High-Speed PCB Routing Guidelines

Graeme Parker

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?

over 4 years ago

0 Ikechukwu Anyiam over 4 years ago

TI__Mastermind 35130 points

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.

0 Graeme Parker over 4 years ago in reply to Ikechukwu Anyiam

Expert 1235 points

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.

0 Ikechukwu Anyiam over 4 years ago in reply to Graeme Parker

TI__Mastermind 35130 points

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.

0 Graeme Parker over 4 years ago in reply to Ikechukwu Anyiam

Expert 1235 points

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.

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High-Speed PCB Routing Guidelines