HDMI output only works with a certain 5m cable - Page 1
Forum section: electronics / beginners

author: nuno
author_rank: Frequent Contributor
timestamp: May 15, 2019, 02:22:25 pm
content: Hello All,So I have this friend that designed a carrier board for a SOM (system on a module), which has an HDMI output.He routed the HDMI pins from the SOM's connector to a mini HDMI connector on the carrier board. He followed best practices for the differential pairs, pair lines close to each other and little further apart from other pairs, always a ground plane underneath, same length on the pairs. The HDMI lines go through an ESD protection and voltage conversion chip (specific for HDMI); the differential pairs go directly to the mini HDMI connector, the chip just hangs on them.Funny thing is, he says the HDMI only works if he connects the output to a monitor through a 5m long (apparently high quality) HDMI cable of an unamed brand. Trying even very short mini HDMI -> HDMI cables doesn't make it work (no image or constant signal loss). If he has the short cable that doesn't work and then adds the "high quality cable", it works.So, what do you think is hapenning here? Is the long cable adding capacitance that slows down the edges? He says that any other cable he has works on everything else, from his PC to a Raspberry PI.Thanks, any ideas will be appreciated!

author: AndyC_772
author_rank: Super Contributor
timestamp: May 15, 2019, 02:34:49 pm
content: Clock / data timing skew, perhaps? Have you checked carefully what the timing specs are for your interface, and in particular, where the clock transitions with respect to the data valid window?Maybe this "high quality" cable actually has a mismatch between the lengths of the clock and data pairs, which has the effect of moving the clock within the data window, putting it into a position where the data actually is valid when the clock transitions?

author: soldar
author_rank: Super Contributor
timestamp: May 15, 2019, 04:14:12 pm
content: Quote from: nuno on May 15, 2019, 02:22:25 pmI would start by making sure all cables are wired/connected alike. That is the first thing. I remember the old VGA cables where no two were alike and would give problems.Next make sure the connectors connect alike. I have found Ethernet RJ45 where one particular cable has a problem with one particular connector but both work well with other cables/connectors. Mysteries of science.I have gotten to the point where all my USB cables are assigned a certain phone/function and they all work in that assigned role but once I start changing them around I start having problems.Only after all these simple connector and continuity issues have been discarded would I start looking for more complex causes.

author: nuno
author_rank: Frequent Contributor
timestamp: May 16, 2019, 02:47:51 pm
content: Thanks for the suggestions.I met with my friend. It is a not a "connection alike" issue, we tried several different HDMI cables and cable sets (with adapters, sometimes 3 or 4 cables in series but not exceeding ~2m) with the board, a PC, monitors, a RaspberryPI. All cables / cable sets work perfectly (full HD) with all the devices except with the carrier board he designed; only the "magic 5m cable" works with it, it is enough to add the cable in series with whatever combination of cables we already have to make it work.We checked trace length on the PCB. The biggest difference there is is almost 3mm from CLK- to DATA0+. We built a small adapter to correct a little for the length difference and to add common mode ferrites to the differential pairs which the carrier board didn't have, but there's no difference at all.Then we put the adapter and a "non working" cable and I stuck a finger over the exposed nodes on the adapter board until I made it work, and I did. I pinned it down to a finger over the DATA2 differential pair. My finger makes it work flawlessly. As I touch the 2 pins (DATA2+ and DATA2-) it starts working (monitor recognizes the image) but with a set of blinky pixels, then I start pressing a little more and the image gets better until it is 100% perfect 100% of the time.My friend has an evaluation board for the SOM and, although HDMI works without the magic cable, it also has some small issues with the image, there are some minor interferences sometimes (a few pixels blinking, an occasional horizontal line).What do you think of this? We don't have how to measure these signals, we have a 50MHz digital scope and that's it.

author: lamabrew
author_rank: Regular Contributor
timestamp: May 16, 2019, 03:07:25 pm
content: Do you have more than one of these SOMs? So far from the description it sounds like what is wonky is on the SOM and trying to swap those might give some more ideas. It could be as simple as a bad solder joint somewhere on that board.Maybe make a second 'inverse' adapter and stick it between some other HDMI things you have on hand.You don't mention what resolution(s) you've tried to see if changing the clock/datarate would make a difference. At lower pixel counts HDMI doubles up the data so lowest resolution isn't the lowest clock rate (I've forgotten the details on where that cross over is).Last thought is DDC (I2C) failure due to all sorts of funny issues with capacitance on the lines is possible (when the design itself isn't robust) but your finger test would seem to have ruled that out. But at least that's something you can look at on your scope.Ultimately a scope with the high bandwidth differential probe is the only way to know you have the interface meeting the HDMI waveform requirements, even a used setup for only basic 1080p HDMI speeds will run you US$ 30K plus so I feel your pain on the scope issue (add in the required TDR and jitter tests and now it's 3x the scope cost).

author: KrudyZ
author_rank: Frequent Contributor
timestamp: May 16, 2019, 03:44:06 pm
content: Since you don't have adequate test equipment your options of truly fixing this will be limited.Your problem is likely an impedance discontinuity causing reflections that look like extra edges or incorrect levels at the receiver. The bits of a particular frame timing will occupy a certain length on the cable. For some length of cable the reflections might line up with bit transitions and be largely benign while for others they might hit in the middle of a bit, where it is sampled. You could try running different video timings to see if they are all affected or not.As to you touching the board and making it "work", what you are doing is changing the the impedance of the traces in that vicinity, so that would support the above theory.You mentioned a "voltage conversion chip". If this is an active repeater, then they sometimes require a minimum cable length for their equalization to work, but that is usually not very long. It would help to see the schematics or at least know the part number of this chip.

author: nuno
author_rank: Frequent Contributor
timestamp: May 16, 2019, 05:45:21 pm
content: Quote from: nuno on May 16, 2019, 02:47:51 pmI meant to add that the "magic 5m cable" solves these small issues on the SOM's evaluation board.Yeah, at least as of today, there's only one such SOM available so we can't test another one. I'll suggest it to him, to get a another SOM.Quote from: lamabrew on May 16, 2019, 03:07:25 pmWhat do you mean by the inverse adapter?To clarify what we have:[carrier board with SOM plugged in | female mini HDMI connector] -> [male mini HDMI connector | small adapter with ferrites and trace len fix | female HDMI connector] -> (HDMI cable / cable set) -> This all works better with lower resolutions (some cable combinations other than the magic one show lower resolution formats, but with lots of image quality problems and frame loss, while at full HD it basically doesn't show anything).By "voltage conversion" I meant voltage level conversion, level shifter, there are other names. It's not an active repeater, it's just ESD protection for the differential pair lines (the chip hangs on the lines, it's not in series) and digital (I2C....) level conversion (SOM -> 1V2, HDMI -> 5V) and 5V HDMI power line current limitation, I don't have the chip reference handy.I understand the reflection theory. I don't have the knowledge to evaluate the PCB layout on that light. As far as I can see, the basic rules were followed, both lines of the same pair are close, more space between pairs, uninterrupted GND plane underneath the pair, all differential lines the same length (within 3mm) (both inside a pair and between pairs)... Most non-ideal situations is that the signals change layer through vias some 3 or 4 times and there's a spot near the (mini) HDMI connector where the GND plane was interrupted because of the layer changing vias, but it's still very close; I'll see if I can get/post shots of that. Maybe the layout should be re-designed to have more direct (less vias) path from SOM to HDMI connector...Many thanks for the help.

author: nuno
author_rank: Frequent Contributor
timestamp: May 22, 2019, 05:29:05 pm
content: The HMDI layout on my friend's board is in attach. The vias at the left go almost directly to the SOM's connectors, and at the right to a mini HDMI connector. GND plane all around but there's a small cut caused by the vias at the right. Putting a finger on differential pair DATA1 makes it all work, including the manufacturer's original eval board for this SOM, which has some minor occasional interferences at full HD.The ESD protection chip isTPD12S016

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 22, 2019, 06:28:03 pm
content: Quote from: nuno on May 22, 2019, 05:29:05 pmFingers add capacitance, but you'll probably need to generate a good eye diagram to figure out how that is "helping" (is it affecting ringing/overshoot; adding a bit of delay; merely loading down the driver a bit more; etc...). HDMI is really f'ing hard to get right, btw. It's basically all the fun and excitement of Gigabit ethernet multiplied by 5.

author: nuno
author_rank: Frequent Contributor
timestamp: May 27, 2019, 12:44:34 pm
content: Thanks, good to know.

author: LapTop006
author_rank: Unknown
timestamp: May 28, 2019, 01:14:48 pm
content: Quote from: nuno on May 15, 2019, 02:22:25 pmIf the cable has explicit direction labels on it, it might have the Redmere chip in it, which can help not-spec-meeting HDMI sources or sinks to work more widely.

author: nuno
author_rank: Frequent Contributor
timestamp: May 29, 2019, 11:28:37 am
content: The "magic cable" is totally symmetric, both in aspect and in usage (doesn't matter which side you connect to the HDMI source or sink).