A Xenon flash will cause the Raspberry Pi 2 to freeze

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?

I want to see this with a bare regulator. I might have some time Monday.

My money is still on an electromagnetically coupled pulse onto either the 3.3V or ground rail.

I have seen that happen plenty of times, but I have never seen a light flash disrupt a power rail like that.

I'm not saying it couldn't happen, just playing the odds...

The historical importance of the photoelectric effect is to demonstrate the relevance of the work function Ω. It works like this:

V ~ E_0 - Ω

where hf is the energy per photon and Ω is the work function of the target. If Ω > E_0, no voltage is produced. This provided the basis for Einstein's demonstration that energy is proportional to frequency (E_0 = hf), and won him the Nobel prize.

What does this have to do with xenon? Xenon lamps produce not only visible light, but also ultraviolet light, in fact, xenon lamps extend further into the UV than natural sunlight:

http://en.wikipedia.org/wiki/Xenon_arc_lamp#mediaviewer/File...

LEDs by contrast produce almost no UV light:

http://en.wikipedia.org/wiki/Light-emitting_diode#mediaviewe...

With this data we can determine that the work function of silicon (in the Raspberry Pi 2) is somewhere in the range of hc/(400 nm) < Ω < hc/(250 nm) or 3.1 eV < Ω < 5 eV. Using data from

http://journals.aps.org/pr/abstract/10.1103/PhysRev.127.150

http://www.sciencedirect.com/science/article/pii/00223697599...

we see that the work function of silicon is around 4.7-4.9 eV, which agrees well with our observations.

draws a little box

Link for the lazy: 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

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.

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

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.

red laser makes it crash too

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.

A xenon tube is not a spark gap, in terms of the early radio receiver 'spark gaps' which were a primitive tuned circuit used to then feed an antenna (it wasn't the spark itself that was used for the transmission).

Sparks produce a broadband radio signal, that decays strongly with increasing frequency. Very little is present in the Megahertz region, which is why lightening doesn't interfere with FM radio that much.

Radio frequencies below the gigahertz or high megahertz region will not interfere with the Pi as the wavelength of these is too long to effectively couple into the short circuit board traces.

Xenon flash tubes are not that noisy in terms of unwanted RF. Apart from anything else, you wouldn't be allowed to sell such radiating circuits in consumer equipment.

A xenon flash puts out an enormous amount of blackbody light (high in IR and NIR) in a very short time frame (as short as 100 ns). Plastics are not very IR blocking by themselves. Silicon has a band gap of 1.1 eV. Any photons above this energy will be absorbed across the gap, and interfere with the circuitry. Firing a xenon flash close to something will result in an instantaneous light flux many hundreds of times larger than direct sunlight. So the circuit might be designed to be resistant to any ambient lighting situations, but not to being flashed.

As a commentator mentioned below, U16 is a switched mode power supply circuit. It's analogue, and switching quite fast. It doesn't take much for it to be disrupted & mess up a few cycles, possibly even then triggering some protection circuitry on the inputs of chips down the line.

Blue tack, being an insulator, will be almost totally transparent to radio frequencies.

Can the original poster just try putting a sheet of cardboard in front of the flash and see if it affects the 'scope results?

On the original thread, official Pi engineers have diagnosed this as light interference on a specific unshielded circuit. Just wanted you to know.

Hi, you should have a look elsewhere in these comments where a HN user has reproduced it with his camera flash, but used a piece of cardboard to block the light and the Pi behaved normally.

Cardboard (and I'll wager the blue tack also) has negligible EM shielding effect, so I'd say that's pretty conclusive that it's a photoelectric problem.

From this large picture,

https://blog.adafruit.com/wp-content/uploads/2015/02/Pi_II_t...

U16 appears to be in a WLCSP package (and others have noted that it is "shiny", which would also agree with the typical appearance of one), and WLCSPs do not have much of a package - they're essentially bare die that have had solder bumps attached to their top surface, and are mounted upside-down ("flip-chip" style) onto the PCB. It won't take much light intensity to cause photoelectric effects on a WLCSP.

More info here: https://ez.analog.com/docs/DOC-2357

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

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?

All chips are photosensitive and a xenon flash produces light from UV through to IR, so my guess would be that the plastic used for the casing on that chip is not entirely opaque to something at the top or bottom end of the visible spectrum, which is why most bright lights don't trigger it. At a bet, I'd say the top end, as otherwise radiant heaters would also probably kill them. Would be fun to play around with some filters and a flashgun and narrow down the frequency.

All semiconductor materials are photovoltaic to some extent.

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.

That's because completely free means different things to different people. Does it include the firmware/bios? Are you only concerned with what runs on the CPU?

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/

>In light of

I see what you did there!

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.

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

Wow there's a band of savages wielding heavy down voting hammers.

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

i'm seeing a market for chip "protectors" coming up, like the heatsinks that started selling for the original Pi

Bugs like these are the reason I never order version 1 of anything new.

Which is completely incorrect. There is a light sensitive chip on the board. They state which chip it is in the link.

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