[Arm-netbook] HDMI High-Frequency Layout: Timing

[mike.valk at gmail.com]

2017-08-01 16:11 GMT+02:00 Luke Kenneth Casson Leighton <lkcl at lkcl.net>:
> On Tue, Aug 1, 2017 at 2:51 PM, mike.valk at gmail.com <mike.valk at gmail.com> wrote:
>
> [trimming a huge amount of unnecessary context on your behalf, mike...
> hint, hint....]

My apologies to all.

>
>>>  there's a lot of other stuff in here which is really good, such as
>>> making sure that lengths on each *layer* are matched, and that even
>>> when turning corners the lengths are matched.  and matching just after
>>> VIAs *not* before... damn
>>
>> Between via's the length should be matched right? Length mismatch
>> should be solved as soon as possible to accommodate eddy-currents
>> right?
>
>  yes.  the toradex article explains it very well, also mentions the
> importance of symmetry (as does the TI post).
>
>> Found a nice post from TI
>> https://e2e.ti.com/blogs_/b/analogwire/archive/2015/06/10/differential-pairs-four-things-you-need-to-know-about-vias
>
>  nice!  dang, above 10ghz the thickness of the via starts to matter
> and needs to be drilled out, post-production.  dang.
>
>  *sigh* unfortunately symmetry is not completely achievable with the
> drastically-reduced amount of space available.  the HDMI signals come
> out right at the bottom of the board.

That might not be to problematic. I've search to the net for talk
about running tracks on top of each other. It keeps hunting me. And
found a knowledable awnser.

http://www.sigcon.com/Pubs/news/2_30.htm

On of the things mentioned is that the differential signals might not
be quite aligned to begin with. So achieving symmetry might look nice
on but can only give limited help in minimizing emission and pickup.

>
> l.