LVDS Interface

[RITRedbeard]

Laptop panels are cheap, especially if you're a frequenter of eBay.

Adding LVDS interface would probably bump up the cost quite a fair bit.

However, interfacing with LVDS panels can reduce electronic waste and give a new lease of life to dead laptops.

Does anyone know cost effective way to interface to LVDS over VGA/DVI/HDMI/S-video/Displayport and what are your thoughts on the matter?

[jamesh]

I've had a quick Google and not found anything that easily converts HDMI or composite to LVDS.

The Raspi only supports HDMI or composite out, so you are out of luck if you want to use LVDS.

To have LVDS output would require at least two more chips (if continuing to use the current SoC), so would put the cost well past the $25 barrier.

It looks like LVDS is the way go so I would expect that will become standard on most SoC in the future, but that's a guess.

[abishur]

Actually the r-pi also has pinouts for mipi DSI out as well.

[jamesh]

Quote from abishur on August 17, 2011, 14:28
Actually the r-pi also has pinouts for mipi DSI out as well.

Not sure that will make it on to the final board. DSI is old tech, so may well be left off. There is also the problem of the drivers for the GPU which need to be tweaked for different panels, and there is no end user access to the GPU code...

[Gert van Loo]

I am curious why you think DSI is old stuff? If you look at the DSI standard it has more bandwidth then most monitors or TVs can consume. The DSI standard supports up to 4 lanes with up to 1.2 Gbits/sec/lane (Rate-1) . That gives you a 2980x1670 screen with 16 bits per pixel, 60 frames per second non-interlaced. I think it will be a few more years before people have that in house. Not much used but defined are the other two rates of DSI: Rate-2 (2.49 Gbits/sec) or Rate-3 (4.9 Gbits/sec)
And the final board will have DSI and CSI connectors. The only thing under discussion is the size and location.

[DaQatz]

More info.
http://www.toshiba-components......gehub.html

[jamesh]

Quote from Gert van Loo on August 17, 2011, 20:12
I am curious why you think DSI is old stuff? If you look at the DSI standard it has more bandwidth then most monitors or TVs can consume. The DSI standard supports up to 4 lanes with up to 1.2 Gbits/sec/lane (Rate-1) . That gives you a 2980x1670 screen with 16 bits per pixel, 60 frames per second non-interlaced. I think it will be a few more years before people have that in house. Not much used but defined are the other two rates of DSI: Rate-2 (2.49 Gbits/sec) or Rate-3 (4.9 Gbits/sec)
And the final board will have DSI and CSI connectors. The only thing under discussion is the size and location.


Just something the boss said when Eben brought the Raspi proto round to show us. It was suggested that there was no need for the DSI pin out - but I'll need to confirm. It may be that the LCD panel manufacturers are dropping DSI for LVDS or similar. Meantime, everyone, take what I said with the proverbial pinch of salt - may have got my lines crossed. Gert's the HW expert here, I stick on the software side!

[RITRedbeard]

http://www.analogix.com/Upload....._14_11.pdf

Maybe?

[RITRedbeard]

Well, since HDMI implements TMDS, wouldn't it just be a matter of converting the HDMI signals to 3.3v friendly ones then using a LVDS transmitter IC?
I'm just looking at DS90C363AMTD by National Semiconductor (quite an old chip, but using quite an old panel) and information gleaned here: http://hackaday.com/2011/02/07...../#comments

[RITRedbeard]

That can't be right. Maybe Gert or Eben can pop in with the HDMI output specs this way have a better idea of what chips for a circuit to convert the signal. TI seems to have some good chips for a solution, the only thing is now prototyping it with cost effective parts that can be bought in small quantities and then making a PCB/schematic so anyone can build one.

Heck, now the Raspberry Pi can be an educational tool in electronics and digital signaling (I'm a software guy ) !

Gert or Eben:

Pixel Clock?
Number of bits per pixel or bits per color?
Single link? Dual link? Both?
Audio out?

[jamesh]

Quote from RITRedbeard on August 22, 2011, 17:31
That can't be right. Maybe Gert or Eben can pop in with the HDMI output specs this way have a better idea of what chips for a circuit to convert the signal. TI seems to have some good chips for a solution, the only thing is now prototyping it with cost effective parts that can be bought in small quantities and then making a PCB/schematic so anyone can build one.

Heck, now the Raspberry Pi can be an educational tool in electronics and digital signaling (I'm a software guy ) !

Gert or Eben:

Pixel Clock?
Number of bits per pixel or bits per color?
Single link? Dual link? Both?
Audio out?

I think it outputs up to 1080p (Just plugged one in to a TV I have here, and it said 1080p anyway!). It does conform to the HDMI spec 1.3, and has audio. Not sure on the bits per pixel/colour formats - will need to check up on that.

[abishur]

So I actually happen to randomly have an old laptop lying around that is only good for parts. Let's assume, simply for the sake of argument that the r-pi team decides to leave the LVDS interface on the board. Using this base assumption, would I be able to (theoretically) hook my laptop monitor (which uses LVDS) to the r-pi? In a scenario like this, do I need to worry about HDMI output? I mean, aren't I circumventing HDMI entirely by using the LVDS interface? I'm guessing I would also need to hook up a power supply to the monitor as well, right?

Would I ultimately be better off just purchasing a prebuilt LCD controller card designed for my screen then trying to match up each wire in the LVDS ribbon?

[abishur]

Okay, so I think I might be confused on DSI v LVDS. Wikipedia lists DSI as "a high-speed LVDS point to point serial bus" does this mean I can connect a LVDS LCD screen directly to the DSI pins or do I need to convert it to LVDS first? If so, does anyone know of a (less than $15) way to make this conversion? I'm not afraid of a little soldering (though I am afraid of a lot of soldering or soldering extremely small points! ). The only other option I can think of is something like this, but at $33 it's rapidly approaching the "not worth it" price point.

[Gert van Loo]

When people talk about LVDS and video the mean a parallel LVDS bus. The LVDS bus replaces the old parallel display bus which would go up to 600x800. But you need higher quality signals if you go over 600x800 so that is where LVDS comes it. Very silicon expensive as you have to double the number of pads on the chip.
The DSI signals are differential and low voltage. But that does not mean that they adhere to what most people call the LVDS standard. It is the same as saying "it is a battery so the voltage must be 12V"

[Motley]

The MIPI DSI standard that is mentioned above as being available on the RasPi header is totally different to "LVDS". When people say "LVDS" with respect to displays, they are actually referring to FPD-Link ( http://en.wikipedia.org/wiki/FPD-Link ) meaning "Flat Panel Display Link". As Gert says, this is the stardard for high resolution display panels inside laptops, tablets, etc. What LVDS actually means is "Low Voltage Differential Signalling" which is a type of electrical connection rather than any particular standard or protocol that uses it to transmit data through wires.

HDMI uses a different electrical connection again, TDMS or "Transition Minimized Differential Signaling". HDMI, when HDCP encryption is not active, is compatible with the DVI-D standard. You can easily connect DVI-D monitor inputs to HDMI cables using nothing more than a socket converter dongle.

[RITRedbeard]

Bingo. I have the sample chips here but I lack time and smd experience. Going to have to buy a few more parts and etch my own board, I think. Let you know how it is going.

[scologic]

every lbds panel pin out is different, and cables cost a lot to make. stick with hdmi it's nice and standard. But perhapshave a header pin so us integrators dont have to use a massive HDMI cable.

another 2p

[abishur]

oh, there's no worry about the HDMI going away. The reason we want LVDS though is because we want to be able to connect a spare laptop screen. If I could hook that screen up directly with HDMI you bet I would! I've seen a possible solution on eBay. You'd hook the r-pi up to it via a hdmi->DVI cable and then hook the LVDS cable to the board. The only draw back here is that it's $30 (more than the r-pi itself!) I know that for the project I wanted to do (make an e-reader) this cost makes the project too expensive, but it might work for others.

[scologic]

Hmm i think it'll get to graphics decoding over load on the processor then.. The othersolution would then be a USB display(im not a big fan of these as you can tell from my other posts, and sufferance with plug computers for the past year). There would probably be a driver issue then.

Also in working with the samsung cortex chips, the HDMI and LVDS use very different on board technology and it's probably difficult from a power state for both to co exist and also considering the cost of the PI.(a much more refined expensive board may be able to do it fine).

[abishur]

So I think I have a bit of a dumb question here. We know that the r-pi GPU is, more or less, a GPU designed for mobile applications, as such I would suspect that the GPU itself supports LVDS output even if we don't bring those pins out to the board anywhere.

If that is true, would it be possible to somehow tell the GPU to output the LVDS signals to some of our GPIO pins?

[abishur]

ah, never mind. I just re-read the second post, which hints to the fact that the SoC does not support LVDS at all.

[na1pir]

I have found something interesting... a boards that inputs vga, dvi,pc-rgb and hdmi and outputs it to LVDS. http://www.njytouch.com/lcdcon.....roller.htm
I bought one(18€) with vga/dvi for my old lcd monitor. I hope it will be compatible with hdmi to dvi cable for interfacing with R-Pi =)

[abishur]

Yeah, I had seen those too. my only hold up is for my monitor it would cost me around 30 USD, which is the price of another r-pi.

[abishur]

So I was talking to a guy on another forum and he was able to make a DVI->LVDS himself and sent me the specs of how he did so. I've uploaded it here.

It uses TI chips TFP401A and the sn75lvds83b and a couple other items, but the specs are labeled (just look for the dark blue next to the chip layout for the name of the chip he used). It's a little more advanced than I'm comfortable with, but I've put it here for anyone brave enough to do it themselves.

[abishur]

Okay so I talked with this guy a little more, turns out that the board uses both the top and the bottom side of the PCB which means the PDF image I uploaded won't do you much good. He sent me the original file which is a Protel 98 .pcb file which won't open in any other free program I've tried, but will open in later version of Protel (such as Protel 99). For anyone who has Protel or who can *ahem* obtain it, I upload the .pcb file to a dropbox here.

I was also able to export it to an autocad file (.dxf) which opens in the freeware version of Progesoft. I've also uploaded that to my dropbox here.

**EDIT** I've also included the Gerber files. Which can be downloaded here. For those who don't know, the extension relate to layers as follows:

GTL - Top Layer
GBL - Bottom Layer
GTS - Top Layer Solder Mask
GBS - Bottom Layer Solder Mask
GTO - Top Overlay
GBO - Bottom Overlay

**/EDIT**

To download, right click the link and say "Save Target As"