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A Xenon flash will cause the Raspberry Pi 2 to freeze

raspberrypi.org

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Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#61

Post by analog31 »

jayrhynas wrote: post #54
noonespecial wrote: post #50

Earlier quoted context omitted.

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?

The most recent post [1] seems to indicate that covering one of the chips with blu-tac prevents the freeze [1] http://www.raspberrypi.org/forums/viewtopic.php?f=28&t=99042...

Not knowing what the IC is, one thought is that the blu-tac could be blocking light, or providing a wee bit of extra capacitance to tame a wicked electrical signal.

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#62

Post by tdicola »

noonespecial wrote: post #50
tdicola wrote: post #33

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 edit 2: A…

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?

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.

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#63

Post by lotsofmangos »

foobarbecue wrote: post #44
lotsofmangos wrote: post #21

Earlier quoted context omitted.

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…

"When they get that short they are basically becoming infra-red anyway." You mean UV. You got your spectrum backwards. You should probably just admit your error here -- it's light, not radio waves, causing the effect. Sure, they're part of a common spectrum, but your statement "radio waves are light" is obviously false. It's a bit like saying "red is blue". Edit: I misread the above, lotsofmangos was talking about wa…

I have been saying all along that it is electromagnetic waves in the visible or near visible spectrum causing the effect, otherwise the bluetack would not work.

As to saying that radio is light, if calculating the propagation speed of a radio wave, you don't talk about its speed of radio, you talk about its speed of light. In many technical contexts, light is one of the default terms.

Also, there would be no need for the term 'visible light' if light could not be used to describe electromagnetic radiation outside the visible spectrum. Infra-red and ultra-violet are commonly thought of as light and you can even create lensing optics for radio waves and for x-rays.

http://www.rozendalassociates.com/Luneberg/index.htm

https://www.imt.kit.edu/english/236.php

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#64

Post by na85 »

swamp40 wrote: post #13

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 lig…

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.

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#67

Post by karlshea »

noonespecial wrote: post #55
nitrogen wrote: post #53

Earlier quoted context omitted.

It's not a mains spike, but an EM discharge caused by the rapid voltage and current changes in the flash, that the parent comment is considering (IIUC).

Yes thankyou. This is what I'm getting at. The momentary power flow in a xenon flash is very large. A great big capacitor charges up and then discharges very rapidly to create the bright flash of light. There is also a high voltage exciter coil that fires to kick the xenon tube into conduction at the beginning of the flash cycle. Both of these could be very disruptive to nearby digital electronics. I thought the orig…

From the original thread it does appear to be a photoelectric effect, since blocking a specific chip on the board causes it to survive the flash.

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#68

Post by swamp40 »

jarvist wrote: post #29
pjc50 wrote: post #19

Earlier quoted context omitted.

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 casin…

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.

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.

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#69

Post by bluehex »

jayrhynas wrote: post #54
noonespecial wrote: post #50

Earlier quoted context omitted.

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?

The most recent post [1] seems to indicate that covering one of the chips with blu-tac prevents the freeze [1] http://www.raspberrypi.org/forums/viewtopic.php?f=28&t=99042...

I'm thinking of trying a more permanent fix with some Sugru[1] when my Pi 2 arrives. Their about says it's insulating and okay for electric repairs under 24 volts.

[1] http://sugru.com/about

Re: A Xenon flash will cause the Raspberry Pi 2 to freeze

#70

Post by noonespecial »

tdicola wrote: post #62
noonespecial wrote: post #50

Earlier quoted context omitted.

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?

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.

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