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From: hondgm on 5 Aug 2010 07:47

I'm using a 320x240 monochrome LCD with a built-in high voltage
generator. As is probably standard, the LCD bias is provided out of
the module, and I have to use a resistive divider to provide this
voltage back into the module for contrast adjustment. This simple two-
resistor circuit is given in the datasheet. The LCD contains a
controller if that helps (RA8835).

Up until now, I've always simply turned the main 5V supply on and off
with
no regard to power sequencing. The only odd thing I've noticed is on
power-down, one dark horizontal line in a random location will appear
for
maybe one second. (Incidentally, my TI-89 calculator does the same
thing!)
If I immediately power the module back up, the horizontal row of
pixels
where that dark line was looks slightly lighter in contrast than other
pixels that have been turned on via the attached microcontroller.
After a
second or two, there is no noticable contrast differences.

Is this bad for the liquid crystal material? I'm surprised the
datasheet
for this specific module makes no mention of any of this, but another
module's datasheet from the same company (Densitron) does talk about
sequencing. How damaging would this behavior be to the LC material in
the
glass panel? I've been using it like this for some time and haven't
noticed any detrimental effects, but maybe I'm slowly killing it.







From: Jan Panteltje on 5 Aug 2010 10:39
On a sunny day (Thu, 5 Aug 2010 04:47:36 -0700 (PDT)) it happened
hondgm(a)yahoo.com wrote in
<9c23db10-62ec-4d98-b745-5af419bd22dc(a)i31g2000yqm.googlegroups.com>:

>
>I'm using a 320x240 monochrome LCD with a built-in high voltage
>generator. As is probably standard, the LCD bias is provided out of
>the module, and I have to use a resistive divider to provide this
>voltage back into the module for contrast adjustment. This simple two-
>resistor circuit is given in the datasheet. The LCD contains a
>controller if that helps (RA8835).
>
>Up until now, I've always simply turned the main 5V supply on and off
>with
>no regard to power sequencing. The only odd thing I've noticed is on
>power-down, one dark horizontal line in a random location will appear
>for
>maybe one second. (Incidentally, my TI-89 calculator does the same
>thing!)
>If I immediately power the module back up, the horizontal row of
>pixels
>where that dark line was looks slightly lighter in contrast than other
>pixels that have been turned on via the attached microcontroller.
>After a
>second or two, there is no noticable contrast differences.
>
>Is this bad for the liquid crystal material? I'm surprised the
>datasheet
>for this specific module makes no mention of any of this, but another
>module's datasheet from the same company (Densitron) does talk about
>sequencing. How damaging would this behavior be to the LC material in
>the
>glass panel? I've been using it like this for some time and haven't
>noticed any detrimental effects, but maybe I'm slowly killing it.

Probably the micro or ASIC in the LCD panal stops scanning.
Then you get DC on the LCD crystals for a moment.
DC will polarize them a bit much one way, and if left in that mode may
be hard to get out, back to normal.
I am not sure it is permanently destructive, clearly in your case
when scanning resumes it slowy returs to normal.
But there are so many types of liquid crystals these days,
that the only way top find out is contact the manufacturer to make sure.
From: Grant on 5 Aug 2010 17:32
On Thu, 05 Aug 2010 14:39:38 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote:

>On a sunny day (Thu, 5 Aug 2010 04:47:36 -0700 (PDT)) it happened
>hondgm(a)yahoo.com wrote in
><9c23db10-62ec-4d98-b745-5af419bd22dc(a)i31g2000yqm.googlegroups.com>:
>
>>
>>I'm using a 320x240 monochrome LCD with a built-in high voltage
>>generator. As is probably standard, the LCD bias is provided out of
>>the module, and I have to use a resistive divider to provide this
>>voltage back into the module for contrast adjustment. This simple two-
>>resistor circuit is given in the datasheet. The LCD contains a
>>controller if that helps (RA8835).
>>
>>Up until now, I've always simply turned the main 5V supply on and off
>>with
>>no regard to power sequencing. The only odd thing I've noticed is on
>>power-down, one dark horizontal line in a random location will appear
>>for
>>maybe one second. (Incidentally, my TI-89 calculator does the same
>>thing!)
>>If I immediately power the module back up, the horizontal row of
>>pixels
>>where that dark line was looks slightly lighter in contrast than other
>>pixels that have been turned on via the attached microcontroller.
>>After a
>>second or two, there is no noticable contrast differences.
>>
>>Is this bad for the liquid crystal material? I'm surprised the
>>datasheet
>>for this specific module makes no mention of any of this, but another
>>module's datasheet from the same company (Densitron) does talk about
>>sequencing. How damaging would this behavior be to the LC material in
>>the
>>glass panel? I've been using it like this for some time and haven't
>>noticed any detrimental effects, but maybe I'm slowly killing it.
>
>Probably the micro or ASIC in the LCD panal stops scanning.
>Then you get DC on the LCD crystals for a moment.
>DC will polarize them a bit much one way, and if left in that mode may
>be hard to get out, back to normal.
>I am not sure it is permanently destructive, clearly in your case
>when scanning resumes it slowy returs to normal.
>But there are so many types of liquid crystals these days,
>that the only way top find out is contact the manufacturer to make sure.

LCDs usually say no DC or they 'burn in', but I know there's cheap
panel meters connect dp to DC instead of adding an inverter.

Some LCD chips specify a powerdown sequence in software to kill LCD
drive before removing supply.

I think OP's calculator example shows it probably doesn't matter,
long term. Looks bad though.

Grant.
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