TMDS181: About DC blocking capacitors location

Michael

You can't AC coupling the TMDS181 input as the input Vih would then be above the 3.3V VCC depending on the input signal amplitude and this would stress the TMDS181 I/O. I would recommend using the DP159 for this application.

Thanks

David

David,

TMDS181 is a HDMI retimer.

As far I understand, all HDMI applications involve DC blocking capacitors on differential pairs before the connector.

You say that having capacitors on the input of the TMDS181 is not good.

What else if we place the capacitors at the output?

Michel

Michael

For AC coupling the output, you would need to place external 50ohm to 3.3V between the TMDS181 and the AC coupling capacitors. 

Please note this creates a double termination network which cuts the output signal amplitude in half, so you may have to adjust the TMDS181 VSADJ resistance to compensate the reduction in signal amplitude.

Thanks

David

So, it is not a good idea.

I'll use the DP159.

I see that it is possible to have capacitors on the input.

I have found a previous post we had with Brian Zhou on the same topic.

We previously used DP159 but Brian Zhou said that it is for DP++ to HDMI retimer.

He suggested to use TMDS181 instead because it is HDMI to HDMI.

I kindly remind that the source comes from DDI chanels on the COM Express module we can configure as DP or HDMI according to the signal DDI_ AUX_SEL.

I don't know anymore what to use.

If you can help...

Thanks.

Michel

Michael

DDI interfaces support dual-mode (DP++) and can be natively used as HDMI or DisplayPort, according to the state of AUX_SEL pins that should be connected to the DisplayPort connector on the carrier board.

So the DP159 is the right solution since it supports DP++ with AC coupling at its input, it will convert from DP++ to HDMI when AUX_SEL pin being pulled to 3.3V through a 100k resistor. 

Since the output is a HDMI1.4/2.0 output, it will be DC coupled without the AC coupling capacitors.

Thanks

David

David, I agree with you.

The last question is about the signals SCL_SRC and SDA_SRC needed by the DP159.

If you configure the DDI channel as DP, the multiplexed signals DDI_CTRLCLK_AUX+ and DDI3_CTRLDATA_AUX- will act as AUX+ and AUX- respectively.

Do you tie them to SCL_SRC and SDA_SRC?

Michel

Michael

If you configure the DDC channel as DP, then DP159 is not the right solution. You would need an active DP to HDMI protocol converter solution. The DP159 is purely a passive DP++ to HDMI level shifter. 

The DDI_CTRLCLK_AUX+ and DDI3_CTRLDATA_AUX-  will be CTRLCLK and CRTLDATA as shown below when AUX_SEL is high. And you can connect them directly to the DP159 SCL_SRC and SDA_SRC with the 2k pullup if the COM Express Module supports I2C clock stretching. If not, then you have to use the DP159 snooping mode.

Thanks
David

David,

To get the CLK and DATA signals from DDI channel, you have to configure it as HDMI.

Is it OK if we use DP159?

Thanks.

Michel

David,

Please find attached the schematics.

I didn't insert the page with the HDMI connector.

Could you please tell me if everything OK?

Thank you.

MichelHDMI_Retimer_1.PDF

That‘s good news!
Thank you David