CN13(LVDS) I2C Bus = DDC?

Discussion in 'Accessories' started by Tohm, Dec 5, 2013.

  1. Tohm

    Tohm New Member

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    I had a similar question to this on another thread, but now that I look deeper into it, I guess it is more of a general board question. I was looking at the pinout of CN13 LVDS socket, and I noticed the I2C bus. Now, for many displays, a DDC protocol is required, so does anyone know if the i2c pins on CN13 are for DDC, or are they just for touch screen?

    Thanks,

    Tom
     
  2. Lifeboat_Jim

    Lifeboat_Jim New Member

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    Great question (I assume you are after EDID rather than DDC?)

    I don't have time to look into it tonight but will try to do so tomorrow.

    In 'the old days' DDI, DDI/CI, EDID etc were over separate lines but of course things have moved on. I know the documents to check (I think I do anyway). Chase me if you don't hear back in a few days on this issue - unless you solve it first of course (in that case, post here)!
     
  3. Tohm

    Tohm New Member

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    http://udoo.org/download/files/schemati ... matics.pdf

    On the HDMI page, there is a DDC clock and data wire, and ground is shared with the rest of the HDMI circuitry. However, on the LVDS output, there isn't(atleast not LABELED DDC). I fear that without the DDC protocol, the display I want to hook up might not function properly, if at all. I guess I'll have to dig deeper into the LCD's datasheet and see what is the exact function of the DDC.
     
  4. Lifeboat_Jim

    Lifeboat_Jim New Member

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    Ok, I've had a quick look around. A lot more reading of standards, datasheets and schematics before I fully grok this.

    Anyway, EDID over DDC2b, utilising I2C. Which boils down to I'm pretty sure it is using Pins 12 & 19.

    Or another way of looking at it, 'in the old days' DDC had it's own wires, now it's serialised/deserialised over I2C... Pins 12 & 19.
     
  5. Tohm

    Tohm New Member

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    Ah, good to know. Thanks, Jim.

    Also, one thing that REALLY threw me off were the channels. When I found that the LVDS had four transmitters and was "18/24-bit" I spent a LOT of time searching for a single-channel 24-bit screen. With little luck. Then, I looked up the screen model for the kits(the ones sold by UDOO.) Single-Channel 6-bit. I think there may be more than one type of "bit" because the screens that come as kits for the UDOO's are very similar to the standard 40-pin LVDS LCD's out there.

    Thanks again for the clarification,

    Tom
     
  6. Lifeboat_Jim

    Lifeboat_Jim New Member

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    I know what you mean.

    When they say 6 bit they mean per colour, RGB = 3 times. So you have an 18bpp Panel in your example.

    That means only 3 LVDS pairs (+ Clock) are needed.

    There are also 21bpp (7 bit) and 24bpp (8bit) panels out there. 4 LVDS pairs (+ Clock) are needed for those. UDOO is configured for 4 LVDS pairs (+ Clock).

    If the UDOO was configured to only support one LVDS panel it could do 1920x1200 @ 60fps, but it's set to support two so there is a sacrifice to be made and that means it tops out at 1366x768 @ 60fps (with 35% blanking) but can drive two of those. The other sacrifice is only one of the two LVDS interfaces is physically exposed (IE CN13 is one interface).

    I'll have a look at the 40pin connector standard you mention (there are 20, 26 and 30 pin layouts too. UDOO is 24 and doesn't correspond to any standard I've read so far - but I'll check the 40 out).
     
  7. Tohm

    Tohm New Member

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    Yeah, I can't seem to find a 40-pin to 24-pin connector, so I'm buying one that has wires connected to all required pins(data, clock, power, etc)

    I'm going to cut the female header and make my own to fit the 24-pin. A good tip for anyone who wants to make their own cable is to have a multimeter on hand, because the wire being bundled up may lead to confusion, and a continuity test will work wonders when determining which pin connects to which wire.
     
  8. Lifeboat_Jim

    Lifeboat_Jim New Member

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    True. I have to make up a cable as well but will do it from scratch rather than cannibalise a commercial LVDS cable.

    Most of those are very short. 12" or so.

    With Cat5 (I'm using FTP) 10M lengths aren't an issue for LVDS.
     

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