Length matching SDIO interface

Layout
1

Hello,

I am trying to length match an SDIO interface using KiCAD’s ‘Tune Track Length’ tool.

I noticed that in my current situation, I am unable to route 2 lines in the interface without using vias. I have 2 queries in this particular case:

  1. Should I consider the via’s length, when length matching?
  2. If the answer to 1. is:
    yes - how can I get this information from KiCAD? I have observed that the tool does not consider via’s length.
    no - why shouldn’t I?

Thank you for reading.

Cheers,
ashare

2

Because you do length matching up to +/- couple of mm if the signals requires it. Very fast signals require tighter matching than slow signals (USB3 very fast signals specifies max. 150mils (~4mm) of mismatch).

So unless you are working with USB3 kinds of speeds or higher, you should not worry about a via-lenght difference between your pairs.

3

Nice reading:

Layout Design Guide - Toradex:

TI High-Speed Interface Layout Guidelines:

http://www.ti.com/lit/an/spraar7h/spraar7h.pdf

This doesn’t answer your question directly but you might find something relevant there. Check especially page 39 in the Toradex document.

2 Likes
4

I read somewhere about being able to specify the additional length from the pad on the PCB to the die in the chip somewhere. I’m not sure if that was related to KiCad though.

And how do you know if KiCad adjusts for via’s? It knows (at least a defaut) for the board thickness.

5

At present, I don’t think it adjusts for the vias as KiCAD does not know the distance between the planes in case the tracks are routed through middle layers.

6

Thank you for the link. It helped.

7

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