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PostPosted: Sun Mar 29, 2015 3:38 am 
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I already made topic about this at gbdev.gg8.se forum, but this forum seems more active.

There is this caution in Pan Docs (http://gbdev.gg8.se/wiki/articles/Video_Display#LCDC.7_-_LCD_Display_Enable):
Quote:
CAUTION: Stopping LCD operation (Bit 7 from 1 to 0) may be performed during V-Blank ONLY, disabeling the display outside of the V-Blank period may damage the hardware. This appears to be a serious issue, Nintendo is reported to reject any games that do not follow this rule.

However, in leaked official Nintendo Game Boy Programming Manual (version 1.0, released 11/09/1999) it's not mentioned that it can cause any hardware damage, only that "a black horizontal line appears on the screen" if LCD turned off outside of VBlank interval (except on Game Boy Color, where it's safe). Also it says that it's 'recommended', not 'required'.

In the end, which one is correct?


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PostPosted: Sun Mar 29, 2015 7:27 am 
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Fwiw, I saw your original post on the other forum last week (I stop by from time to time)

Anyway, I'd go with official documentation. I'm not a hardware guy, but if you flipped the POWER switch on a DMG while it was rendering mid-frame, wouldn't this be the equivalent of telling the MMIO register (LCDC) to do it via software? If that is indeed the case, I doubt you can damage DMG LCDs by simply shutting the system off. I haven't touched my GameBoy Pocket in ages, but iirc, you could see the "line" the official docs talk about when turning it off.

Note that this isn't going to be an issue on the GBC afaik.


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PostPosted: Sun Mar 29, 2015 8:01 am 
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I've seen it too: a black line appears for a second and then fades. I guess if it happens once every 5 minutes, it's OK, but if you do it every frame or every 10 seconds, it can wear that line of the LCD down.


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PostPosted: Sun Mar 29, 2015 8:18 am 
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Shonumi wrote:
Note that this isn't going to be an issue on the GBC afaik.

Yes, that's true according to Official Game Boy Programming Manual.

tepples wrote:
I guess if it happens once every 5 minutes, it's OK, but if you do it every frame or every 10 seconds, it can wear that line of the LCD down.

But why would it wear LCD's line down?


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PostPosted: Sun Mar 29, 2015 8:30 am 
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Have you ever heard of a concept on CRTs called "screen burn-in"?

This is just a conjecture, but perhaps the drivers for each scanline were designed to pass a specific amount of current. Normally, the PPU drives each line for 109 microseconds (source: Pan Docs) before going onto the next line. Thus the average current over the course of, say, 10 seconds remains small. But if one line is driven constantly over a long period of time, it could cause the driver to pass more current than it's rated for, and things overheat.


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PostPosted: Sun Mar 29, 2015 11:07 am 
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LCDs work by applying an electric field to a very thin layer of liquid crystal.
When so affected, the liquid crystal reorients, reducing (or increasing) the amount of light that can go through.
HOWEVER, the liquid crystal itself additionally moves a little bit from one plate to the other.
So every LCD necessarily has to alternate whether the electric field is "up" or "down" to prevent net migration.

When you shut down the game boy's driver that advances which row it's refreshing, it almost assuredly also shuts down this other alternation. Leaving an LCD with a net DC bias for too long will result in elements that are stuck on.

Prolonged DC operation may cause electrochemical reactions inside the displays which will cause significantly reduced life. The initial indications of display degradation because of excessive DC current is a loss of alignment along the edges of some of the characters. The visual indication will be a "fuzzy" appearance of some of the characters.
LXD Inc wrote:
The initial indications of display degradation because of excessive DC current is an electro-plating of liquid crystal components onto the surface electrodes. These will appear as a mirror-like "burn-in" of the electrode pattern that was energized with DC. Elevated temperature will accelerate this effect.


So, how long is too long? Dunno. If what you're really asking is "I need a whole frame's time to upload something to the screen and I don't want to wait for vblank" or "I want to use mid-frame OAM refresh", it's probably fine.


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PostPosted: Sun Mar 29, 2015 4:09 pm 
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The real question is why did they literally make it shut off the screen instead of just making it output blank pixels (like everything else did).


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PostPosted: Sun Mar 29, 2015 4:36 pm 
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Presumably power savings; the same reason that shutting off the sound hardware not only silences everything but also loses register state.

Not clear what the use case for the DMG was; I don't remember seeing any GB games that used this "shut everything down to allow extended sleep" like I did with the GBA. (Maybe I just wasn't paying enough attention?)


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PostPosted: Sun Mar 29, 2015 8:11 pm 
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tepples wrote:
But if one line is driven constantly over a long period of time, it could cause the driver to pass more current than it's rated for, and things overheat.

lidnariq wrote:
When you shut down the game boy's driver that advances which row it's refreshing, it almost assuredly also shuts down this other alternation. Leaving an LCD with a net DC bias for too long will result in elements that are stuck on.

That would be true only if current still flows through LCD after LCD is turned off by CPU. Would that black line quickly fade out after turning LCD off by CPU like when you shut off whole Game Boy off or not? Unfortunately, I don't have flash cartridge for Game Boy to experiment.


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