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We've been attempting to do some graphics over HDMI using an FPGA. I've built a board that has two HDMI connections. One of these links the required video lines directly to the FPGA. The second HDMI connection is wired to a TMDS chip - The TMDS141 from Texas Instruments https://www.ti.com/product/TMDS141 - and then to the FPGA.

We managed to get 720p video out of our direct FPGA connection, which seems pretty good. However, it would be nice to use the TMDS buffering chip to get a more reliable connection and higher resolutions. Sadly, the TMDS connection appears not to work.

I've had a fair bit of trouble analysing what is wrong. I don't have any test points, the chip package is awkward and the speed of HDMI is too fast for any logic analyser I have access to. I'm wondering where I've gone wrong.

Here is the schematic (I'll admit it's not the best - I am a beginner) :

enter image description here

And here is a closeup of the TMDS on the board. You can see two FFC sockets - one on the right does HDMI video via the TMDS chip in the centre. The one at the top connects directly to the FPGA. enter image description here

The direct connection looks like this:

enter image description here

and the FPGA relevant parts look like this:

enter image description here

Some of thoughts on this are:

  1. The chip is on the wrong way around. I think the dot is in the right place - it being the corner where pin 1 is. But maybe I got it wrong. Not sure how to test that.
  2. The package didn't solder on right. I baked it with some solder paste, then ran over the joins with flux, a bit more solder and my iron. All the connections look good as far as I can tell but it's hard to test. Again, not sure how
  3. Timing and resistance / cap values. Maybe I'm a bit off and too cavalier when it comes to getting the accuracy. This is high speed stuff afterall.
  4. The differential paths are too different. I did check this in KiCAD and the normal HDMI connector works so I doubt it's this.
  5. Something else needs to happen to the TMDS signal coming out of the FPGA. I need to check with my FPGA colleague but using the same signal for both the direct and TMDS chip HDMI outputs may not be correct.
  6. The FPGA didn't solder properly when it was baked. I mean, we use different pins on the FPGA for each HDMI output (in theory, we could have two signals coming out of this thing), so it could just be that one connection didn't work.

Sometimes, when I plug a monitor in, we get a signal but the monitor claims it can't decode it, use it, or otherwise display anything. Sometimes I don't get that message but it stays 'on' - a little green light and no sleep mode. Othertimes it claims there is no signal and it just goes to sleep. Sounds like a loose connection perhaps? But I'd have thought I'd have managed to get at least something on screen. Perhaps it's noise? But then, surely our direct connection would not have worked.

Really, I'm a tad stuck on where to go next and how to get more information to diagnose the problem. I may have to redesign another board with more test points.

I'm a bit out of my league here. Any help at all would be great. I know I'll have to go back to the drawing board at some point but that's fine. Hopefully I've just done something silly and it's an easy fix.

Cheers B

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  • \$\begingroup\$ The TMDS141 looks like a repeater, and not an output buffer. The inputs on the TMDS141 already behave like a HDMI receiver (including embedded receiver termination), and shouldn't work directly with the logic levels from the FPGA. Basically, inputs to TMDS141 should be treated as another HDMI connector, with all required serial caps and/or resistors. The whole purpose of that chip is to repeat HDMI in between boards inside the TV/etc. \$\endgroup\$
    – Vlad
    Jul 20 at 10:59
  • \$\begingroup\$ Is the other HDMI connector handled similary? Why are pins that should not be grounded connected to ground? Why is the connector shell not connected to ground but floating? @Vlad what serial capacitors you are talking about, HDMI does not use serial caps. \$\endgroup\$
    – Justme
    Jul 20 at 11:03
  • \$\begingroup\$ @Justme Not being an expert here, but looking at the Vidor 4K for reference, there are 100nF caps in series with hdmi lines. Few other designs I quickly googled also tell about AC coupling and mention the same 100nF caps and some pulldowns. \$\endgroup\$
    – Vlad
    Jul 20 at 11:13
  • \$\begingroup\$ @Justme Although now I do see that the HDMI is DC coupled by design, and it's in the spec. \$\endgroup\$
    – Vlad
    Jul 20 at 11:22
  • \$\begingroup\$ Hello. Thanks for the replies there. To the best of my knowledge, no pins are left floating. All pins that are not required for simple HDMI video are grounded. The connector shell is not grounded - I was tempted to do that but decided against. The direct HDMI connector is handled in the same way. It is a little simpler - it has no I2C lines - these are grounded. Will add the image to the question. \$\endgroup\$
    – Oni
    Jul 20 at 11:57

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