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

Richard Wilbur richard.wilbur at gmail.com
Fri Aug 4 03:24:09 BST 2017

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

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).

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?

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.

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.

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.  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.

I'm not sure what I'm looking at as you mentioned the ground reference
planes were solid under the HDMI differential pairs, but it looks like
they have voids under the signals in the pictures.  Am I seeing a
negative, 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.

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

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