LCD + Touchscreen Interfacing

First - Sorry for the lack of posts here. I was down in the hospital with acute appendicitis (had to remove the appendix and a small part of the large intestine where it attached due to extreme infection!). Needless to say that was not fun!!!

Now - One of the items I am looking to use with the gumstix computer is a LCD display with touchscreen capability. Gumstix sells a Samsung 4.3 inch (diagonal) LCD display (LTE430WQ-FOC) for the gumstix computers. The description and specifications of the LCD display are:

Description
LTE430WQ-F0C is a TMR(Transmissive with Micro Reflective) type color active matrix TFT (Thin Film Transistor) liquid crystal display (LCD) that uses amorphous silicon TFT as a switching devices. This model is composed of a TFT-LCD module, a driver circuit and a back-light unit and a Touch Screen Panel(TSP). The resolution is 480 X 272 X RGB dots and can display up to 16.7M colors.

Features
- Transmissive with Micro Reflective type and back-light with 10 LEDs are available.
- Visible in outdoor & back-light off condition with reflectivity.
- Using the Touch Screen Panel (Film to Film-Glass type).
- VA(Normally black) mode.
- 24-bit RGB Interface (8-bits per color)
- DE(Data Enable) & SYNC mode - DE, Vsync, Hsync, DOTCLK

Applications
- Display terminals for PMP(Portable Multimedia Player) , Portable CNS(P-CNS)
application products.
- Display terminals for AV application products

In order to use the LCD display with the computer you also need the LCD interface board which is also sold by Gumstix. The Palo-43 board contains all the LCD interfacing circuits, the power supply for the LCD and Gumstix Com board and voltage level shifting hardware needed to use the Samsung LCD display with the Gumstix COM computers.

Not only are the basic interfaces on the board to drive the LCD display there are also other interfaces for audio in and out, USB OTG connectivity, USB console connectivity and interface connections that allow access to some of the other interfaces available on the Gumstix COM computers (I2C, SPI, A/D input PWM output and so on). The description and specifications of the LCD display are:

LCD-ready for 4.3" Samsung touch-screen Touchscreen controller Provides 16 bits per pixel USB mini-AB with OTG signals USB mini-B connector with USB Host signals USB console Signals available on 0.100" through-holes at 1.8V logic ---Two (2) two-wire serial ports ---One 1-wire port ---6 PWM lines ---I2C port ---SPI bus 6 A/D input lines and processor control signals.

4V to 5.5V input

(2) 70-pin AVX 5602-24 connectors

118.3mm x 67.25mm

Four (4) x #0 mounting holes for direct connection to an Overo COM Four (4) x #2 mounting holes to secure Palo 43 inside a casing

OK - Great! There is an LCD display and interface board for the Gumstix Overo line! I thought that was wonderful, until I started using it that is. Don't get me wrong - it works as advertised and does a decent job. It's just that I wanted something with a wee bit more resolution than is afforded by the Samsung display. 480 X 272 may be great for a smart-phone but this is a full blown computer system and I want MORE resolution!! (grin).

In my search for a higher resolution LCD display I ran across a question asked by someone else on the Gumstix forum. For those who don't know the Gumstix forums contain a wealth of information about the Gumstix Overo COM line of computers and the expansion boards. This is a meeting place where developers and hardware designers go to bounce questions off each other and where the Gumstix software developers hang out (and answer questions too!). To really glean the benefits of the forums I would strongly suggest using the "search" capabilities of the forums to find answers to questions FIRST! From what I have seen there are more repeat questions because people don't search for the answer first - it probably has already been asked and answered multiple times on the forums!

The question on the forum was about the possible use of a different Samsung LCD display with touchscreen capabilities but this display is 800X480 instead of 480X272. The part number of the display is LMS480KF02 with the following specifications:

Description
LMS480KF02 is a TMR(Transmissive with Micro Reflective) type color active matrix
TFT (Thin Film Transistor) liquid crystal display (LCD) that uses amorphous silicon TFT
as a switching devices. This model is composed of a TFT-LCD module, a driver circuit and a back-light unit. The resolution of a 4.8" contains 800 x 480(RGB) dots and can display up to 16.7M colors.

Features
- Triple-Gate Technology applied
- Transmissive with Micro Reflective type and Back-light with LED is available.
- TN (Twisted Nematic) mode
- 24bit RGB Interface
- Back Light with 14 LEDs (Light Emitting Diode)

Applications
Display terminals for PMP(Portable Multimedia Player) , Portable CNS(P-CNS) ,
AV , UMPC (Ultra Mobile PC) application products.

Hmmm - this may be the answer I am looking for!

Looking at the two display's specifications and drawings indicated they are basically the same in terms of electrical connections - they both use a flexible circuit board with a 45-pin connection for the electrical signals. Both have the same electrical connection layout (with one or two exceptions for the signal definitions but they look equivalent). The biggest exception is the Gumstix Samsung display uses a single string of 10 LED's to provide the LCD back light whereas the possible replacement LCD uses two strings of 7 LED's for the back light. I have not determined if this will be an issue yet or not. The LED back light power supply on the Palo-43 board is rated for up to 40-Volts maximum and up to 1.2-amps of current. I suspect I will need to wire the backlight LED strings in parallel with a ballist resister in each string to run all the back light LED's on the replacement board. I suspect wiring in series will raise the total forward voltage required too high approaching the maximum voltage rating of the PWM Boost-Switching Regulator on the Palo-43 board. Both Blue and White LED's tend to have around a 2.5V - 2.8V forward voltage drop so fourteen LEDs in series would present a 35V to 40V or greater voltage requirement to drive the LED string. This is too high for the boost-voltage regulator to handle. Wiring the LED strings in parallel with a ballist resister in each string would lower to total voltage requiement to approximately half that of the series string. Increasing the total current to drive the two strings is not a problem - all that is required is to replace the 10-ohm feedback resister with something a smaller value resistor - say 5 to 7 ohms which would give around 30 - 40 ma of LED current total. Driving each string at around 20-ma is a good compromise for light intensity vs longivity of the LEDs.

For those interested the regulator is the Micrel MIC2297 and you can find the specification sheets on the Micrel website (gotta love the Internet!). Here are the condensed specifications for the device:

Features:

2.5 to 10V input voltage range
Output voltage up to 40V
1.2A switch current
600KHz PWM operation
Trimmed 200mV feedback voltage
OUtput over voltage protection (fixed or adjustable)
PWM Brightness Control
DAC Brightness Control

The following chart shows the pin out of the LCD display sold by Gumstix along with differences between the 4.3-inch LCD and the 4.8-inch LCD displays.
Note the pins highlighted in RED.

Gumstix 4.3-inch LCD electrical pin out chart

Pin#    Symbol    Function       Remark
1       VSS      Ground
2       VSS      Ground
3       VDD      Power Supply   (2.5 or 3.3V)
4       VDD      Power Supply   (2.5 or 3.3V)
5       R0       Red Data       (LSB)
6       R1       Red Data
7       R2       Red Data
8       R3       Red Data
9       R4       Red Data
10      R5       Red Data
11      R6       Red Data
12      R7       Red Data       (MSB)
13      G0       Green Data     (LSB)
14      G1       Green Data
15      G2       Green Data
16      G3       Green Data
17      G4       Green Data
18      G5       Green Data
19      G6       Green Data
20      G7       Green Data     (MSB)
21      B0       Blue Data      (LSB)
22      B1       Blue Data
23      B2       Blue Data
24      B3       Blue Data
25      B4       Blue Data
26      B5       Blue Data
27      B6       Blue Data
28      B7       Blue Data      (MSB)
29      VSS      Ground
30      PCLK     Pixel Clock
31      PON      Display on/off
32      HSYNC    Horizontal Sync Signal
33      VSYNC    Vertical Sync Signal
34      DE       Data Enable
35      PWRSEL   VDD power select (High:3.3V / Low:2.5V)
36      VSS      Ground
37      Y2       Y_Up
38      X2       X_Left
39      Y1       Y_Bottom
40      X1       X_Right
41      VSS      Ground
42      VLED1-   LED 1 Cathode
43      VLED1+   LED 1 Anode
44      NC       Non Connected
45      NC       Non Connected

The three red highlighted lines in the pin-out listing above are the differences between the two displays. Pin-35 on the Gumstix LCD is used to inform the circuitry on the LCD that the power supply voltage is 2.5 or 3.3VDC - on the 4.8-inch LCD pin-35 is ground! DO NOT CONNECT the 4.8-inch LCD directly to the Palo-43 as this will short out the 3.3-VDC power buss on the Palo-43 board. I am looking into a mod for the board that will allow either LCD display to be connected without causing problems with the 3.3-VDC buss.

As for pins-44 and 45. On the 4.8-inch LCD display these pins are used for the second back light LED string. I have not checked to see if one string will light up the LCD panel - if it will then the use of the second LED string may not be required - otherwise I will need to connect the two pins correctly in parallel with pins-43 and 43 to power the second LCD string on the backpanel. Of course this will require a ballast resister in series with each LCD string to keep a current balance between the LCD strings but this is not a very difficult thing to do.

Stay tuned - more to follow....

Update!

I modified the 4.8-inch LCD display by cutting the copper trace running to pin-35 on the LCD connector (quick fix) to test and see if the LCD display would indeed work. After cutting the copper trace near the connector (end of flex board) I checked for any possible short-circuits I might have caused (.5mm traces are really small!) and after I was curtain I had not inadvertently caused any issues I inserted the 4.8-inch LCD flex board connector into the Overo Palo-43 LCD interface board. I have an Overo Earth COM installed on the Palo-43 board for testing and the original factory firmware is still installed in the NaNd Flash memory on the Overo Earth board.

I powered up the Overo boards using a 5.0-VDC power supply (obtained from Gumstix) and the

4.8-inch LCD panel lite up using just one LED string (there are two in this LCD display) but it could be brighter (grin). After a couple of seconds (about 15 or so) the Overo Earth GUI displays on the LCD panel! Success! This panel works with the Gumstix Palo-43/Overo COM computer modules with minimal modifications! Pin-35 is the only thing that actually has to be modified in order to get this LCD panel to work!

As you can see in the image on the right the display settings within the uboot and kernel have not been changed from the default 1024 X 768 display settings but that is something else to do so the displayed resolution matches the LCD display resolution. One other thing to notice from the images - at the bottom of the display you can make out a slight darkening along the bottom edge of the display. If you look really close you can see there are seven slightly darker areas along the bottom edge. I am thinking these are the un-lite LED's in the second string. For normal operation I don't see it as a problem but if you are in a well lite room you may want the second LED string powered up.

Now, instead of a 480 X 272 LCD display I have a 800 X 480 Display (4X more display info) so can now do many more things with the computer system (better, fine-resolution graphics comes to mind).

Hopefully this blog posting will be useful to others working with the Gumstix Overo line of COM modules!

As a side note for those of you "Green" people (grin) - a Gumstix Overo Earth mounted on the Palo-43 board using the 4.8-inch Samsung LCD display with one LED string lite draws about 2-watts of power at 5-VDC. Not bad at all.

Let me know what you think! Leave a comment (good, bad or indifferent).