
A Xenon flash will cause the Raspberry Pi 2 to freeze - voltagex_
http://www.raspberrypi.org/forums/viewtopic.php?f=28&t=99042
======
tdicola
Oh neat, I just reproed it with a Pi 2 and a Canon Speedlight flash. I'll put
my scope on the power lines and see what's happening when you flash the board.
Sounds like from the thread one of the power ICs is photo sensitive.

edit: Wow yeah, here's a look at the 3.3V power line when you flash the board,
it drops almost down to 0V and then wildly fluctuates for about 100
nanoseconds: [http://imgur.com/hG86pRy](http://imgur.com/hG86pRy)

edit 2: Another interesting measurement, with the board _totally unplugged_
and flashing it you can see a big voltage spike on the 3.3V rail. Up to 6-7
volts or so for a few nanoseconds:
[http://imgur.com/td262QK](http://imgur.com/td262QK)

I guess not only can you learn about electronics but also Einstein's
photoelectric effect with the Pi 2!

~~~
noonespecial
Are you sure that's not induced current from the power spike required to
operate the flash or the spark-gap inside the xenon tube?

You've got the gear set up... would you mind terribly repeating the experiment
but blocking the _light_ with cardboard or something?

~~~
tdicola
Alright I put the Pi 2 in a cardboard box and held the flash at the same spot
(but the box obscures the light). It keeps running just fine and doesn't have
the nasty spike. Definitely is something light related.

~~~
noonespecial
Thanks! Well done. Photoelectric it is. Now I wonder what color its most
susceptible to...

~~~
jakobegger
Since the effect seems to be caused by a silicon component that isn't properly
shielded from light, anything with a photon energy higher than the bandgap
should do. The bandgap of silicon is 1.1eV, which means the effect should
start in near-infrared.

~~~
psgbg
What about glass, then that should work, right?

~~~
jakobegger
Not sure I understand you correctly.

The problem should happen with near-infrared, visible, and UV-light. Glass is
transparent in the visible range, and depending on type, also in IR / UV, so
glass won't work as a shield.

The easiest, reliable way to shield would be wrapping in aluminum foil (also
helps against RF-interference).

~~~
psgbg
Yeah you are right. I was assuming that the chips would block visible light
but I forgot about UV.

------
ChuckMcM
That is fun, reminds me of the 'yelling at the drives slows them down'
video.[1]

[1]
[https://www.youtube.com/watch?v=tDacjrSCeq4](https://www.youtube.com/watch?v=tDacjrSCeq4)

------
teddyh
From _The Devouring Fungus_ , Karla Jennings, 1990, chapter 10, _The Monster
Turns… and Falls to its Knees_ , p. 211:

 _Another legendary debacle triggered by light hit at a highly publicized
affair thrown by IBM, ironic considering that IBM is the master of the
seamless image. D. E. Rosenheim, who helped develop the IBM 701, the first
mass-produced modern commercial computer, recalled the famous faux pas, which
occurred when the company held a dedication ceremony for the 701’s
installation at its New York headquarters. Top-level executives, the
engineering team, and a gang of reporters crowded the ceremony room_

 _“Things went pretty well at the dedication,” said Rosenheim, “until the
photographers started taking pictures of the hardware. As soon as the flash
bulbs went off, the whole system came down. Following a few tense moments on
the part of the engineering crew, we realized with some consternation that the
light from the flash bulbs was erasing the information in the CRT memory.
Suffice it to say that shortly thereafter the doors to the CRT storage frame
were made opaque to the offending wavelengths.”_

Those who do not know their history are doomed to repeat it.

~~~
BuildTheRobots
Link for the lazy:
[http://en.wikipedia.org/wiki/Williams_tube](http://en.wikipedia.org/wiki/Williams_tube)

Note for the kids: Yes, that's right, it says CRT memory. That's the same
Cathode Ray Tube as found in non-flat-panel TVs and monitors, except we're
using it as a high speed storage device.

Ironically, the wikipedia page for Selectron tubes actually has more useful
information on Williams tubes than it's article does...
[http://en.wikipedia.org/wiki/Selectron_tube](http://en.wikipedia.org/wiki/Selectron_tube)

------
exDM69
This reminds me of an old Finnish engineering legend from the early days of
Nokia. The guys had just built an important prototype of some network
equipment (early GSM base stations IIRC), which was going to be demonstrated
for the press. All tests and previous demos had gone fine.

But as soon as the demo for the press started, the machine crashed. The
management was upset. Later, the reason was found to be some old EPROM chips
that are erased using UV light, and the photographers' cameras had strong
flashes that went through the tapes covering the "window" on the chip. This
caused the program memory to be corrupted when a photograph was taken.

~~~
teddyh
Interesting that the Finnish version of this story is about Nokia, GSM base
stations and EPROM memory. The version from 1990 from _The Devouring Fungus_
which I quoted in a separate comment is about the IBM 701 and CRT memory.

------
Johnythree
One of the standard tests to gain an EMC Compliance Certificate is a spark
discharge test.

Any experienced engineer will have a Spark Generator (Car Ignition coil, spark
gap and short Dipole) to test to see if his latest project misbehaves when
confronted with Impulse Interference.

As an EMC Investigator I would always carry a spark generator to demonstrate
to newby engineers why EMC Compliance is so important.

I've seen a spark from 50ft away crash or reset a microprocessor system. Just
the static discharge from walking on carpet is often enough.

~~~
hueving
This is not the same thing. The cause is from the burst of light, not EM.

~~~
TheSwordsman
What if I told you that light is EM?

~~~
hueving
I would tell you that pretending there isn't an implied distinction for
everyone involved in a modicum of applied science is as counterproductive as
arguing about the editor chosen for a coding tutorial.

------
dietrichepp
Reminds me of old EPROMs. You can buy special "light sensitive" transistors,
but they're really just ordinary transistors with a window in the case, since
ordinary transistors are light-sensitive. You can even use an ordinary 1N4148
diode as a solar cell, it just doesn't generate much power.

The fix is simple: apparently, you just have to cover U16, which controls the
power supply.

~~~
adestefan
LEDs can also be used for narrow band detection. It's called the Mims effect
since Forrest Mims was the first to publish about it.

------
Animats
There's nothing mysterious about this. Semiconductor gates are light-
sensitive. There's usually carbon black in the plastic of plastic-packaged ICs
to prevent interference from light. The opacity isn't perfect, though. For
that you need ceramic or metal-encased ICs. Still, this is a rare enough
problem that IC data sheets don't specify a maximum tolerated illumination
level.

Try some laser pointers, especially towards the blue end of the spectrum where
the photons have more energy. You may be able to trigger this effect by
pointing at a specific IC.

~~~
rasz_pl
red laser makes it crash too

~~~
Animats
OK, that part is a reject. It will probably crash in bright sunlight, too.

~~~
poizan42
Apparently the U16 IC is completely missing its casing

~~~
rasz_pl
its not missing it, its just a specific packaging type (flip chip). Its
supposed to be used in products with full enclosure, or under a heatsink.

For example almost every single CPU and GPU nowadays uses some fcbga packaging
(flip chip on ceramic/pcb carrier), but they are all hidden under heatsinks
and often additional heat spreaders.

[http://www.maximintegrated.com/en/app-
notes/index.mvp/id/400...](http://www.maximintegrated.com/en/app-
notes/index.mvp/id/4002)

------
mholt
Shortcut to a video of this phenomenon:
[http://youtu.be/wyptwlzRqaI?t=1m29s](http://youtu.be/wyptwlzRqaI?t=1m29s)

~~~
alephan
[https://www.youtube.com/watch?v=4huRNCrwfNw](https://www.youtube.com/watch?v=4huRNCrwfNw)

~~~
mholt
Is your cursor nyan cat?

~~~
jstalin
How do you do that? I want a nyan cat cursor!

~~~
bluehex
It appears to be Nyanology[1] on Terminology[2]

[1] [https://github.com/anisse/nyanology](https://github.com/anisse/nyanology)
[2]
[https://www.enlightenment.org/p.php?p=about/terminology&l=en](https://www.enlightenment.org/p.php?p=about/terminology&l=en)

------
swamp40
A xenon tube is a spark gap.

If there's anything in this world noisier than a spark gap, I don't know what
it is.

I think the first radio transmitters were spark gaps.

The energy flies thru the air, and is coupled onto the power line.

The power supply doesn't cope well with the oscillations, and hiccups.

I see the notes about U16 being photosensitive, but if it is a black epoxy
like most IC's, I'm not buying that light gets into it.

It's possible that blue tack shields the EMP a bit.

~~~
lotsofmangos
A light flash is an EMP. Radio waves are light. They also, unless they are
very short, pass easily through blue-tack. When they get that short they are
basically becoming infra-red anyway. Then we have a wide range through the
visible spectrum, up through UV, that blue-tack stops, until we start getting
towards x-rays and blue-tack starts to become transparent again. This is
caused by an electromagnetic pulse, but of one in or close to the visible
spectrum, otherwise blue-tack would be useless.

edit :

The word light is commonly used as shorthand for variously the whole EM band,
the near visible EM band and just the visible EM band.

If you are talking about the speed of light, it includes radio and gamma.

Discussing the colour of light on the other hand and you are referencing the
visual system.

As soon as you start talking about visible light however, you are widening the
definition of light again to include IR and UV and more, otherwise you would
not need to clarify with the word visible.

edit 2 - darkmighty. I am not talking about the xenon bulb producing an RF EMP
from the lamp flashing circuit, I am merely continuing the terminology used
elsewhere in this thread, that a flash of visible light is also quite clearly
a form of electromagnetic pulse.

edit 3 - foobarbecue. Radio as it gets shorter becomes microwave, then IR,
then red, through green, past blue, goes to UV, then xrays, then gamma.

~~~
krenoten
To the downvoters of the above comment: if it's technically incorrect, say so.
I don't know if it is or not, but it's certainly not an offensively worded
comment. When you downvote rather than respond, you are actually spreading
ignorance. A downvote is an effort to mute. A half-muted incorrect technical
comment will encourage far more ignorance than a properly corrected, visible
one. Don't encourage ignorance.

~~~
_almosnow
True that, I always see downvotes without replies as nonsense. If they could
argue something reasonable about the statement then they would post it, but
since they don't do it then it's probably a downvote "just because I don't
like the guy".

------
tonteldoos
A computer with actual strobe-induced epilepsy. Looks like the singularity is
closer than we thought.

------
agumonkey
The new wave of single board computers really exposed me to the amount of
failure that can happen at the electrical level. Growing up with large ATX
boxes I'd never expect so many things to go wrong.

btw: anyone tried to light-freeze other devices (banana, orange, cubie, etc) ?

~~~
IgorPartola
As my electronics professor said: "every digital circuit is really an analog
circuit."

------
mikerr
Here's a pic of the chip in question (so you can cover it up)
[https://pbs.twimg.com/media/B9Ut_QwIQAACrp_.jpg](https://pbs.twimg.com/media/B9Ut_QwIQAACrp_.jpg)

------
wolfgke
Question to HN: Does also the Raspberry Pi 1 B or B+ have this problem or is
the Xenon flash problem specific to the Raspberry Pi 2 B? Is somebody willing
to do this experiment/has done it?

~~~
AlyssaRowan
The B doesn't (at least according to the lovingly-hand-resoldered, first batch
board I've got handy).

Don't have a B+ to hand, sorry. It might?

------
thought_alarm
Why would a switched-mode-power-supply chip be photosensitive?

~~~
pjc50
All silicon is photosensitive to some extent. The power supply will be one of
the few chips which is primarily "analog"; in particular it will have a
bandgap voltage reference. That's exactly the sort of thing to be badly
affected by a charge pulse from the flash. Glitching the power supply will
then destabilise the digital logic it's powering.

The real question is how the light manages to get through the epoxy casing.

~~~
jarvist
Epoxy resins are pretty transparent by themselves, opaque ones have additional
materials or dyes added.

Most dyes (also aromatic organics) are not active (i.e. transparent) in the
near-IR.

~~~
swamp40
Yes, but if you were tasked with designing the epoxy that was going to be used
in every IC everywhere, I would _hope_ that you would go through the extra
effort of adding dyes that were specifically opaque to UV and IR.

------
sqren
Video demonstrating the issue:
[https://www.youtube.com/watch?v=wyptwlzRqaI](https://www.youtube.com/watch?v=wyptwlzRqaI)

------
mikerr
The problem IMO that the chip in question doesn't have a plastic cover, look
how shiny it is in this video:
[https://www.youtube.com/watch?v=c7p2OcQ7G58](https://www.youtube.com/watch?v=c7p2OcQ7G58)

A laser (no EMP!) shone on that chip will also crash the Pi.

------
yuhong
As a side note, the power supply chip directly uses the 5V from USB, right?
Wonder if it is tolerant of 3.3V as common when running from batteries.

------
pervycreeper
Slightly OT: is it now possible to run a completely free OS on this new
version? I've been getting contradictory info on this so far.

~~~
morganvachon
Not yet. Closed source binary blobs are necessary for booting and for
accelerated video. In fact, it's the GPU that bootstraps the rest of the board
when you first power it up.

There is work being done to write an open source driver for the GPU, with the
blessing of the RPi Foundation and BSD-licensed code provided by Broadcom[1].
But I don't know if that will include the bootstrapping code, or how far along
it is. Given that it is BSD licensed, it may not meet everyone's definition of
"Free", but at least there is a good chance of having fully documented,
source-available drivers in the future.

[1] [http://www.raspberrypi.org/a-birthday-present-from-
broadcom/](http://www.raspberrypi.org/a-birthday-present-from-broadcom/)

~~~
pedrocr
_> Given that it is BSD licensed, it may not meet everyone's definition of
"Free", but at least there is a good chance of having fully documented,
source-available drivers in the future._

3-clause BSD (what's common these days) meets pretty much everyone's
definition of Free:

[https://www.gnu.org/licenses/license-
list.html#GPLCompatible...](https://www.gnu.org/licenses/license-
list.html#GPLCompatibleLicenses)

( _The following licenses qualify as free software licenses, and are
compatible with the GNU GPL._ )

~~~
poizan42
Not if you ask RMS: [https://www.gnu.org/philosophy/open-source-misses-the-
point....](https://www.gnu.org/philosophy/open-source-misses-the-point.html)

~~~
Turing_Machine
"the GNU GPL qualifies as an open source license and most of the open source
licenses qualify as free software licenses."

There are open source licenses that RMS considers non-free, but the modern (3
clause) BSD license isn't one of them.

------
bitwize
Someone commented "Fire photon torpedoes..."

In light of Heartbleed and Shellshock, I propose calling this the Photon
Torpedo vulnerability.

~~~
seba_dos1
>In light of

I see what you did there!

------
DalekBaldwin
Looks like there's some truth to the belief that a photograph steals its
subject's soul.

------
arnie001
It's interesting this was not caught before..

------
nh
Good find OP! I wonder how many electronic devices would have similar problems
if we took out the covers?

~~~
thrownaway2424
All of the badly-designed ones and none of the well-designed ones :) Often
you'll see audio equipment with LED-biased voltage amps where the LEDs have
some opaque glop slathered on them so they don't make popping sounds under
bright lights, and to a lesser extent so multiple channels inside the same box
are optically coupled, which is a concern. LEDs are photovoltaic like any
other semiconductor but for obvious reasons they are not sold in opaque
packages.

You could probably find devices that are disturbed by loud sounds and physical
shock, because ceramic capacitors are microphonic.

------
andruby
Awesome. "Flash" your board just got a whole new meaning :)

------
ozy23378
That Pi isnt very photogenic.

------
nacs
This was clearly designed to sell more opaque cases. /s

Or to look on the upside, the Pi now comes with a free photodetector.

------
Alupis
I just ordered two of these, and they will arrive on Tuesday.

Makes me sad because I'm imagining a Raspberry Pi 2.1 release in the near
future now...

~~~
jonah
The update will be a piece of black tape or epoxy over that component.

------
thrownaway2424
That comments thread contains a head-smacking quantity of ignorance. "Is it
the light or the EM pulse?" What?

~~~
eternauta3k
He's asking if it's RF interference or just the light.

~~~
lotsofmangos
RF interference is long wavelength light and the successful filter was blue-
tack, so probably not.

~~~
eternauta3k
RF is not light and that's a pointless remark. But right on your second point.

~~~
lotsofmangos
RF stands for radio frequency, and defines a band of electromagnetic
radiation, popularly known as light, that is on a continuum of frequencies
that above RF pass through visible light and on towards gamma rays.

In simple terms, whenever something possessing charge moves it generates an
electromagnetic wave associated with the motion and at right angles to it.
Visible light is generated by motions of electrons at an atomic scale, radio
frequency is motion of electrons at the scale of antennas, but they both
generate electromagnetic waves quantized as photons.

edit - darkmighty. Electromagnetic radiation at radio frequencies is popularly
known as light in contexts such as speed, but not in contexts such as colour.

~~~
darkmighty
RF refers to specific frequencies of radiation, which are not popularly known
as light at all. Radio is usually in the MHz range to a few GHz (10^6-10^9
Hz). Light starts a hundreds of Terahertz (10^14 Hz). The behavior is
qualitatively different at usual scales due to quantum mechanical behavior
(light is difficult to emit/absorb coherently -- it's more particle-like) at
high frequencies.

A photon striking a semiconductor will probably displace an electron _if_
given an adequate bandgap, creating a voltage spike (which is nonlinear --
i.e. the problem isn't a propagating terahertz wave obviously). An incident
radio wave will cause voltage spikes through coherent induction in the wires
(i.e. they act as an usual antenna).

~~~
lotsofmangos
Here's some quantum optics with microwaves :
[http://phys.org/news/2013-05-quantum-optics-
microwaves.html](http://phys.org/news/2013-05-quantum-optics-microwaves.html)

 _Even if there is no fundamental reason to believe that quantum theory would
make a distinction between "microwave photons" and "optical photons", this
demonstration puts this equivalence across a huge frequency range on a firm
experimental footing. Moreover, the lower frequency of the microwave photons
enabled a more complete characterization of the effect than has been able so
far with optical photons, opening up new possibilities to characterize
radiation sources. Finally, the new experiment highlights how quantum optical
effects can be exploited in experiments with microwave sources, which may lead
to practical applications of "microwave optics"._

------
psgbg
Relevant. Red vs Blue "EMP"
[https://www.youtube.com/watch?v=SMCsRkH-9AA](https://www.youtube.com/watch?v=SMCsRkH-9AA)

------
pmalynin
Explanation: Camera's have capacitors that charge up in order for the flash to
happen. They are usually quite powerful. Now during the discharge (aka flash)
what you have is very high energy electrons flowing across the wire creating
aa magnetic field, coupled with the electric field of the electron you get a
mild EMP.

And if it is light sensitivity then it should be tested with a bright
continuous light

~~~
thrownaway2424
The entire purpose of a camera flash is to create a directed electromagnetic
pulse.

~~~
djmanning
The entire purpose? Not really. I think it has something to do with a mess of
directed photons.

~~~
falcolas
For a "that's the joke" \- light is on the EM spectrum, so a flash directing
light will direct an EM pulse. :)

