TMDS171: Where should TMDS171 retimer be placed in a 2:1 switch configuration along with HD3SS215?

Hi,

Since the compliance is done at the connector output, I would recommend putting the TMDS171 at the output of the HD3SS215. 

Thanks

David

Would there be issues with the sink or repeater tests to do the adjustments made by the retimer? Stuff like swing adjustment and fixed equalizer settings?

Hi,

What compliance failure did you see with the internal HDMI source? 

I was thinking the source compliance testing with my initial response. So for the source compliance testing, you basically use the TMDS171 to shield the HD3SS215 since the HD3SS215 is just a passive MUX without any signal conditioning capability. 

If you run the sink side compliance testing and with the TMDS171 on the HD3SS215 output, I don't think VSADJ and equalization will be the problem here. I would also recommend to use the adaptive equalizer instead fixed equalization. But you may get Test ID 8-8 sink differential impedance to fail since the sink differential impedance is the impedance of the TMDS171 RX 50ohm termination plus the HD3SS215 Ron. 

I don't believe the repeater test would be applicable here because a repeater includes both sink functionality and source functionality which the current design does not have. 

Thanks

David

Here are our source electrical testing results:

According to the HDMI ATC, we are subject to repeater, source, and sink tests due to the nature of our device. It sounds like we should place the retimer before the switch then? Also one more question related to this. Can the polarity be swapped on the HD3SS215 in addition to the lanes as long as everything is kept consistent between the two inputs and 1 output? For example:

Dan

Dan

You can switch the lane polarity as long as the polarity is maintained between the HD3SS215 input and the output. The TMDS171 also supports receive lane polarity swap in retimer mode only. 

For the internal HDMI source, I would still recommend putting the TMDS171 on the HD3SS215 output. You can let the TMDS171 RX equalizer compensating the loss from internal HDMI source + HD3SS215 to pass the eye mask test, and tune the TMDS171 VSADJ to pass the VL test. If you don't have common mode choke on the output, then I would also recommend adding common mode choke between the TMDS171 and the output connector to pass the intra-pair skew.

For the external HDMI source, I would recommend putting the TMDS171 between the external source and the HD3SS215 input so you would not run into the Test ID 8-8 compliance failure. 

Thanks

David  

David,

I contemplated doing the polarity swap on the TMDS171, but we can only use the IC in pinstrapping mode and cannot guarantee we won't fall below the threshold for retimer mode. 

Are you suggesting using two retimers for our design? There a couple problems with that solution. The supply of these ICs is very low, lead times are long, and the ICs are going for 10x on the broker market. Do you think its feasible to get by using the retimer before the switch IC only? We had no issues with 8-8 in testing where we simply had External Source -> HD3SS215 -> HDMI Out. 

Dan

Dan

I understand your concern with a two retimers design proposal, but I see this as the solution that gives us the max margin in passing the compliance. 

The HD3SS215 insertion loss itself is around -2dB at 3G, and the TMDS171 max de-emphasis is -2dB only. So my concern is with TMDS171 before the switch, the TMDS171 TX will not able to provide enough compensation to pass the compliance test.

Thanks

David 

Do you think we could get a better idea if 1 retimer would be enough if we use the evaluation module in the various configurations we're discussing here? I know there will be some differences due to the discontinuities introduced by the connectors on the EVM, but is it worth a shot?

Dan

Dan

I would agree with you. We have both the HD3SS215EVM and the TMDS171EVM in stock, so you can definitely try out the different configuration.

Thanks

David