Re: PXA270 WinCE LCD shutdown query

<adntaylor@xxxxxxxxx> wrote in message
Hi all,

I'm working with a PXA270 driving a small LCD, running CE 5, and we're
trying to get the transition to deep sleep mode to look nice, rather
than having the screen left with a nasty residual image.

LCD datasheets all recommend outputting a few frames of completely
black or white data, but I can't think of any way around this without
filling the frame buffer and somehow forcing windows to re-paint the
screen on resume. Nasty.

I'm guessing that just disabling the LCD controller will stop it
outputting anything at all, and so the LCD will be as stuck as it
would be if you just disabled it or removed power.

Anybody got any bright ideas on this one?

Can you change the colour depth of the screen on the fly and can you
change the frame pointer to point to a frame buffer elsewhere? If you
can change the frame pointer then you can point to a black/white frame
buffer in memory and leave the original screen intact. You won't need
that memory because you'll be asleep. If you can change the resolution
then you can reduce the colour depth to 1 bit per pixel and dramatically
reduce the size of the frame buffer. You can then reverse this on power
up and get your screen back very quickly.

Another (probably very tricky) way would be to store the 50 pixels or so
in front of where the frame buffer is reading and temporarily replace
them with black pixels. Put them back once the scan has passed Thus the
screen goes black but the frame buffer still holds your original image.
Timing on this would be a bit horrific but, if all you are doing is
waiting to sleep and can dedicate the processor to doing it then you
might be able to continually read the frame pointer faster than it is
moving through the memory. This assumes a lot though. Caching could
interfere, pointer might not move linearly, reading the pointer may
alter its behaviour, etc. All good fun.

Alternatively, how much brightness/backlight control do you have? You
might be able to turn off the backlight or dim to 0% before entering
deep sleep and avoid messing with the frame buffer at all.