LCD / LVDS Interface for Raspberry Pi


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by cnxsoft » Mon Jan 16, 2012 5:53 am
For those interested, one company (chalk-elec.com) is apparently developping a LCD touchscren and LVDS LCD interface board for the Raspberry Pi.

I don't know the status, but they already have solutions for Beagleboard and Pandaboard.

Edit: The hardware for existing boards is open-source. http://www.chalk-elec.com/?page_id=1284
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by nullstring » Mon Jan 16, 2012 9:00 pm
I guess we have must have confirmed LVDS on our boards?

A bit expensive, but any development in this category is good development.
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by jimmy » Thu Jan 26, 2012 8:06 pm
The problem with this board is that its input is a parallel video interface (18/24 bits of RGB info, H Sync, V Sync, etc.), the Rpi doesn't provide this type of interface.  Maybe, though, it can be 'demuxed' from the MIPI DSI connection on the Rpi board.  Alternatively, Texas Instruments makes a chip (part number eludes me at the moment) that will demux a dvi-d interface into a parallel stream.
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by ceteras » Fri Jan 27, 2012 2:08 pm
I couldn't find not one MIPI DSI lcd panel using google search.

There is a chip made by Toshiba that can convert MIPI DSI to LVDS, but it's a  P-TFBGA package, impossible to DIY.

Anybody knows at least one compatible display for the MIPI DSI connector?
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by kasperl » Fri Jan 27, 2012 2:44 pm
According to rumour, the one in the Nokia N900 which can be had on eBay for around 50 quid, apparently. On my N900 this looks pretty decent, but it's a phone display, not a 10 incher.

If anyone would want to spend time and frustration talking to Toshiba about sourcing that chip in small numbers it would allow a lot of old laptop displays to be used without blocking the HDMI port, pretty handy for laptops/netbooks. Maybe, just maybe, they'll sell them less than a trainload a go. Is there any chance of maybe the foundation or Gert or somebody sourcing these things and putting them on a bit of PCB with a header for the DSI and a header for an LVDS ribbon cable?
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by ChalkElec » Sat Feb 11, 2012 11:26 am
Well, we are developing PCB to convert DSI to LVDS. And yes, it uses this IC from Toshiba. It was hard to get NDA with them and MOQ is really high, but we believe that this board will become very popular. I hope we will manage to release it in March. Stay tuned!
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by BertM » Sun Feb 12, 2012 3:12 pm
Just wondering, how will you be managing the drivers involved for using DSI at all?
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by jamesh » Sun Feb 12, 2012 4:04 pm
BertM said:


Just wondering, how will you be managing the drivers involved for using DSI at all?


I'm intrigued to know how they are going to manage that as well, since the DSI is only accessible from the GPU.
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by Gert van Loo » Sun Feb 12, 2012 4:54 pm
I spoke about DSI output to my neighbor at work (he designed the interface after all). He said that, yes it requires a bit of tinkering but is much, much simpler then the CSI interface (which he also did). He thinks that getting a reasonable signal out of it should be no more difficult then the HDMI interface.

And before anybody asks: Yes, if you get the DSI interface working it will be independent from the HDMI one. (In contrast to HDMI and composite which are mutual exclusive.)
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by nullstring » Mon Feb 13, 2012 12:17 am
Sounds very promising. =)

But it sounds like it still requires modifications to the sources of the GPU library and therefore, any driver would have to be supplied by broadcom. Is this a correct assumption?
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by Evine » Mon Feb 13, 2012 9:14 pm
Hey i was wondering what kind of resolution output we could expect over the DSI output/ LVDS interface?
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by ceteras » Tue Feb 21, 2012 1:24 pm
The DSI interface seems to be quite capable, I believe you can expect up to 1080.

It depends on the actual implementation on the board (1, 2, or 4 lanes), which we don't know yet.

As for the LVDS (actually it's FPD link), it also depends on the chosen solution.

I guess we can say it's "big enough".

It's great news that a DSI -> LVDS adapter is in the works.

In the mean time I had another idea, I think the nokia 5800 express music display could be used with a simple adapter. The resolution is 640 x 360, and according to its service manual, it has a two-lanes DSI interface.

Edit: can we have the DSI connector pinouts?
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by cloudsloth » Tue Feb 21, 2012 11:45 pm
Gert said:


And before anybody asks: Yes, if you get the DSI interface working it will be independent from the HDMI one. (In contrast to HDMI and composite which are mutual exclusive.)


This is so exciting, thanks for the info Gert! Does this mean the displays will be at the same resolution, mirrored?
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by zydeco100 » Sat Mar 03, 2012 4:20 pm
I design capacitive touchscreens by day and would love to adapt one of my designs for the Rpi.  But PLEASE tell me you have I2C (or SPI) as well as bus power on the S2 connector.  A couple of GPIO pins wouldn't hurt either.

If those aren't on the connector, it's going to be very awkward to make anything fit smoothly into the board.  You'll need to solder wires on or jump on the GPIO connector (blocking anything else from attaching).

Attaching an LCD is also going to need a lot more power than a typical 500 mA on the USB bus.  So you're going to have to attach to GPIO anyway.  So much for that.
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by kasperl » Sat Mar 03, 2012 4:27 pm
Zydeco: I think that for most users, it would be quite OK to solder any needed input pens to the GPIO port. As for power, I think external power wouldn't be that much of an issue. A screen that can fit on the DSI soldering pads while using some GPIO pins would be great.

As far as I know, I2C can have more than one device on the bus, no? If that's so, blocking the pins wouldn't be too much of an issue?

Now that I think of it, the GPIO is delivered as solder-holes anyway, so soldering will always be needed when attaching anything there. As far as I know, the DSI is also just solder pads. There is no such thing as a solder-free screen installation without using the HDMI, so I don't quite see the problem?
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by zydeco100 » Sat Mar 03, 2012 5:06 pm
I'm confused.  The photos I've seen show a connector on S2.  Is this not in the final product?

If I'm designing a display/touchscreen for the Rpi, I want to be able to instruct the owner to "plug this flex cable into S2 and you're done".

If that can't be done, then it's "Plug this flex into S2 and this cable into the GPIO header".  I'm okay with that too.

If there's no GPIO header, or S2 doesn't have the signals needed, and we don't have the power required (my cheapest LCD has a 700 mA draw on the backlight ALONE), then we're soldering pins.

Now it's not worth my time dealing with people that want to add an LCD/procap assembly to this board.  People will wreck their boards.  They'll need more tech support than I can handle.

The hackers will do just fine and they'll find ways to adapt old Nokia scrap or iPhone replacement parts, or plug in some 22" LCD/touch assembly they bought at the store. I'm thinking about designing a clean (and cheap, I thought) solution that works off the shelf and can be used in an embedded design.
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by kasperl » Sat Mar 03, 2012 8:24 pm
There are no connectors on the GPIO or on the DSI port. It's all soldering (at least, as far as I know, I would love some confirmation on that). There will probably never be a connector on the GPIO, but the Gertboard will use th GPIO and expand/clone quite a few pins. As far as the DSI (and the camera port) are concerned, I am not so sure, but most of the posts here didn't seem to assume that there would be a connector.

Drawing 700mA off the GPIO is not an option. There's a 1A fuse on the input (AFAIK), USB devices run off the same bus, and the board itsself needs some juice as well. However, you can assume that most custom builds that include a screen would also have some extra 5V lines available, or even some higher voltages at whatever current you specify.

A screen that works, and has soldering instructions, would greatly help the Pi users, and I think that for most people not having a warranty is part of the deal once you break out the soldering irons.
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by zydeco100 » Sat Mar 03, 2012 8:39 pm
I think there's a real disservice being done to all the people interested in this board.  Type "raspberry pi" into Google Images, every image you see has GPIO and DSI connectors attached.

Most DIY types will have no problem attaching a .100" center header to the GPIO area.  Attaching a fine-pitch locking flex connector (like what's shown at S2) is much harder.

I have a hunch, and this is just me, that most people clamoring for the Rpi want to just hook up a bunch of ready-made parts and make it do something.  People are lazy.  If they really had the ideas and initiative, they would own Pandaboards and be making things already.  People are eager for Rpi because it's cheap, and it can run some kind of media playback on HDMI out of the box.  I think for 90% of the early owners, that's as far as it's gonna go and it'll be in a desk drawer before the year is out.  Sorry.
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by kasperl » Sat Mar 03, 2012 8:51 pm
Note, I am not in any way connected to the Foundation, and I'm simply reposting what I think I read elsewhere on these forums. I do hope the ribbon connectors get included, it'd be a pain to solder them on as a home user.

The pictures are all of beta boards, where connectors were included for testing purposes. Most images have been labelled as such.
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by khulat » Sat Mar 03, 2012 8:58 pm
Gert answered the question about possible current here. So your idea seems to be impossible since he suggests to limit it to 5mA/pin. I think there are 17 GPIO pins so about 85mA.
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by kasperl » Sat Mar 03, 2012 9:29 pm
khulat said:


Gert answered the question about possible current here. So your idea seems to be impossible since he suggests to limit it to 5mA/pin. I think there are 17 GPIO pins so about 85mA.



Mind you, that's when drawing off the data pins. I think this would draw directly off the 5V0 bus, but that's limited by the fuse as explained above. You could of course power the Pi from the screen, saving you a plug and bypassing the fuse, but that would require the screen to ensure that the power is clean.

None of that is a showstopper issue though, but I agree that the status of the DSI connector is quite important for ease-of use reasons.
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by khulat » Sat Mar 03, 2012 9:43 pm
Of course, i should have looked at the wiki before posting, then i would have seen my error.
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by MikeOToole » Sun Mar 04, 2012 3:46 am
I've never had to google so much to read a thread before... any chance you could add acronym mod and some common acronyms... ;)

A single reference post with common acronyms might do?

Mike
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by domesday » Sun Mar 04, 2012 5:19 pm
I can't see how adding LVDS would help further the foundations aim of getting an affordable experimenters computer into the hands of school kids. The HDMI port makes perfect sense as most households have at least one TV.

Assuming the foundation follows through with the promise to make the design open then perhaps a third party might be able to modify and produce a Pi compatible board with the required interface if there are enough people interested. But I broadly support the foundations aim of building a system that goes with a TV and don't think they should spend their limited resources going in many different directions to support different applications such as portable computers and in car systems.

The exciting possibility is that the Pi system could offer the base system design to establish a standard while other people can carry the design in other directions.
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by nullstring » Sun Mar 04, 2012 11:29 pm
Don't be so narrow-minded.

The RPI foundation has made the device while recognizing it's capabilities in the DIY crowd. Almost none of it's current massive interest comes from schools or kids.

The RPI Foundation is planning on exploiting the interest from the DIY crowd in order to get documentation, software, and drivers created and to a stable state.

This means that the RPI Foundation has a incentive to listen to the demands of the community and help make this device the most useful for all.

Further, the device has a DSI output interface, which can be either be used with DSI displays or converted to LVDS and then used with LVDS displays. The capability of the device is already there.

The only thing we need from RPI/broadcom is the development of the drivers/firmware required to use the DSI interface. One of the broadcom employees implied that this wouldn't be too difficult, it just needs to be done.
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