Why does an LCD need a reset line?

[caphair]

Is it to reset all the pixels to a known state when the MCU isn't generating information?

I'm wondering because if the panel gets display information constantly why does it need to be reset at power up?

[jiroy]

Re: Why does an LCD need a reset line?

Sometimes , many wrong options lead to a confused and abnormal view , without knowing how to re-calibrate colors , Frequency , Horizantal , East-West correction , etc , so , here it goes .. Reset .

In Repairs , Reset is vital to define the health situation of an LCD .

[stj]

Re: Why does an LCD need a reset line?

data is clocked in serially,
if the counters start in a random state then the initial image could look like a mess.

also, if the mainboard takes a few seconds to boot, the reset signal prevents a screen full of colourfull snow.

[caphair]

Re: Why does an LCD need a reset line?

I see. I ask because I have an iPhone 6 with the lcm_reset line shorted. It's tied directly to the CPU so it's a CPU fault.

Normally this line is high at 1.8v. So what I tried was injecting 1.8v into the LCD connector (after removing a filter to separate the short)

I got the display to work but it required timing on my part of when the 1.8v needed to be injected with the phone being turned on.

When the phone goes to sleep and powered back on the display doesn't return so this led me to wonder why that is if it was just on with a static image that didn't change when it went into sleep mode

[goontron]

Re: Why does an LCD need a reset line?

Garbage data being sent to an LCD is interesting. On building coreboot for my thinkpad i ran into the Vbios not loading correctly. This lead to a black screen on boot, slowly fading in a sorta washover pattern to basicly a gray screen with every other line being a single pixel colour (R,G or B). Then when Unix finally booted and went graphical, the screen would reset and would have a rolling flicker for about a minute afterwards.

I don't think it will damage anything, but that rolling flicker afterwards does suggest it screws with the logic...

[caphair]

Re: Why does an LCD need a reset line?

So basically less display errors occur when the display is turned back on from a known state and quickly instructed on what to display?

For example if a known state is a white screen the CPU/GPU quickly tells the display what the last image was before it was turned off to display that

[stj]

Re: Why does an LCD need a reset line?

it's possible that the display chip is not powered off, but simply sent a command to shut the glass drivers down.

[goontron]

Re: Why does an LCD need a reset line?

^Yep.

[caphair]

Re: Why does an LCD need a reset line?

Is there anyway to replicate the reset command in my case so the display keeps coming back on

[gabiz_ro]

Re: Why does an LCD need a reset line?

Hard to tell.
Since touchscreen and display activity are somehow tied together you may use some power line from touchscreen or maybe if exist reset from touchscreen.
Since touch must be active while display is on and stay active as long as display is on there may be some singnal low-high for touchscreen on-off and you may use it somehow for lcd reset.

[stj]

Re: Why does an LCD need a reset line?

maybe you can get something from the backlight drive.

[gabiz_ro]

Re: Why does an LCD need a reset line?

That one too.
Now depends on delay.
Timming seems to be required.
One logic analizer could help to see what gets low or high and when is need to reset LCD to be high.
But that mean you need to add some extra parts small enough to fit in available space.

[caphair]

Re: Why does an LCD need a reset line?

I tried getting the 1.8v from lcd_enable rail which is also 1.8v.

That worked for initial power on but again when phone goes to sleep the display doesn't come back after waking it up. Only on initial power up if have it powered off.

Do it's all about timing? Even though it's just 1.8v the reset line needs to be activated at a certain time?

[gabiz_ro]

Re: Why does an LCD need a reset line?

Timing is critical I think.
First time is enable signal then with a little delay reset must come then maybe all OK.Or may be reset then enable.
Best chance is like I said before to use some multichanel logic analyzer on working unit.
You may see when and after or before reset signal is generated.

Edit
On most CPU related problems,reset signal must be after all conditions are OK to make proper initialization.
So LCD enable power the LCD,then there must be a reset to start fresh communications,but that could be wrong,LCD must be reset then power enabled.
My bet is on first variant,power enabled after that reset occur after that LCD is feeded with data.

[caphair]

Re: Why does an LCD need a reset line?

Timing is critical I think.
First time is enable signal then with a little delay reset must come then maybe all OK.Or may be reset then enable.
Best chance is like I said before to use some multichanel logic analyzer on working unit.
You may see when and after or before reset signal is generated.

Edit
On most CPU related problems,reset signal must be after all conditions are OK to make proper initialization.
So LCD enable power the LCD,then there must be a reset to start fresh communications,but that could be wrong,LCD must be reset then power enabled.
My bet is on first variant,power enabled after that reset occur after that LCD is feeded with data.

Thanks. So to help understand this better, I measured a working one and the reset 1.8v is present the entire time the display is on. So how exactly does that voltage reset the display to avoid display errors? I pictured it like a quick 1.8v that goes away to arrange the glass drivers before receiving display information

[stj]

Re: Why does an LCD need a reset line?

the transition from 0v to 1v8 is the signal
you may need a fast scope to catch it.

[caphair]

Re: Why does an LCD need a reset line?

the transition from 0v to 1v8 is the signal
you may need a fast scope to catch it.

So at 1.8v the glass drivers would be "reset" the entire time the screen is on to properly display correct images?

[stj]

Re: Why does an LCD need a reset line?

maybe.
i dont know if the reset signal is active-high or active-low.

but the magic takes place on the transition.

[caphair]

Re: Why does an LCD need a reset line?

maybe.
i dont know if the reset signal is active-high or active-low.

but the magic takes place on the transition.

On a working one 1.8v is there during screen operation and 0v when the phone is sleeping/off

[caphair]

Re: Why does an LCD need a reset line?

maybe.
i dont know if the reset signal is active-high or active-low.

but the magic takes place on the transition.

So if this line is shorted then it's the transistor within the CPU that's gone bad holding it at 0v correct? (If its normally active high that is)