The Layout Design guide V1.0 specifies 90-ohm differential trace impedance for HDMI TMDS signals in section 7.7. The general industry specification for HDMI 1.x is 100-ohms differential. Is this a typo or is there something special about the HDMI PHY on the Apalis SOM modules, specifically the TK1?
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The value that you see in the Layout Design Guide is actually not a typo. We recommend our customer to use this impedance which is the same used in our modules. Having an impedance discontinuity could result in a bad signal quality.
Said that, honestly, I am not sure why we have started using this value in our modules. I would assume that we got this information from a design guide or we had to use this value for routing reasons. Then we kept using this value in order to avoid different behaviors between different modules.
I really hope that it clarifies your doubt. Thanks one more time for your punctual feedbacks, they are really appreciated. Please don’t hesitate to use again the Developer Community if needed.
Unfortunately, an impedance discontinuity cannot be avoided as the external HDMI plant is all 100-ohms (connector/cable/monitor input). Additionally, the PHY of the SoC device, if HDMI compliant, will mismatch with the 90-ohm PCB tracking, yielding two points of discontinuity. I would think that in order to minimize the effect, you want to minimize the traversal length of the aberrant region, in this case the short hop from the SoC to the MXM3 connector pins. If the region of discontinuity can be reduced to 1/10 wavelength of the signal fundamental (about 12mm in this case), the discontinuity becomes virtually transparent. Recommending that the tracking remain at 90-ohms out to the connector on the carrier extends the discontinuity region out to as much as one wavelength or more.
Realistically, 90-ohms is not a terrible mis-match to the 100-ohm target, but I see this as a zero-cost opportunity to minimize the effects.
I definitely agree on all the points you mentioned. We have already in our system a note about this possible improvement and we are looking into these tickets all the time we work on products redesigns.
Again, @RPHILLE, your feedbacks are always really valuable and useful. We are really glad of having customers so competent.
I wish you a nice day.