Salvaged 7" TFT display, but how to interface with it? - Page 1
Forum section: electronics / beginners

author: polemon
author_rank: Contributor
timestamp: May 14, 2016, 01:15:44 pm
content: I recently pulled a 7" display from a portable DVD viewer (I don't know the correct term for those. It's a netbook-shaped thing, that can play DVDs and files off of SD cards), that I've found in the dumpster.Now, I wasn't interested in the DVD player, but the display seemed interesting. I've powered up the device, and the display works fine, it displays a sort-of greeting screen with a "JAYtec" logo, and seems to wait for a DVD to be inserted.So the display works fine, but I'd like to use it with other stuff. Before I can go on, I need to know what sort of interface it uses. There's a sticker on the back saying "CPT 7.0-LED9+R3" with a barcode on it. I was suspecting this is something like a part number or product designator, and googled for it. The closest thing I could find is this:https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-130-00-00-00-26-02-52/Chunghwa-Display-CLAA070LC0ECT.pdfthe specification seems about correct, the backlight takes 9V, and 800×480px seems about right when it comes to resolution.The datasheet suggests the display has a 30-pin LVDS connector, but this is where it doesn't match with my screen. My screen has just a 26-pin flatflex connector.The connector reminded me of the DSI port on a Raspberry Pi, but that one has only 15 pins, about half as this display has.So now, I'm a little at a loss. I don't know what sort of interface this display uses. I can power up the backlight, but I don't know even what sort of interface it has, so I can't even really begin to read on about how to interface with it. Since this is a free display, I'd love to make a DSI -> LVDS adapter for it, possibly get a VGA/DVI -> LVDS interface adapter.I've attached a couple of pictures, showing the display and the markings.I hope someone could steer me in the right direction, etc..Cheers,--polemon

author: sokoloff
author_rank: Super Contributor
timestamp: May 14, 2016, 01:28:03 pm
content: Watch this Ben Heck episode:

author: amyk
author_rank: Super Contributor
timestamp: May 14, 2016, 02:53:19 pm
content: 26 pins and that size is likely to be analogue, pins 10 and 11 being joined together is a clue (they're both VCOM which is a relatively high-current signal):https://www.eevblog.com/forum/projects/re-purposing-a-photo-frame-looking-to-identify-the-lcd/msg704798/#msg704798

author: bitwelder
author_rank: Super Contributor
timestamp: May 14, 2016, 03:15:39 pm
content: Is it generally worth to salvage small LCD screens from portable DVD players, photo frames, old laptops and the likes?

author: JacobPilsen
author_rank: Regular Contributor
timestamp: May 14, 2016, 05:04:46 pm
content: Is it generally worth to breath???

author: polemon
author_rank: Contributor
timestamp: May 14, 2016, 05:12:37 pm
content: Quote from: bitwelder on May 14, 2016, 03:15:39 pmI don't see why not. Getting a new display like this is usually not exactly viable. Since this screen is absolutely immaculate, there are no scratches, the backlight is in perfect condition, I don't see why it wouldn't make sense salvaging this thing.

author: VEGETA
author_rank: Super Contributor
timestamp: May 14, 2016, 05:23:39 pm
content: Quote from: polemon on May 14, 2016, 05:12:37 pmso can you use it with raspberry pi or something?

author: polemon
author_rank: Contributor
timestamp: May 14, 2016, 05:49:45 pm
content: Quote from: VEGETA on May 14, 2016, 05:23:39 pmNo idea. I could probably make some sort of composite interface for it, at least I guess so. Since this is an analogue display, I'll see what sort of weird things I can do with it. Maybe it'll accept a VGA signal, or something.

author: bktemp
author_rank: Super Contributor
timestamp: May 14, 2016, 09:01:59 pm
content: Using an analogue TFT is quite difficult because it does not use normal video levels. You need to generate the voltages for driving the LCD cell. This means inverting the voltage every second frame and applying a gamma correction curve.If you don't have much experience with driving TFTs, better get a digital TFT. It is much easier.

author: aurmer
author_rank: Regular Contributor
timestamp: May 14, 2016, 09:23:47 pm
content: If you are looking for more clues on how it operates, plug it back into the DVD player, and find the power pins with your multimeter. Then use a protocol analyzer on the data pins to get an idea of what each pin is doing on still screens and what it does on moving screens. You can get a better idea of what you are looking at this way, and maybe even find a protocol which matches your test results.Either way, to the nay-sayers, I definitely think this is a worthy project; not just for the free screen, but for the learning opportunity. Why else do we tinker if not to learn?

author: polemon
author_rank: Contributor
timestamp: May 14, 2016, 11:03:02 pm
content: Quote from: zetharx on May 14, 2016, 09:23:47 pmYeah, that's pretty much what the Ben does in the video Sokoloff linked.

author: polemon
author_rank: Contributor
timestamp: May 14, 2016, 11:27:56 pm
content: Quote from: bktemp on May 14, 2016, 09:01:59 pmI was looking at various sites to see if I can find driver boards, that could interface with a 26-pin TFT screen. It seems they're pretty expensive. The cheapest one I could find, and which seem to interface with this TFT perfectly goes for around 20 bucks if ordering only one. However Alibaba.com doesn't want to exactly ship to individuals. The driver boards I could find on ebay go for around 30 bucks plus shipping, which is kinda on the boundary of me wanting to shell out that kind of money for something that is basically just a very small play project here...

author: bktemp
author_rank: Super Contributor
timestamp: May 15, 2016, 06:00:17 am
content: Quote from: zetharx on May 14, 2016, 09:23:47 pmI drive displays for a living, so I have driven hundreds of different tfts. But I have driven only 1 analogue TFT. I have done it once, because I could get the displays very cheap but I don't want to do it again, because it is simply not worth it:Look at a datasheet for a similiar tft how the video signals VR,VG,VB look like. You need to generate an inverted waverform in every second line/frame and also toggle VCOM. That's at least 1 opamp + 1 analogue switch per colour -> 2 ics. Then you need the gamma correction. Thats either a nonlinear amplifier or an analogue lookup table built from some diodes. To convert analogue video signals from standard RGB video levels to the required levels you end up with at least 2 opamp ics + 1 analogue switch ic + many passives. And that is only the video signal. The display also needs many voltages: DVdd, AVdd, VGH, VGL, VCOM. That's 5 voltage rails.The digital section is no less complex: First you need to generate the pixel clock from the horizontal sync signal (or maybe even extracting it from composite sync) using a PLL. Then you need to derive all timing signals from that clock. A CPLD or a small FPGA is the easiest solution, but it should also be possible using 74hc logic ics. You probably need >10 ics to generate all timing signals.Compared to a digital tft it is insanely complex if you do not have any asic that integrates all these functions. There are some nice Chinese ics designed for driving analogue tfts from a composite video signal, but I couldn't find a datasheet or where to buy those parts. They also need some firmware, impossible to do without any information. Most manufacturers of analogue video tfts (like Sharp and Sony) made their own asics. So the market for such ics is quite limited, they are therefore hard to find.From my experience the image quality of analogue tfts is inferior to a digital tft: It shows more line flicker and has a lower contrast unless you tweak all signals carefully.If you still want to drive that display to gain some experience, go for it. Otherwise the huge effort is simply not worth it. You can get complete video tft modules for much less it would cost to design or buy a driver for that display.

author: amyk
author_rank: Super Contributor
timestamp: May 15, 2016, 02:53:17 pm
content: I'd say the analogue design skills practice is enough reason to give it a try. For extra fun, use discrete transistors only, no ICs.It's definitely going to be harder than a digital LCD, but easier than a CRT...

author: polemon
author_rank: Contributor
timestamp: May 15, 2016, 07:59:18 pm
content: To be honest, all I'm really looking for, is just a small interface card, that'll accept a CVBS signal. If this thing is analogue, I don't think it'd be feasible as a short project to make this accept digital data, etc. RGB and/or CVBS would be fine, whichever is simpler.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 16, 2016, 05:48:39 am
content: Quote from: zetharx on May 14, 2016, 09:23:47 pm- good diagnostic spiritQuote- good learning spiritWelcome to EEVblog

author: sjd.aliyan
author_rank: Contributor
timestamp: May 16, 2016, 06:17:58 am
content: You definitely could not use it with a raspberry.DSI connectors is for Digital LCDs and you need a external chip (DSI to LVDS) for connecting a LCD because DSI LCDs are very rare than Parallel LCDs and LVDS ones.If you dont have a datasheet then forget it it is waste of time and has no benefit for you.You can buy a Digital LCD for 20 bucks or spending your time to use High tech Cellphone or tablet lcds.Look:http://andybrown.me.uk/

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 16, 2016, 06:29:58 am
content: https://youtu.be/Cl7rshPe-x4