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High-Speed PCB Routing Guidelines

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Replies: 4

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Hi

I'm doing an HDMI board design and I've encountered what I think is a very large intra-pair skew requirement. The HDMI spec states 0.15Tbit which at 6GHz is 25ps. As a rule of thumb this would equate to about 3.8mm of FR4 (150um/ps). In TI's doc http://www.ti.com/lit/an/slla414/slla414.pdf section 2.4 (displayport) suggests 5mil (125um) for 20ps. I do not know why 125um is suggested for a 20ps skew when it only needs to be 3mm!

Have I made a glaringly obvious mistake?

  • Hi Graeme,

    I'm not quite sure I understand. Why are you concerned about the DisplayPort intra-pair skew requirements when your design is for HDMI? 

    Regards,

    I.K. 

  • In reply to I.K. Anyiam:

    Hi

    I referenced the displayport figure of 20ps and 5mils (125um) as this is comparable in speed to HDMI. Your doc doesn't give any figures for skew in terms of ps or mil for HDMI, only in terms of Tbit.

    The quoted figure of 0.15*Tbit for HDMI is equivalent to about 25ps.

    So I could've rephrased my question and only mentioned displayport, and then asked why do you recommend 5mils for a 20ps skew when this need only be 3mm?

    Thanks.

  • In reply to Graeme Parker:

    Hi Graeme,

    Understood. Less than 5 mils of intra-pair and interpair skew is our generic, rule-of-thumb recommendation for most high-speed designs across all frequencies (LVDS, DisplayPort, etc.).

    It is just a recommendation, and system designers are welcome to stray from it if they have a solid understanding of the requirements for this system.

    Regards,

    I.K. 

  • In reply to I.K. Anyiam:

    Thanks. That's our rule of thumb too. I just wondered if there was a reason it was far less than 3mm.

    Thanks for your help.