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

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68


On Fri, Aug 4, 2017 at 3:24 AM, Richard Wilbur <richard.wilbur at gmail.com> wrote:
> HDMI Layout Notes
> for EOMA68 Cards
> by Richard Wilbur
> Thu 3 Aug 2017
>
> Recommendations for this Layout
>
> Source (processor) end:
>
> Could we shrink the via pitch between constituents of a differential
> pair (bring the vias of a differential pair closer to each other)

 maaayyybeee?  PADS does the distances automatically, so it would
involve manual editing (and second-guessing of the automated rules /
best-practices for diff-pair routing in PADS)

> and
> combine the anti-pads into an oval shape on each layer?  This reduces
> fringing fields and thus parasitic capacitance.

 https://e2e.ti.com/blogs_/b/analogwire/archive/2015/06/10/differential-pairs-four-things-you-need-to-know-about-vias

 ok found it... hmmmm yeah i can see how that would work.

 ok see attached little image: turns out  that the GND copper
flood-fill clearance is enough to *automatically* create the
equivalent of what you're referring to.

> I like the ground vias close to the signal vias between differential
> pairs.  It would be lovely to get a ground via close to the via on
> HTX2P and possibly move the one between HTX2N and HTX1P closer to the
> signal vias (if the signal vias  of pairs can be moved closer with
> combining the anti-pads).

 yeah i think i can do the one on HTX2P... but it involves: *deep breath*

 moving HSCL and all those other signals further over

 inverting the XTAL-IN and XTAL-OUT signals so that one of them goes
the *other* side of its BGA pad

 moving and re-routing the PWM and EINT-0 signals (which are too close
anyway) with those vias, to the XTAL signals

 possibly routing HSDA round the *back*... no that takes it past the
diffpairs... HHPD routing *right* instead of down... that would be
okay.... it would go past the USB diff-pairs though.... i think i can
tolerate that..


> What is the intra-pair skew from the processor lands to the first
> signal vias?  I wonder if we could move the vias on the short lines a
> little further from the processor and make up some of the skew in that
> segment before we leave it?

 yes i was considering that - maybe just staggering the vias so for
example HXTX1N and P are inverted as to how they really should be.

> Sink (connector) end:
>
> Same thing for differential pair vias--HTX0 and HTX2--it would be
> lovely to shrink the via pitch and combine anti-pads (if possible).
> Again, I like the ground vias close to the signal vias HTX2P, HTX2N,
> and HTX0P.  It would be lovely to be able to either put a new ground
> via closer to the signal via on HTX0N or move the one on the ground
> shield trace closer.

 it's virtually impossible to get anything in there, because of the
three Rclamp0524p components (anti-static protection).

 i'm going to move one of the rclamp0524p's so that it's directly
above the other, and it *might* then be possible to fit some GND vias
in there.

 also i realised that the path is shorter to the DC3 connector because
of the via staggering, so i will have to put a small "wiggle" into the
shorter path right at that point.  why? because the signals should be
properly matched right up to that point.


> I like what you were showing on the video with the signal vias at the
> connector lands:  putting a neck on the trace between the via and the
> land should dampen the spirits of the solder but not the signals.

 yehyeh.

> If we could reduce the signal via pitch by combining anti-pads at the
> connector, we might be able to move the HTX1P and HTXCN signal vias to
> the other side of the lands next to the other side of the differential
> pair, thus equalizing the skew on the segment between the ESD chip
> pads and the connector pads.

 yehyeh i get it.

 nomnomnom....

 it might just be doable. i'd have to shrink the size of the two GND
pads, 16 and 4, then the vias for 14 and 6 could be moved to the other
side then *diagonal* (right).... shrinking the size of 10 as well
would allow the existing vias to the right of 8 and 12 to *also* be
moved diagonally to the right... changing them to 0302s would give
some extra clearance, it's risky but what about this isn't...


>  If that worked the final touch might be
> to add a ground via between DC3 pin 10 (GND) and the board edge for
> return current paths.
>
> Other than that, I would try and move as much of the skew compensation
> close to the source of the skew as possible.

 yehyeh *sigh* i missed that.  frack.  gonna have to redo the whole
frackin lot, one path at a time, so i can make sure each segment is
matched.  frack!!

> I'm not sure what I'm looking at as you mentioned the ground reference
> planes were solid under the HDMI differential pairs,

 yes.

> but it looks like
> they have voids under the signals in the pictures.

 no,

>  Am I seeing a negative,

 you're seeing the board pre-flood.  when flooding is done it f***s
things up in PADS, causes it to be very unstable (especially if you
switch it to "invisible" with SPO and PO / PD keystroke commands).
also massively increases the file size.  also gets in the way as you
can't see a damn thing.

 also if it was a real-time feature the entire system would grind to a
halt as it takes about ten SECONDS to recalculate the flood-fill.

 and yes layers 2 and 5 are solid GND planes.

> that there are only little strips of conductor in the ground
> reference plane directly under the high-frequency lines?  Neither of
> these interpretations is very satisfactory, nor do they seem to
> represent reality.

 you may be referring to the little ground tracks i added.  these are
there because the copper-to-everything-else clearance i set to
around.... i think... 7 or perhaps 10mil, so that it doesn't get
absolutely everywhere.

 however i *want* the GND plane (on 1) to go into nooks and
crannies.... reach the parts that other beers can't reach... only way
to do that is manually.

> Please let me know what can and can't be done and I will adjust
> recommendations accordingly.

 appreciated.

well, let me try the DC3 experiment of moving the VIAs to the other
side.  that i feel is really important.

l.