View Full Version : Digital RGB

Ken Peterson
09-01-2007, 09:53 AM
Has anyone tried driving an LCD display with digital RGB with the Propeller?· I have some ideas about this - including using the VGA object.· I'm sure if I tie the two bits of output to every other bit on each channel, I should get a good proportion for the color.··

For example on 8-bits per channel (24 bit RGB):

00 = 00000000
01 = 01010101
10 = 10101010
11 = 11111111

I was thinking of generating a dot clock signal with another counter, but how do I·sync the phase with the video hardware?· Another thing I'm unsure about is the alternating polarity issue with LCD's.· VCOM, etc.· Do I need to keep inverting the data lines every frame?· How does that work?· Anybody have experience with driving LCD modules?




The more I know, the more I know I don't know.· Is this what they call Wisdom?

Ken Peterson
09-01-2007, 09:57 AM
by the way....an example would be nice.· Here's an example data sheet for an LCD display to which I'm referring.


The more I know, the more I know I don't know.· Is this what they call Wisdom?

Marcel Majoor
09-01-2007, 12:54 PM
I have done some interfacing with a LCD from the same manufacturer, but which is controlled differently. I have included the code, and in the code is also a schematic of the connections. I did not use the VGA object, but control it directly. The LCD you refer to uses one byte per color. 'Mine' does 3-colors per byte, which makes it slightly more difficult.

09-01-2007, 01:05 PM

I'm interested in display options... but I am mostly in the cheering section right now.

When you guys get talking about this stuff, I get a little winded trying to keep up. So, please allow a newbie/near-tech type question.
It seems to me that the display issue always gets down to memory and bandwidth, with memory being the real killer. I've seen a couple of solutions in various stages of development... one by Baggers that uses interleaved cogs. So that approach would fall into the tried and true in my book. What that gets a person in terms of ultimate performance is beyond me... throwing additional Props would seem to follow that same strategy and give unlimited bit depths.

In looking at this display... my question would be what advantages do you or other forum experts forsee over the approach taken by 4DSYSTEMS with their uOLEDs?


09-01-2007, 01:24 PM
rjo_ said...
.. my question would be what advantages do you or other forum experts forsee over the approach taken by 4DSYSTEMS with their uOLEDs?

(1) Choice! There is a limited offer of such "special" displays, with draw-backs:" Buy or die" http://forums.parallax.com/images/smilies/smile.gif
(2) Price! Not only due to missing competition, but also because of additional parts needed and low volume, price is extraordinary high for such "convenient displays". A naked 234x480 TFT display as used in car monitors (6" to 8") cost around $20.
(3) Quality! OLED is not yet "mature". At the moment the OLEDs are expected to fail after 10,000 hours (around 3 years), but maybe not so relevant for hobby use
(4) Form factor! Some near sighted persons like me have mixed feelings WRT 1" displays...

Post Edited (deSilva) : 9/1/2007 6:29:37 AM GMT

09-01-2007, 01:32 PM
Ken Peterson said...
....including using the VGA object.

I think this is a wrong conception. The relevant part of the "VGA object" consists of 30 lines of machine code... I should suggest:
(1) You understand that part
(2) You understand your monitor (sorry, I cannot help much with the datasheet..)
(3) Code your driver

Ken Peterson
09-01-2007, 08:20 PM
I've been looking at the screen requirements and the VGA driver, and it looks like I would end up writing something very close to the VGA driver anyway, So why re-invent the wheel?


The more I know, the more I know I don't know.· Is this what they call Wisdom?

09-02-2007, 02:37 AM
I know baggersis a busy man, but I would PM him and see what his take is on this..

E3 = Thought

http://folding.stanford.edu/ (http://folding.stanford.edu/)·- Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats. (http://vspx27.stanford.edu/cgi-bin/main.py?qtype=teampage&teamnum=78528)

09-02-2007, 03:35 PM
Hi All, sorry for the lack of responses and updates lately, but my wife's Nan passed away, so that's gonna take up my time for a good few days.


In the imortal words of the Terminator...

I'll be back. :)

09-03-2007, 12:17 AM
Holy cow, you guys should definitely check out Marcel Majoor's code posted above. It's an awesome example of a well-documented program, making excellent use of the propeller font. The only problem is it makes me ashamed to post any of my own code now.

09-03-2007, 02:12 AM

Thanks for your response.

About the form factor... the uOLED is a wearable monitor. To make a head mount out of it, you just add a plastic 35mm slide viewer, hinge and wear it as a jewler's loop. Then to work on our Prop at the theatre, all we need is a keyboard built into a pair of knee pads:)

But seriously, I was asking about the technical limitations of the two basic approaches. I take from your answer that it is "6 of one and a half dozen of the other?"


09-03-2007, 02:45 AM
Just to clarify the comments about the "life expectancy" of OLED displays. The 10,000 hours noted in some data sheets refers to the blue pixels·and actually refers to the "half life' of the pixels. Which means that the intensity levels of the blue pixels will be deminished by half after ~10,000 hours. The red and green pixels are much better with a half-life betweem 50,000 and 100,000 hours. The new active-matrix (most OLED displays available today are passive-matrix)·TFT OLED displays·not only have better half-life numbers, but provide overall brighter displays PLUS significantly reduced power requirements.

If you're interested in some good technical·(rather than anecdotal) information on the current state of OLED technology, take a look at the following:


http://electronics.howstuffworks.com/oled3.htm·(and the other pages in this article)


Ken Peterson
09-03-2007, 03:27 AM

That's all well and good, but this thread·is dedicated to driving Digital RGB LCD devices.· Please start another thread if you want to talk about OLEDs.· I know they're the latest thing and will eventually replace LCDs but let's stick to the topic.




The more I know, the more I know I don't know.· Is this what they call Wisdom?

09-03-2007, 03:37 AM
@Duffer: A well known German distributor still warns his customers, that they buy OLEDs "on their owen risk" http://forums.parallax.com/images/smilies/smile.gif
Japanese "Global Players" promise large displays for years now http://forums.parallax.com/images/smilies/smile.gif
Of course there is steady development and I expect us all using nothing but OLEDS from 2010 on... Seriously!

@rjo_: I think an interestig question is:

What is a display?

The tendency for years now is to use more and more intelligent devices, controlled through sophisticated protocols (as USB e.g.) This is fine. When you look inside e.g. an ADC you will be shocked by the amount of circuitry in it, but it is still basic electronics, and you can most of it build-up discretely. I call such devices "unintelligent", as they can be very complex and the term "simple" does not seem appropriate...

In the field of "micro controller applications" you generally connect an intelligent controller to unintelligent devices - this is most of the fun!

Connecting intelligent devices is no fun at all, but work. You could do it as well from your PC. Or buy some "smart phone" and program it in JAVA...

Of course there is never black and white.
But I would hazitate to connect a device to my Prop which is (considering the mass of code it uses) 1000 times more intelligent than my own program in the Prop and say "Look Ma, what I have done"...

Post Edited (deSilva) : 9/2/2007 9:33:43 PM GMT

Ken Peterson
09-03-2007, 04:25 AM
Exactly, deSilva! What's the point of that? Many of us want to use hardware that we obtain out of who knows what, instead of paying big bucks for hardware that is designed to do it all for us! Prop fans for the most part are into doing that!

OK...getting back to my original post in this thread....many devices (some obsolete in themselves) have LCD modules with touch screens in them with basically no drivers other than digital RGB inputs. I am trying, with this thread, to gain some understanding about the technical requirements for driving such devices. If anyone wants to create a "cheap PDA" of sorts, or perhaps a wireless home automation terminal, or whatever, the Propeller can make it happen!

If you would like to discuss the pros and cons of different display technology, Please choose, or even start, another thread. I would like the focus here to be related to driving various types of TFT LCD modules.

Thanks again!



The more I know, the more I know I don't know.· Is this what they call Wisdom?