External HDMI output

Hello there,
I am currently working on a motherboard for the BeagleBone Black. One of the requiments is to have a HDMI output on the side of the motherboard. I Know that there are LCD pins on the BeagleBone Black header and there is a possibility to connect a LCD cape. I was wondering either it would work to duplicate the NXP HDMI framer circuit that is on the board to the motherboard and use it for hdmi output? If I am correct the only difference would be that there is no acces to I2C0 from headers so I would have to use I2C1/2 for the NXP chip. That way at least I wont chave to worry about addressing 2 chips at once (not sure If you can change address of the hdmi framer yet).

My question is, would this solution be bard to implement in software, or is it easy to change the I2C bus config for hdmi? Are there any obstacles that I am not aware of? I would really aprichiate any help in here.

I doubt it will work but you are free to try. Adding two loads such as that on those pins is asking for trouble.


There are resistors on the lines, worse case scenarion I can take them off. But tgose lines are used for lcd cape already so it does work, whats the problem?

I am talking about you adding two NXP parts on those lines as you indicated that you were doing. LCD cape uses buffers do boost the signal. The NXP pins are 1.8V inputs.


I could do buffers as well, thats not a problem. I am building the board from scratch. Besides, arent the nxp input very high impedance?

It is the low voltage level as these are 1.8V inputs. The issue is making sure you get low enough, fast enough due to capacitance and at the correct frequency to yield a good data integrity. You can try using a translator buffer from 3.3V to 1.8V. That should help.


Cant I just power the NXP from 1.8V? If the lines are 1.8V I would need no translation then. Also there are no buffer on the bbb itself so why would I need such?

It already is powered from 1.8V. The processor however, is powered from 3.3V. The NXP part is 3.3V tolerant.

You need the buffer to clean up the signals and provide better drive as is being done on the LCD boards…

I just designed the BeagleBone Black. You are designing your mother board so do as you see fit.


I am only asking, no hard feelings :p. Ive just noticed that there were no buffers on the lcd lines between nxp and mcu on the bbb. Thats why I asked. Why are there no buffers on the bbb? By buffers do you mean that a voltage follower would do if the inputs are 3.3V tollerant?

No there are not. That is why I am saying you need them on the expansion board. The reason there is no buffer on the board is because it had to be cheap and fit in an Altoids box. I could not do that and add the buffer,

By buffers I mean like the one used on the LCD board. https://github.com/CircuitCo/BeagleBone-LCD7-RevA3/blob/master/BeagleBone-LCD7-RevA3-schematic.pdf?raw=true

Look at U1. Replace the VDD_LCD with 1.8V.


I see now. Thank you a lot, i will take this under consideration when I get to the hdmi part in my board. I already aprichiate your help, but do you have any other tips that might help? Any software config problems you predict?

If you do it right, you should have nothing to do. But, be warned, the expansion connectors are not what you call high speed. So it is possible that your HDMI may not be able to get to the same resolution as the one on the board.


I understand. Thank you.

*aside from the I2C bus change in the software somewhere I understand? The I2C0 is not on the expansion header.

Hello there again Gerald,
I was trying to get a piece of TDA19988 chip but he is not avaible in a single distributor :|. I was trying to find some simmilar chips from other distributors but I am not sure either they would be compatible. Would you mind helping me out in this case?

W dniu środa, 8 października 2014 21:03:58 UTC+2 użytkownik Gerald napisał:

You are kidding.


Whats there to be kidding about? I have obtained AD9889B from analog devices, but I am not sure either it would work. I havent worked with HDMI interface before. I dont want you to get me a certain chip, just a hint either it can be replaced easilly.

W dniu 2014-10-17 o 17:50, Gerald Coley pisze: