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From GBATEK, writes to a certain I/O port for power management may damage hardware:

4000304h - NDS9 - POWCNT1 - Graphics Power Control Register (R/W)
  0     Enable Flag for both LCDs (0=Disable) (Prohibited, see notes)
  1     2D Graphics Engine A      (0=Disable) (Ports 008h-05Fh, Pal 5000000h)
  2     3D Rendering Engine       (0=Disable) (Ports 320h-3FFh)
  3     3D Geometry Engine        (0=Disable) (Ports 400h-6FFh)
  4-8   Not used
  9     2D Graphics Engine B      (0=Disable) (Ports 1008h-105Fh, Pal 5000400h)
  10-14 Not used
  15    Display Swap (0=Send Display A to Lower Screen, 1=To Upper Screen)
  16-31 Not used
Use SwapBuffers command once after enabling Rendering/Geometry Engine.
Improper use of Bit0 may damage the hardware?

Emphasis mine. The document does not go into detail and provides no concrete information. How could improper use of the enable flag for LCDs cause hardware damage? Under what circumstances would it cause damage, and what types of damage to the hardware would it cause?

I've posted this after writing a meta question about the NDS and receiving the answer that it may finally count as retro.

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    Possibly enabling both at once would overload some part of the power supply chain. Or, it might exceed the fan-out of some chip trying to drive both at once, possibly corrupting other signals generated by the same chip with potential collateral damage to anything. IMO this needs circuit diagrams and data sheets to give a good answer. The power management page linked to by your link suggests the design is a bit fragile - e.g. look at the comment on issues with backlighting. – alephzero Feb 9 at 2:25
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    @alephzero As far as I know, there are no datasheets for the relevant parts. The NDS main processor is an ARM946E-S with custom display components that aren't public. However the author of the referenced document doesn't have access to any confidential information (as far as I am aware) but still wrote that improper use of the display enable bit may damage hardware. – forest Feb 9 at 2:30
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    @forest we can only speculate, but sometimes you find out the hard way that "improper use" of some bits in some registers can cause damage to the hardware: someone tried it, and the hardware was damaged. (This has actually happened to me, though with other hardware). And "improper use" usually means something like "you'll need to set up other things in other registers to make this work without damaging anything, but without documentation no one knows how". – dirkt Feb 9 at 6:16
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    An LCD does not like to be driven by DC, perhaps this bit switches off the AC generation. – the busybee Feb 9 at 19:48
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    I wrote an email to the author of that document. He said that he wrote that because Nintendo states that that bit is "prohibited". He hasn't done any empirical testing but says that one could try to randomly toggle it on and off and see if it causes any damage or just temporary glitches. – forest Feb 21 at 23:45

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