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HDMI connection

altay , 04-12-2019, 04:52 AM
I have two problems with HDMI connection.
In most circuits there is a level shifter to adjust 3.3v HDMI signals up to 5v connector.
Normally, in some circuits schematics there are coupling capacitors in between the level shifter and graphic cards output
however, in some others there is no coupling capacitor.
As a rule of thumb, should I have to put these caps or it depends on the shifter design?
( My design shows exactly nothing, so I am suspicious the reason is that I did not use any coupling cap)

HDMI connection has 4 lanes. Does any one know if these lanes are interchangeable or not?
Accidentally I connected Lane 4 into lane 1 and vice versa.


robertferanec , 04-17-2019, 08:19 AM
1. Many ESD chips support level shifting for the DDC bus. Have a look at our open source projects at https://www.imx6rex.com/ for example here (page 16): https://www.imx6rex.com/wp-content/u...Production.pdf

The chip used there is CM2020-00TR https://www.onsemi.com/pub/Collateral/CM2020-00TR-D.PDF

2. I do not think you can switch lines.
Paul van Avesaath , 04-25-2019, 05:42 AM
I can 100% confirm you are not allowed to swap channels on the hdmi connector.. Just ask me how I know.. 😳
altay , 05-17-2019, 02:05 PM
I have solved the HDMI problem of mine. Newcomer can take a look at the attachment. I have tested the MAX chip but the other one, I mean PTN, is added to test but I did not implemented on my circuit- just added as an alternative if the MAX does not work.
The output is HDM and the input is the signals the come from graphic card - in my case it was Computer On Module Express (COMe) Type 10.

Hope it give you some idea.
robertferanec , 05-20-2019, 06:55 AM
@altay, please I can not see the atachemnt
altay , 05-20-2019, 09:12 AM
Hope this time it is attached.
Btw, Altium version is 17.
robertferanec , 05-21-2019, 12:35 AM
@altay, is it working? Can I create a youtube video where I would comment on the board and point out some things what could be improved?

PS: You know, that the layout is not properly done, right? Also maybe schematic would need some improvements.
altay , 05-21-2019, 04:01 AM
Hi @robertferanec, yes sure you can use it as you wish.

For anyone novice to HDMI and has the problem with the HDMI level shifter:

Please first take a look at the images attached to this post.

I could not get any output from my HDMI, but the current consumed by my circuit was normal.
> Question: What is my processing unit? Its actually standard Computer On Module Express (COMe) Type 10 mini, made by Kontron
> Question: What is my processor? Celeron J1900 2Mb cache

I messed up with HDMI level shifter
> Question: what was my level shifter? Max9406
> Question: Why you did not follow the datasheet demo circuit? There was no such a golden bullet on the datasheet! You can find evaluation kit but I think it is still a little bit complex

I developed a patch circuit to learn how to deal with HDMI level shifters ( Find the attachments). A simple 2 layers HDMI level shifter. I designed it for two chips: MAX9406 and PTN3381B. if the MAX did not work than I would have chance to test PTN. I designed it as generic as possible. All HDMI optional paths are provided and one can select one with zero ohm resistors. Beside this, if the circuit failed one more time, I would need a way to trace whats going on the routs with a oscilloscope. For this, I added so many unnecessary zero ohm resistors among the paths.

As you see on the attached images my MAX works fine!
> Question: What did I learn?
  • HDMI and DVI are mapped over each other in COME type 10 mini standard. So you have to select it with AUX_SEL selection pin
  • HDMI pairs are not swap-able! Be careful, otherwise you would see nothing on the screen
  • HDMI differential pair length matching is not a should! it is a MUST.
  • Do not forget coupling capacitors 100 nF! in some guidelines you may not see them but please keep it there in the input pins of the level shifter
Now you can see the sample output screens.

Hope it works fine for you.
Btw, The final implemented design would be available on my SyncBox project soon (with no unnecessary resistors)
robertferanec , 05-21-2019, 04:20 AM
If I understand right, this board is Display Port to HDMI adapter ....and you tried two different chips, correct?
altay , 05-21-2019, 05:21 AM
yep, its an adapter but the input is not standard pcı express
robertferanec , 05-21-2019, 11:36 PM
but the input is not standard pcı express
- you mean, you are connecting it to the processor display interface, right? I see you soldered the wires directly to baseboard. I am not sure what interface it is, but COM Express 10 provides Display port output ... so I am guessing, that could be the interface. Or did you solder the wires to a different interface?
altay , 05-22-2019, 01:29 AM
- you mean, you are connecting it to the processor display interface, right?


I see you soldered the wires directly to baseboard. I am not sure what interface it is, but COM Express 10 provides Display port output ... so I am guessing, that could be the interface. Or did you solder the wires to a different interface?

No no, just COM-e standard interface. actually the green PCB is a patch to the blue PCB. we have tested the green one. its connected to the COMe Display Port output on the blue PCB.

Lakshmi , 05-23-2019, 01:50 AM
I was going through the Gerber, I have few question on HDMI as am novice to the HDMI Design,
1. No need of ESD protection for the HDMI?
2. I was going through this TI's Document in which they directly connect the 5V to the HDMI connector via ESD.

3. In this Data sheet of AM571xx Sitara series based processor
http://www.ti.com/lit/ds/symlink/am5716.pdf (Page No 142),they mention these:
Core I/Os -0.3 1.5 V
Analog I/Os (except HDMI) -0.3 2.0 V
HDMI I/Os -0.3 3.5 V
I/O 1.35V -0.3 1.65 V
I/O 1.5V -0.3 1.8 V 1.8V
I/Os -0.3 2.1 V 3.3V
I/Os (except those powered by vddshv8) -0.3 3.8 V 3.3V
I/Os (powered by vddshv8) -0.3 3.6 V

in the Above why is it I/O voltage 1.3V,1.5V,3.3V. I didn't understand those.
As I have seen in most of the case, HDMI TMDS signal runs at the Voltage of 3.3V(If am not wrong)
robertferanec , 05-27-2019, 10:17 AM
- there always should be an ESD protection on HDMI
- I am not really sure what you mean by 1.2V, 1.5V, 3.3V. Usually IOs are organized in blocks based on interfaces .... and it depends on what voltage you need, that is what you can use. For example, if you are connecting your CPU to a chip which is using 1.5V interface, then you can power the block of IO CPU pins which are connected to this chips by 1.5V.
- in some chips TMDS can be configured to different operations and maybe then different voltages can be used?
Lakshmi , 05-28-2019, 05:15 AM
Okay Thank you.
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