[Oberon] HDMI question

D EMARD vordah at gmail.com
Sun May 17 23:26:51 CEST 2020

Our 1st prototype had no C and directly to the pins.
One board outputs are fried during plugging of connector without GND on shiled
board still works no video anymore. So the C came and no further
boards are fried.

ULX3S has been tested to drive 30-m overhead projector so I'm pretty
shure it's well done.

Caps represent 0-ohm AC impedance so the impedance will be the same
with or without caps but it doesn't matter.

I'm not further expert in electrical standarts but my guess is that TMDS is
current mode differential, not voltage mode like LVDS so final termination in
monitor may be 0ohm (theoretically)

On 5/17/20, Skulski, Wojciech <skulski at pas.rochester.edu> wrote:
> D.Emard:
>> 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,
> Wojtek
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