LCD interfaces

[D Yuniskis]

Hi,

Most of the things I design don't have traditional
displays, so... :<

Aside from mechanical differences in connectors
(a consequence of the number of "color bits"),
can I safely design a generic LCD interface
without preselecting the actual display panel?

I'm looking at small-ish panels (3" - 8") with
an emphasis on low power consumption.

Is there a magic cutoff point between LED backlighting
vs. CCFL's? (i.e., this would impact the controller's
design as well)

Thanks!
--don

 

[D Yuniskis]

You can have Intel or Mot styles, 1 or 4 bits serial, 8, 16 or 32 bits

Huh? I'm talking about the bare *panel*, not controller.

parallel. With / without backlite. With / without touch panels. So,
the short answer is probably not.

Touch panel is a separate issue and addressed via touch
panel interface (from what I have seen, the touch panel
is a seperate assembly, seperate connector, etc. -- even
if it is purchased "married" to the display panel)

CCFL needs 40V to 50V minimum. So, LED is easier for most people to
handle.

Most of the CCFL inverters I have seen were ~12V powered.

Regardless, my question was trying to determine if there
is a *size* beyond which LED is impractical (inefficient
in terms of cost).

 

[D Yuniskis]

Hi Lasse,

there's a step between bare panel and full controller with build in
framebuffer etc.

signals are something like; vsync,hsync, R,G,B(~6bits each) and a
clock. either as seperate lines or serialized onto a few LVDS lines

Exactly!

I want to know how "standardized" the signals presented via
the "flex circuit" tail are. Aside from differing numbers of
"color bits", do all panels use the same basic signal sets?
I.e., how reasonable is it to expect a controller to talk
to a wide variety of panels (assuming the connections can
be mechanically accommodated) without requiring significant
hardware (electronics) changes.

E.g., controlling CCFLs is different than an LED backlight.

(4/5 wire resistive touch is different than SAW, etc.)

Thanks for clarifying that!
--don

 

[Martin Riddle]

as far as I can tell they are all more or less the same, only
difference is
is timing and that should be programmable in the controller

the parallel to seriel are standard chips such as:
http://www.national.com/ds/DS/DS90C365A.pdf




-Lasse

The Thine Controller seems to be more popular, 18bit and 24bit LVDS.

Cheers

 

[Nico Coesel]

Hi,

Most of the things I design don't have traditional
displays, so... :<

Aside from mechanical differences in connectors
(a consequence of the number of "color bits"),
can I safely design a generic LCD interface
without preselecting the actual display panel?

You cannot. Selecting an affordable panel is a world of pain. Make
sure you can get the cables as well!

 

[Nico Coesel]

Hi Lasse,



Exactly!

I want to know how "standardized" the signals presented via
the "flex circuit" tail are. Aside from differing numbers of
"color bits", do all panels use the same basic signal sets?

More or less. If you have hsync, vsync, pixelclock and RGB data
you'll be fine to drive CMOS interface style displays (up to about
40MHz pixel clock). Expect displays to have different timing
requirements though. Some displays have a data-enable signal but this
is usually more trouble to create than its worth.

 

[Spehro Pefhany]

You cannot. Selecting an affordable panel is a world of pain. Make
sure you can get the cables as well!

I'm in the middle of trying to find sources for a small QVGA panel,
there seem to be some "almost compatible" COG chips (from Taiwan). The
chips are a bit strange since they don't need to be packaged, they are
shaped like matchsticks. Anyway, several seem to use a common FPC
pinout but even then all the modes may not be available (and the most
tempting modes to omit probably the ones *you* would want- the ones
that are not used for video display).


Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

They are designed to be mounted directly on the glass, usually 0.1mm
pitch.

Hence I use the term "COG" = Chip On Glass.

LCD does not need hsync and vsync. There is no beam to retrace
anyway.

They have sync signal inputs, presumably to reset internal counters.
"Video" input is RGB digital parallel. Gamma and so on is dealt with
by an auxilliary SPI interface.

 

[D Yuniskis]

Hi Dimiter,

more or less they do, lately I am witnessing them to also
specify a *minimum* clock frequency. On older TFT displays
one could lower the clock frequency at will until the colours
began to degrade (because of TFT gate leakage), now this
seems to have changed, perhaps they have some PLLs or sort of.

Or, perhaps an onboard buffering just preserving a "classic
interface"?

But I am not sure, have not fought any such wars, just
an observation while considering various parts.


Fairly reasonable, if you get a controller, that is. The last
monolithic one on the market was the b69030 by Chips/Intel/Asiliant,
now there are some hybrids by a Taiwanese manufacturer (this
applies if you are looking for VGA and above resolutions).
Some SOC parts do come with a display controller as well
(unfortunately none I like is still really on the market,
the closest was the MPC5121 but one of its errata said
"the SATA interface won't work; workaround: don't use SATA....).
I am eyeing one of their new QORIQ parts now but it seems
to be still far from anything really usable, let's see if
they will make it (I do hope they will).

I'm just trying to reduce the number of variables in the
design process. I.e., if I can pick a controller and
design around that and *later* pick a display panel, that's
much easier than waiting until *all* of the choices have been
made and then rushing to accommodate all of them in a cohesive
design.

Usually backlight is not considered part of the "controller"
interface, the controller has some TTL output to drive it
on/off, I usually use some GPIOs to make up some sort of
DAC to drive the intensity and that's it. If you want to be
able to use both LED and CCFL backlit panels you will have
to design in both power sources somehow (not a huge issue since
CCFL usually need a tiny convertor board anyway, one could
simply change it as needed).

Yes. In my case, I want to put all of the display interface on a
small board (LCD interface, backlight, touchpanel, etc.). So,
if I end up with a panel that has LED backlighting, I need
to have provisions for that on my board. Likewise, if I
aimed for a CCFL backlight, then I would need the hooks to
turn that backlight on, vary it's intensity, detect faults
in the inverter, etc.

I'm just trying to gauge what approach is safest to follow.

--don