[Oberon] HDMI question

Skulski, Wojciech skulski at pas.rochester.edu
Sun May 17 22:57:11 CEST 2020

> Signals are DC balanced so if you want to do only output
> to monitor, just use C in series to every TMDS wire
> value doesn't matter some boards use 100nF  I use 220nF

On a further thought, this may be problematic. TMDS needs to be DC terminated with 50 ohms to 3.3V. If you insert the cap then DC termination needs to be before the cap, which is on your board. Then the monitor will provide its own termination, again with 50 ohms to 3.3V. It means that you are driving two terminations in parallel, which is 25 ohms. It is out of TMDS specs according to the data sheet. The FPGA may be able to pack enough current and reach the prescribed swing, or it will keep the current and the voltage will get reduced by two. It probably depends on the details of the driver which are hard to guess outside the specs. 

My conclusion is that I either provide the HDMI connector w/o any caps or I provide the redriver chip. Not sure yet. I will look how much space I still have on the board.

Thank you,

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