[Arm-netbook] HDMI High-Frequency Layout: Recommendations--Taper

Richard Wilbur richard.wilbur at gmail.com
Fri Jan 5 08:25:23 GMT 2018

On Fri, Jan 5, 2018 at 12:44 AM, Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
> On Fri, Jan 5, 2018 at 7:32 AM, Richard Wilbur <richard.wilbur at gmail.com> wrote:
>> Any
>> chance we could use another trace to even up the little dimples that
>> are left in the edge facing the differential pairs?
>  always best to screenshot / arrow-mark it, richard.  round-trip
> delays getting critical here, we're up to 5th january already.

I have attached a screenshot with red arrows.  It is taken from the
long straight differential microstrip transmission lines on layer 6.
These are otherwise so clean and straight.

> attached green arrows, i am guessing you mean, i remember you said
> that causes... something-or-other... capacitance-related?

The ones you marked weren't the ones I was looking at.  I think they
are probably fine as they aren't too sharp, they mind the 15mil
clearance, and we are turning 90 degrees out of some signal vias onto
the top of the board.  (If they were pointing at a straight side I'd
be more worried.)

>  there's a few more like that back at the source diff-pair vias, i'll
> think about whether to drop a plain square keepout in place (to
> preserve 15mil) or a straight manual track.  track would cut clearance
> to around 12mil.  thoughts?

It is kind of tumultuous with the pairs coming from close quarters on
layer 1 and still to be sorted through the intra-pair
skew-compensating wiggles.  What you've done at that end looks much

>> I glanced at parts of the rest of the board but it would be much more
>> straightforward with the schematics.
>  not so concerned about the rest of the board, it's mainly the HDMI.
> PDF version of schematics should be here
> http://hands.com/~lkcl/eoma/DS113-V2.7-2017-02-17.pdf

I'll concentrate on the HDMI, then.

>> Do you have a way to compare
>> changes with previous versions of the board to see if PADS did
>> anything surprising behind your back?
>  the only way would be visual pdf diffs, and i've made several minor
> changes as well that would show up and need to be reviewed, one by
> one, and eliminated.  i've found it's easier just to go over the board
> making minor tweaks, because each time i do so i find one more "little
> thing" such as a place where there's a capacitor that doesn't have a
> nearby GND via and so on.

Sounds good.
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