chose the Galois one because pistols at dawn.

That seemed to work well for the basic generation of random numbers, but the Arduino always started up with the same seed, and so gave the same sequence of colours each time. Luckily, there are 512 bytes of EEPROM on this microcontroller and so I used two other those to generate and store a seed to be used next time.

Initially, I generated two random bytes at power-on, and stored those into the EEPROM. However, this rapidly proved to have a really short period: there were only two different patterns being displayed by the goggles in this mode!

So, next, which seems to work better, the code now initialises the PRNG from EEPROM, takes a single bit from it (and discards it) and then writes out the PRNG state into the EEPROM. That means that the start state advances by one bit every boot.

Code for the goggles is here on github: https://github.com/benclifford/goggles.

Lua Fibre-optic Christmas Tree

In 1998, I was given a fibre optic christmas tree, lit by two coloured light bulbs powered by 2 x AA batteries. In the intervening years, the electrics broke. Finally today I got round to doing something about it.

The tree now has a bunch of neopixels where the bulbs used to be, and an ESP8266 microcontroller providing the flashing (as well as a Wifi/telnet accessible Lua command prompt).

Unfortunately, I can't see much of a different between the illumination provided by the seven pixels - I was hoping that each one would light the fibres up very differently, but that hasn't turned out to be the case. No big deal though, and maybe more optical isolation between the different LEDs and the base of the fibre bundle would help.

Interestingly, the fibres manage to project an image of the layout of the LEDs on the board (see link above) onto the wall behind the tree! and all the fibres emit a very slight blue light because there's a blue LED on the controller board.

spherical camera

This week I've been playing with a Theta S spherical camera. I'm pretty impressed at what it does, given the price. Here's a gallery of images that you can scroll around.

sdr

I got a NooElec R820T Software Defined Radio.

Only got a kernel stack trace once, trying to unload the automatically loaded digital TV drivers. And I didn't have to compile anything.

About half an hour after unboxing, I was using gqrx to get a waterfall plot of various things - BBC Radio 1, Capital, PMR446, the London bus trunked radio system, GB3LW, some local business users, and some (not-decoded) ADS-B plane traffic and some classical music being played in the CB range.

I think it needs a better antenna though - I can pick up stuff ok on my Baofeng handheld that I'm not getting through this.

Also might be interesting to see if a Raspberry Pi 2 is powerful enough to run this.

pebble time retrospective

I got a Pebble Time smartwatch, but it turns out I don't wear it much, despite wearing my original (OG) Pebble all the time. I don't even keep it habitually charged.

There isn't one particular reason:

• There are two reasons why I have to keep taking the watch off, something I never had to do with the OG Pebble which I wore 24/7:
  • The charge port is inaccessible while wearing the watch. I didn't realise before hand how much of a problem that would be: I tend to take it off to charge and forget to put it back on.
  • The wristband and/or shape does something weird to my skin that neither the OG pebble nor the 10 wristbands I wear do; especially if the Pebble gets wet (which it otherwise would often, several times a day).
• Application loading is heavily coupled to the phone. I got the Pebble because it has long battery life, but I've found myself in situations where I can't use (e.g.) the compass because my phone has gone flat (which is does by the end of every day). That turns the compass from something I can rely on into a gimmick.
• Not really an ongoing reason, but as I'm in grumble mood, I found the software upgrade experience from OG to Time quite frustrating: I needed a different app from the Pebble app that I already had installed, and it seemed to conflict in a weird way with the existing app.

I have been hoping someone would make a smartstrap to expose charging, but to no avail. Now there's an SDK available for the serial port (which is also the charge point), I might start fixing that problem myself. One day.

Payment Wristband on the London Underground

I previously blogged about making a paytag sticker into a wristband. Later Barclays Bank released a variation: bpay, a prepay mastercard already in a wristband.

The wristband holder is pretty shitty and falloffable: it is bulky and I know two people (one being myself) who have lost their bands accidentally. I've rehoused mine on a woven bracelet.

Being a pre-pay band, this chip does an online authorisation for every transaction, making it sometimes a little slower. But for the same reason, they expose authorisations (not just cleared transactions) in their live online statement.

I recently made my first journey on the London Overground using bpay (I've been on their contactless payment trial for 6 months but using a different card) and I got to see an initial authorisation that I hadn't seen before with my previous (post-paid) card:

0908 Enter train system at Wapping station

0915 bpay sees this authorization:

     Auth: TfL Travel Charge,TFL.gov.uk/CP,GB  29/09/2014  9:14:50  Posted On: 29/09/2014  GBP 0.10

0922 Leave train system at Shoreditch High Street

then around close of business on day+1, that Auth gets replaced with the actual charge:

    Fin: TFL.GOV.UK/CP,VICTORIA,TFL TRAVEL C   30/09/2014  18:07:58  Posted On: 29/09/2014  GBP 7.20

Interesting that they charge 10p for authorization rather than the minimum single fare. Also note that the description of the transaction changes (to something less readable, IMO) - that seems to happen from other merchants too. Weirdos.

paytag wristband

A year or so ago, one of my credit cards sent me a PayTag: a sticker with the contactless payment bits of a regular credit card, but without the other stuff (contact chip, embossed number, etc).

Their stated usecase was for sticking on your phone, as a sort-of low tech upgrade for phones which don't have NFC.

I didn't find that use case particularly compelling, and aside from comedy ideas like putting it inside a fairy-wand, I've been waiting for a use.

A few days ago I made it into a payment wristband. My right-hand wrist has loads of bracelets on it already, and I took one of those, some plastic packaging and some superglue and made a payment wristband.

I was a little wary at deploying this in use at first. My initial test was deliberately in a stationer's shop which had unattended self-checkout terminals. (I've used that same branch for RFID fun as a teenager in the past, where I had a anti-shoplifting coil in my pocket and set the alarms off every weekend). For 75p I ended up with a new card case and a successful initial test.

Next I went to Waitrose to buy my groceries. I was wary here because the daytime staff are angry dinner ladies who have confiscated stuff off me in the past(!). They didn't seem to bat an eyelid at me waving my jacket arm at their payment terminal.

Thirdly I went to buy coffee. This was more awkward. Their payment terminal was stuff under a shelf and looked like it was probably quite awkward to use even with a regular contactless card. The dude was a bit confused at me putting my empty hand towards him and waving.

My final test was in a pub. The contactless user experience is a bit different in most pubs: they use hand-held terminals and usually you hand over your card, they notice it is contactless and then use that. So I had to work around that a bit. I handed over my regular payment card and when he noticed it was contactless and went to swipe I drunkenly shouted "WAIT! LET ME USE MY WRIST!" which he did. At that point the card reader decided it needed to do dialup verification so there was a tense few seconds where I hoped I didn't look like a knob. But it worked.

I'm surprised at how unsurprised staff are at seeing this. I need to figure out the right way to start a contactless payment in a hand-held reader environment. I'm looking forward to being able to use this on the London Underground ticket gates later in the year too...

later: someone sent me this article about a contactless payment suit.

BST

I wired in a rugby MSF clock into my raspberry pi, and wrote my own software to interface it to NTP.

Unfortunately I wrote it in winter when they were transmitting approximately GMT. Now its summer time and so they are transmitting British Summer Time not GMT; this is indicated by a bit in the time message. That I ignored. Oops.

Bed time project today was to add that in. Now I've got my stratum 1 server back.

radio clock NTP server

I've got a couple of Raspberry Pi(e?)s. One I'm using for real server stuff like DNS. The other is for hacking around on. I got a gertboard for it, but hadn't wired anything else up. Then I saw this MSF radio time receiver (for 9 GBP) which is intended to receive 60kHz time broadcasts from the UK National Physical Laboratory.

When I was a teenager, a radio clock was a cool thing that I never had. Then later I got an internet connection, and with it NTP, and so I got geek-time a different way. So I never got an MSF clock until now.

The receiver plugs into the Gertboard (actually it can maybe plug directly in to the Pi's GPIO pins without a Gertboard in the way - I'm not sure) and presents a 1-bit serial signal that is the demodulated 60kHz signal. This is a slightly strange looking signal mostly consisting of 100ms of no carrier, two bits at 100ms each then 700ms of carrier, with 60 seconds of data being enough to transmit the current time.

There was some fiddling soldering to do to get the pins connected - turns out 10y old crocodile clips don't cut it. My father, being better than me at soldering, put those on for me. I cut two of the Gertboard's jumper cables in half so that I'd have wires which connect straight onto Gertboard pins.

The layer above that needs to run in software, at least in my setup.

The first program was a scope program that output * for 1 and . for 0 to the console 80 times a second or so, with a CR every second, to give a line-by-line view of the time. That showed the signal pretty clearly, like this:

.*******************............................................................
.**********.....................................................................
.*********************..........................................................
.**********.....................................................................
.*********......................................................................
.******************.............................................................
.******************.............................................................
.************...................................................................
.***********....................................................................

The short lines are when there's a sync pulse only, and the long lines are when there's a 1-bit immediately following the sync pulse.

The second program used a minute-long circular buffer as a shift register (thanks to Dave_H from the #a&a for that suggestion), and tries to decode time when one end of the buffer looks like the start-of-minute sync pattern. This was able to pretty accurately decode the time. I didn't implement parity checking or other error checking (for example, checking the sync pattern is exactly right) because of that accuracy, which gave trouble later on when I moved the antenna around. Thats something I could implement, and give out some accuracy metric. This took a few hours to code in C (yes, I wrote something in a language that wasn't Haskell). The code is pretty inefficient because it hard polls the GPIO pin. There is some edge triggering available in the Pi's GPIO implementation so I could investigate that. So that ended up as a program which outputs the current time every minute, to the console.

Next, I wanted to connect this with ntpd, so that I could compare the time with other internet time sources, and share the MSF signal with other hosts on the same LAN. ntpd has a facility for connecting "reference clocks" which are sources of time that are not other ntp servers. One of those, the SHMEM driver, communicates with a separate process using a shared memory buffer. So I got to learn about Linux shared memory (indexed using integer keys, I discover, rather than as nodes the file system). For that protocol, basically you stuff the current system clock time and the current received radio time into two fields in that shared memory, every time you get a clock reading. That was a straightforward addition to my second program.

So here's what the hardware setup looks like. The big board is the gertboard. Under it with the big wires is the Pi. The tiny tiny board held in the air by the jumpers is the MSF receiver board, and the big metal rod at the front is the antenna.

And here's what I see in ntpd:

 $ ntpq -pn
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+2001:8b0:0:53:: 195.66.241.3     2 u   38   64  177   27.872   -4.596   8.680
-2001:8b0:7c:1:2 61.69.233.69     3 u   39   64  177    0.597   -2.285   8.710
+2001:8b0:7c:1:b 90.155.53.93     3 u   37   64  177    1.000   -3.868   8.395
*127.127.28.2    .MSF.            0 l   44   64   17    0.000  -23.730   6.885

where NTP has recently synced to the MSF driver (with fake IP address 127.127.28.2). Its about 20ms different from the network-connected NTP servers (see the offest column). Its difficult to know how much of that is from NTP over-the-network inaccuracies and how much is from my code, but I suspect the bulk of it is from my MSF code - my polling just isn't accurate enough at the moment, and my parsing has some weird time alignment stuff in it.

All in all, good fun and blinkenlights!

p.s. you're welcome to point your own ntp servers at this for fun. Add server tyne.cqx.ltd.uk to your /etc/ntpd.conf

p.p.s. is it sad that I can decode those octal reachability fields in my head.

undervoltage

It sounds like a common cause of problems sticking peripherals onto a Raspberry Pi come from unsufficient power supply.

They have a USB port for feeding power in, and I think that encourages people (including myself) to use any old USB compatible supply (even though they explicitly tell you to not do that in the docs...). I tried mine first feeding from my laptop and then from my mifi power supply.

That mifi supply seems to work for regular usage but when I start plugging things into USB it gets all wanky. So I measured the voltage with my trusty old multimeter - it seems to be down around 4.6v, (varying by about 0.1v depending on what I have wired in). That's well below the 4.75v minimum that apparently I should be seeing.

Off the the RS online shop to buy a grown up power supply then...

first impressions of raspberry pi

I just got my Raspberry Pi, left on doorstep from Farnell in a package labelled "tshirt"; indeed inside the package was a (free) element14/pi tshirt, and wrapped inside that was my actual raspberry pi. Its taken a while to arrive (like most people's) - it was ordered on the 9th of May.

The first afternoon I spent playing with it came with an excited nostalgic feeling to my teenage years, scrabbling to put random pieces of hardware together scrounged around the house in order to get a working system: a keyboard off one host, a usb hub stolen from someone, a long ethernet wire borrowed from someone because the TV is in one room and the ethernet ports are in another. It was made more nostalgic because my best friend from secondary school who used to be my main partner in this kind of activity turned up to have a play too.

I fairly easily got the example debian distribution downloaded (my first linux distro downloaded by bittorrent) and running on a 4gb SD card; and once that was in place, it was easily to get a browser up and watch the bunnycam.

After giving up on getting flash running in order to get BBC iPlayer, I discovered get-iplayer (on the way getting annoyed by this blog post by a BBC dude about how people shouldn't use 3rd party clients). I can just about play BBC radio audio at the same time as the bunny webcam is streaming video. But it didn't seem to have enough power to decode an iplayer video stream instead of the bunny webcam, which was a bit disappointing.

There seem to be a few funny presumably hardware related problems: the device doesn't reboot at all, and sometimes when I boot, sound does not work at all. I haven't had time or inclination to track these down at all. Most concerning for me is that the experience of different devices seems slightly different - for example, other people seem to be able to reboot just fine.

After downloading the recommended Debian distro, I discovered that there was a beta release of a much more recent version, wheezy. This was a lot smoother experience than the recommended distro - it resized the image automatically to fill the whole 4g disk space, and just seemed more polished. Either way, those distributions are debian based, so the amazingness (if you're being nostalgic about 80s/90s era hacking) of apt comes into play to get a big range of software installed.

It was cool that the wheezy distribution came with Scratch - a visual programming language that is sort of a year-2012 version of Logo. (That's cool - one of the first things I thought about when I heard of the goals of the raspberry pi was to think: why don't you get people programming scratch instead?)

Something interesting I discovered was that the ARM architecture does floating point emulations completely differently to intel: there are different calling conventions for software floating point and hardware floating point, and it seems that all the packages in your distro need recompiling to make use of hardware floating point, if they were originally built for software floating point. I'll have a play with that later.

The GPIO port had pins soldered on, which i wasn't expecting. I'm not sure what to use those for. I've had a user port on a computer before, but never used them much - just for flashing LEDs and making model trains drive around. maybe some geeky weather station thing?

So what do you need to get started, in addition to the pi? Well a keyboard and video (composite or HDMI) and a power supply - I'm using the USB-based power supply I stole from my mifi. Probably you'd like a mouse and an ethernet connection.

If I get up to anything cool, I'll write about it on this blog, no doubt.

cheapish mifi

I got a three huawei mifi from store.three.co.uk.

It cost 50 quid for the mifi and 1 gb prepay sim card that I didn't really want as I already had a sim card.

put in sim card, plugged it in to charge, chose network on my laptop, put in preset password, and it worked without any further fiddling.

lots of menu options to play with, esp to do with port forwarding and the like. but for what I'm using it for I don't think I need that. maybe if I want mobile SMTP one day(?)

they didn't send a vat receipt which is annoying but not disasterous. I eventually found an email form on their website to ask for one.

cool to get on train, and almost seamlessly get a network connection. marred by the fact that my mifi is lower in my OS's priority list than a nearby broken BT Openzone hotspot, so it went there first.

bunny webcam

I got this webcam for looking at pet rabbits for my girlfriend's birthday. (webcams, dear children, are something still fascinating for people born before, say, 1985)

The set up was pretty straightforward. I plugged it into the local ethernet and it appeared at 192.168.1.239. The local ethernet uses 192.168.1.0/24 but I think maybe have been just chance that it configured itself that way - it wasn't using DHCP out of the box. When I switched on DHCP it reconfigured itself to a DHCP-allocated address in the same range.

The supplied manual (on paper) describes downloading and installing some setup.exe to configure, but I ignored that, and using nmap discovered it was running an http server on port 81. Logging into this as admin, with no password, I found myself able to view and control the camera without need for any software.

I couldn't get the wifi working, but only spent 10 mins or so on that - we decided it would be best placed right by a wired ethernet port, so there was no need for that.

We attached it to the underside of an Ikea table using electrical tape. There was a balance to strike between being low enough to get a good angle, but high enough that they can't eat the camera or the wires. I would have loved to have put it in the cage with them, but the lifetime of the wires would have been measured in minutes, or possibly seconds.

The camera has motors to pan and tilt, though where its installed that isn't really necessary, and the whirring of the servos seems to scare the white rabbit a bit. That one likes to sit looking at the camera on the other side of the fence, hanging off the underside of the table.

There's a night vision mode too. The camera is surrounded by a ring of infrared LEDs - actually also a bit visible red too. These are turned on by a CdS cell above the camera lens (so you can trigger them with your finger rather than needing to put the camera in the dark).

The user interface is clunky but functional. The main page looks like this, with arrows at the top left to drive the servos. There are admin menus too, which appear even if you aren't authorised - they just don't work for non-authorised users. This clunky interface means its not a good camera for streaming to the public at large.

So, for £40, was this worth it? yes

(btw, not all the pictures in this post were made with the webcam - for example, the pictures of the webcam were taken with an iPhone rather than a complicated mirroring optical arrangement)

♫ if i could turn back time ♪

Cooker has a one-way-only knob for setting time. You wind forwards until you reach now. If you miss now, you wind forwards another 24h (luckily the cooker doesn't have a date too) until you reach now again. On TZ shift (i.e. end/start of summertime / daylight savings), once per year, there's a wind-forward 1h event. fairly easy. you make the clock run faster than real time. but once per year there's a once per year wind-forward 23h event. BUT! Turns out if you can stop the clock, then you can let real time wind forwards faster than clock time. and wait an hour. and then start the clock again. It involves more real time (you have to wait an hour) but a lot less tedious winding.

new photo frame

On sale, €33 at Media Markt, I got a digital photo frame.

It has a slot for a USB stick, and for various memory cards. It looks like an upgraded version of one I bought for my parents a year ago, where the USB interface worked well but the card reader seemed unreliable. So I put one of the many free conference USB sticks in my collection in.

People seem to have an instinct to unplug these things continuously. Why? You don't put curtains over the print photos on your wall and only open the curtains for a few minutes at a time to look at the photos behind.

The frame is 16:9 aspect ratio. My camera makes files of ratio 4:3 (or 3:4 in landscape). (apparently print photos are usually 3:2. my camera has a helper mode that overlays grey bars on screen to show you 3:2 ratio. But it doesn't have a similar mode for 16:9)

On my laptop, I have gimp. But I found that Preview is good enough for rough cropping of photos to size. That's the technical side of cropping easily dealt with. On the artistic side, though, its hard to get in the habit of evaluating photos for such cropping - photos that I think of as "too much space at the top / bottom" are ones that are perfect for this frame, and photos that fill the frame nicely are too full for cropping.

As the frame is so low resolution (~ VGA) its possibly to crop smaller details from a frame, so that you end up with an entirely new picture rather than something that looks like the original photo but a bit cropped.

I've had a desire but not enough motivation to actually look at my photos from a cropping perspective. Perhaps this will provide the motivation.

pink panther usb hub

I got this pink panther USB hub from mediamarkt - it was the same price as a USB extension cable (which is what I was looking for) - €7.99

Now I have three extra USB ports too, which is useful for attaching (for example) bamboo encrusted memory sticks, or my wide SD card reader.

One problem: when there are two of them chained with my ext hd on the end, the ext hd makes terrible clicking sound and doesn't start up properly. maybe thats a power problem?

slide controller

I just brought my third one of these:



(my first got lost at an event a few years ago, and I lost the USB dongle for my 2nd a few months ago)

If you want something to change your PowerPoint (or powerpoint-like) slides, this is the tool to get. Its worked with every computer I've tried it on, and every piece of presentation software (PDF viewers and HTML slidy in addition to PowerPoint and Open Office).

No software to install either - the dongle appears as a USB keyboard - the four keys send PgUp and PgDown to change slides; the laser beam button sends an F5 (which is start-presentation in PowerPoint) and the bottom button sends 'b' (which is blank screen in PowerPoint).