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clarify TFT chip photosensitivity?

Posted: Sun Aug 14, 2022 9:58 am
by argonblue
This guide page says that the controller chip for the TFT display is embedded in the connector cable and can be damaged by bright light. However, it shows a blue colored plastic covering that area of the cable. My Clue has black plastic in that area. Is that sufficient to mitigate the photosensitivity? Are newer revs of the Clue with black plastic covering the left edge of the TFT still photosensitive to the same degree?

I submitted a feedback form about this, but I haven't seen an update to the page yet.

Re: clarify TFT chip photosensitivity?

Posted: Wed Aug 17, 2022 9:18 pm
by adafruit_support_mike
The display driver is an IC die mounted directly to the glass. There's an array of transparent conductive traces that run to all the TFT's rows and columns.

All doped silicon is photosensitive, and will emit current when exposed to bright light. It usually takes something like a camera strobe to generate enough of a pulse to make a chip glitch out. The principle came into general attention when the RasPi started using some raw silicon dice on the PCB and people found that the board would crash when they took a photo of it.

It's hard to make universal statements about undefined behavior, but in most cases a light-induced glitch doesn't cause any damage. You just have to power cycle the board to put the IC back into a well-defined operating condition.

Normal light levels don't have any effect on the chip, which is mounted face down on the glass. An external light source has to be strong enough to reflect off the back of the glass and onto the underside of the chip before it can cause a voltage glitch. Because of that, the plastic stiffener bonded to the flex cable has no measureable effect under normal operating conditions.

If you plan to run the display near high-intensity light or strobes, cover the driver with something opaque. A piece of electrical tape should work, or you could use foil tape to be really sure.

Re: clarify TFT chip photosensitivity?

Posted: Wed Aug 17, 2022 9:47 pm
by argonblue
Thanks! I was asking specifically about what appears to be a design change from the translucent blue plastic strip depicted in the guide photos to the one used in the Clue unit that I have, which apparently has opaque black plastic instead. Is this a change intended to mitigate glitches from intense light, or is it unrelated?

Also, are there any reports of permanent damage to the TFT driver chip from camera flashes and other intense light? Or are the observed effects temporary as far as you know?

Re: clarify TFT chip photosensitivity?

Posted: Thu Aug 18, 2022 8:35 pm
by adafruit_support_mike
argonblue wrote: Wed Aug 17, 2022 9:47 pmIs this a change intended to mitigate glitches from intense light, or is it unrelated?
It's unrelated. If anything, it indicates a specific version of the driver chip.
argonblue wrote: Wed Aug 17, 2022 9:47 pmAlso, are there any reports of permanent damage to the TFT driver chip from camera flashes and other intense light?
I've never heard of a permanent effect. Silicon is photoelectric, but the stuff used in ICs isn't very efficient. On top of that, the PN junctions are microscopically small, which limits the amount of photocurrent they can generate.

In practical terms, unless you specifically set out to force glitches with a strong strobe, this is far less of a concern than cosmic ray impact (which happens all the time and has far more effect because the photons are so much more energetic).