[Oberon] HDMI question

[D EMARD]

OK then I'd say it's best to spend a little ¢ to
place the transciever and inside will be a circuit
to fit the spec


On 5/18/20, Skulski, Wojciech <skulski at pas.rochester.edu> wrote:
> For the record:
>
>> Caps represent 0-ohm AC impedance so the impedance will be the same with
>> or without caps but it doesn't matter.
>
> The point is not the AC impedance of the caps, but whether or not the TMDS
> output transistors expect the DC level to work properly. I still remember
> the old school open emitter ECL or the new school open collector outputs.
> You cannot operate these topologies without DC connection to the power rail.
> Since your board works it must be somehow different from the ECL or OC, but
> it is clearly violating the specs. It works though it should not ;-)
>
>> 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)
>
> LVDS is current mode. It steers 3.5 mA current into the far end termination,
> which is 100 ohms between the LVDS+ and LVDS-. This develops the nominal 350
> mV voltage difference at the receiver. Unlike TMDS, there is no specified
> connection to either the ground or power.
>
> The TMDS specifies 50 ohms from each TMDS+ and TMDS- to 3.3V. Differentially
> it is still 100 ohms, but the middle point of the termination is supposed to
> be centered at 3.3V. This topology resembles the open collector, but it is
> differential. I bet that the monitor is providing these resistors. It cannot
> provide 0 ohms because this would be a short.
>
> Thanks
> Wojtek
> --
> Oberon at lists.inf.ethz.ch mailing list for ETH Oberon and related systems
> https://lists.inf.ethz.ch/mailman/listinfo/oberon
>