Re: SED1335 LCD Controller: Screen Disturbance

"Richard Phillips" <raphillips@xxxxxxxxxxxx> wrote in message
news:e78lk8$7k0$1@xxxxxxxxxxxxxxxx
Hello all,

I've been using this chip (built into an LCD module) for a while and have
the LCD inverted, i.e. background "black" and characters "green".

I find that when I write info to the screen I often get very slight
disturbances (tiny horizontal lines) appearing on the screen that vanish
again almost immediately.

This is caused by writing in the active refresh time.

I have written my code so that it only updates the SED's memory inbetween
LCD screen-line writes (if I don't do this, the problem is MUCH worse!) to
minimise flicker.

This is a common problem. If you are writing to the controller when it wants
to read for refresh it substitues 0x00. So the problem is much worse if you
have mostly 0xFF. Doing the writes in the line and frame sync is definitely
the right thing. But be careful if you use the line sync because this can be
very short, so you don get much chance for the update. Think interrupts!
Suppose an interrupt comes along when you have just decided there is time to
transfer another byte. The transfer gets delayed in the active part of the
line and...

I found it better to only use the frame sync period since this is longer. I
program the controller for a relatively long sync (slightly slower refresh)
to give a larger proportion of the time available for updating.

I find that if the LCD is not inverted, the effect cannot be seen at all
(at
the moment, the spec I am following requires inversion).

It is often possible to by panels that are inverted or not, in the same form
factor.

Does anyone have any experience of this problem with this chip? I have
poured over the device spec and can't see anything I can do, it's not a
terrible problem because the disturbance is only slight, but I'd like to
get
it as good as possible!

Its an old chip. I used used it years ago.

Regards,
Richard.

Peter


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