Connecting RGB-LCD Display with 1 meter long cable to microcontroller - Page 1
Forum section: electronics / projects

author: sophokles
author_rank: Newbie
timestamp: March 15, 2024, 09:45:13 am
content: Hi,we are planning to connect a 4.3 inch RGB TFT Display likehttps://www2.mouser.com/ProductDetail/Newhaven-Display/NHD-4.3-480272EF-ASXP?qs=MyNHzdoqoQLPUrqKOKliMA%3D%3Dto a microcontroller with a cable of a length of 1 meter.Do you have any suggestions how make this possible with a low cost solution?We were thinking of converting the signals to FPD Link and back to parallel, but the FPD cables and connectors seem very expensive.Another idea was converting the signals to HDMI, but this would require to pay license fees.Can you think of another cheaper solution for consumer products?Thanks in advance,Sopho

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 15, 2024, 09:53:42 am
content: Quote from: sophokles on March 15, 2024, 09:45:13 amWhat sort of data are you displaying? If it's not video then maybe put a processor at the display for local rendering so you only need to send text/graphics primitives over the cable with TTL UART or RS485.There are chips specifically designed for this , e.g. DWIN :https://youtu.be/hFrc4C_QvE0?t=686and FTDI's EVEhttps://www.ftdichip.com/old2020/Products/ICs/FT801.htmlIf you need video rates then you could encode/decode LVDS with a small FPGA at each end, or maybe look at a MIPI display with some LVDS buffersI'm not familiar with FPD-link but if the issue is just the cable cost maybe look at running it over standard HDMI,Displayport or SATA cabling.Dispayport is an alternate to HDMI which I believe is license free and free FPGA cores are available.Similarly DVI is basically the same as HDMI - DVI over HDMI connectors shouldn't need a license as long as you don't call it HDMIEthernet may be another option - you may get away with 100mbit, and you don't need all the protocol layers - just raw packets

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 15, 2024, 10:01:24 am
content: Internal or external cable? What is he srouce of the video?

author: sophokles
author_rank: Newbie
timestamp: March 15, 2024, 10:23:07 am
content: Quote from: mikeselectricstuff on March 15, 2024, 09:53:42 amThank you for your considerations, the microcontroller has to sit at a defined position so putting it at near to the display is not an option.Running FPD Link over HDMI, Displayport or SATA cabling sounds interesting, unfortunately we cannot find any information / reference project about someone successfully doing it that way, which makes us suspicious as to whether this is the right way to go.LVDS seems a way to go, but the cables also seem quite expensive compared to HDMI or Display Port.Maybe we will look deeper into display port bridge ICs as the cables and connectors are cheaply available and license free.Thanks a lot for your help!

author: sophokles
author_rank: Newbie
timestamp: March 15, 2024, 10:23:53 am
content: Quote from: Ice-Tea on March 15, 2024, 10:01:24 amInternal cable, source is a microcontroller with integrated LCD graphics controller.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 15, 2024, 11:51:15 am
content: You can run LVDS or another differential signaling over fairly long distances over FFP. Not sure about a meter but I would pick a microcontroller with display engine stat supports a differential protocol and match the display to it. You know, KISS.As an example (and nothing more): STM32F765 with MIPI display.

author: sophokles
author_rank: Newbie
timestamp: March 15, 2024, 12:20:43 pm
content: What do you mean by "over FFP" ?We also though of using MIPI DSI but it seems the cable length is limited to 25 cm.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 15, 2024, 01:03:51 pm
content: Quote from: sophokles on March 15, 2024, 10:23:07 amI meant put a second display processor at the displayQuoteThey are all differential signals, with similar cable requirements. I'm sure you can find the specs somewhere and compare with what FPD specifies.QuoteWhat cables exactly ? LVDS is a generic standard with no defined connector/cable type. You can run LVDS over cat5 if you want.

author: sophokles
author_rank: Newbie
timestamp: March 15, 2024, 01:46:39 pm
content: Quote from: mikeselectricstuff on March 15, 2024, 01:03:51 pmThat's what we are trying to avoid, as it makes firmware updates / protocols / etc. way more complex.Quote from: mikeselectricstuff on March 15, 2024, 01:03:51 pmGood to know, nevertheless nobody seems to use these cables for transmitting video data, at least we cannot find any reference designs. So does not feel like the right way to go.Quote from: mikeselectricstuff on March 15, 2024, 01:03:51 pmIf you search for "LVDS cable" you typically get some 25 cm long cables like this one:https://www.alibaba.com/product-detail/LVDS-Cable-Video-Display-Screen-Wiring_1600132514456.html?spm=a2700.7724857.0.0.6dd745b6SBH5s5I have to correct myself, it seems you can get cheap LVDS cables like these, they are just not consumer goods.So we will look into running LVDS over cat5 as these cables are easier to get.Still a reference design of someone running LVDS video over cat5 or USB would be great.

author: tooki
author_rank: Super Contributor
timestamp: March 15, 2024, 03:39:03 pm
content: Quote from: sophokles on March 15, 2024, 09:45:13 amLook at the controller datasheet:https://support.newhavendisplay.com/hc/en-us/article_attachments/4414852703383Page 46 has the timing specs for RGB operation. For parallel RGB mode, the maximum clock is only 12MHz. That’s not particularly high, and there’s a decent chance it’ll just work on quite ordinary wiring. Even the serial RGB mode (i.e. 8 data lines, RGB bytes sequential) is just a 30MHz clock maximum. The signaling types you’re looking at are for multi-Gbps signals. Yours isn’t even remotely close to that.Page 74 has the corresponding bus timing diagrams.I just hooked up a square wave from my old, 12MHz max function generator. The pink trace is the signal via a 1m BNC cable. The yellow trace is through over 3m of 24AWG speaker wire, just ordinary untwisted twin wire. No shielding. One picture shows 5MHz, the other 12MHz. Is it pretty? No. Is it good enough to probably work flawlessly? Yes.The key will be to properly terminate the bus to eliminate reflections. You don’t need to resort to differential signaling (though you certainly could). You could use single-ended line drivers. Regardless, proper termination should make it work.Before doing anything more complicated, I’d try using your normal signals over simple ribbon cable (which has around 100 ohm impedance), with pads for terminator resistors.(In the test setup, the output is split at the function gen, with a 50 ohm terminator, and then each scope input also has a 50 ohm terminator, but who knows what that cable’s impedance was!)

author: inse
author_rank: Super Contributor
timestamp: March 15, 2024, 04:43:10 pm
content: Remember how fat 40 pin ribbon cable is?Reminds of 80‘s tech :-)

author: langwadt
author_rank: Super Contributor
timestamp: March 15, 2024, 05:06:05 pm
content: Quote from: sophokles on March 15, 2024, 10:23:07 amsomething like this,https://www.ti.com/product/DS90CF364Ayou could use HDMI/DVI cables

author: langwadt
author_rank: Super Contributor
timestamp: March 15, 2024, 05:10:25 pm
content: Quote from: tooki on March 15, 2024, 03:39:03 pmand most likely the only one where reflections are really critical is the clock

author: zapta
author_rank: Super Contributor
timestamp: March 15, 2024, 10:42:41 pm
content: Quote from: sophokles on March 15, 2024, 10:23:07 amMike's approach can also be done with a smart display that requires slow serial communication. E.g.https://nextion.tech/

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 16, 2024, 03:37:37 am
content: The resolution of that display is really low, maybe consider using a display with composite or Svideo input, commonly sold as an automotive "backup LCD".

author: MarkT
author_rank: Frequent Contributor
timestamp: March 17, 2024, 09:35:56 am
content: You can use a compromise termination that's not perfectly matched but uses less current from the driver - for instance two 330 ohm resistors for each incoming signal to ground and Vcc, that gives 165 ohm termination with a max current draw of only 10mA for 3.3V logic. Still going to take a lot of power for a wide bus (which is why LVDS exists) - perhaps your microcontroller cannot source/sink 10mA? LVDS converter chips exist so you could convert to LVDS and back at each end, and use twisted-pair ribbon cable or CAT5. IIRC you get 7 signals encoded onto each twisted pair.