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

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Aug 16 07:31:17 BST 2017

On Wed, Aug 16, 2017 at 6:11 AM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:

>> yehyeh.  i could then move them slightly away from the edge of the board.
> I'm curious, what would you move?  The goal of this was to get >=
> 15mil between any differential signal trace and any trace not
> from the same differential pair.

 ahhhh ok.  i'm glad you're paying attention :)

> The ground shield traces with 5mil spacing, 5mil trace width,
> and another 5mil spacing enforce this spacing on the differential signal traces.
> So if we remove the ground shield traces, and don't move anything closer,
> we get that spacing for previous effort.




 if i just take *out* the ground intermediary traces that would do the
trick of bringing the impedance back up, is that right?

 what would you suggest, here - leave the intermediary GND traces in
or take them out.

 also, i think i "Get It" about the intermediary wiggles.  when the
transmit end does automatic compensation that results in the signals
coming out in such a way that, really, the inter-pair length-matching
should be done from the *OPPOSITE* end i.e. from the CONNECTOR.


 because the automatic compensation will result in the signals coming
out with a small delay, which by the time they go round that big set
of bends they *WILL BE IN SYNC*.

 ok they'll be in sync as long as all pairs are exactly the same
length from that point up until they meet the connector.

 so the only bit that would be out-of-sync would be that huge set of
bends just after the transition from CPU-layer-1 onto layer 6, where
i've had to put in huge amounts of bend-compensation.

 by adding in the down-stream inter-pair compensation just before the
rclamp0524p's) that *entire straight section* is out of sync... and
the set of bends is also out-of-sync so it's no improvement.

> Are you talking about moving the differential pairs further
> from the edge of the board?

 yes. but from what you're saying it's not possible anyway.

>>> Another interesting reference on high-speed HDMI PCB layout is TI's SLLA324[2].
>> nnniiiiiice.  i love it.  that's exactly the same connector being
>> used.  hmmm iinteresting, they bring the vias up from underneath on
>> all 4 diff-pairs...
> I think that is to keep the path as similar for all 4 pairs as possible.


> Vias add delay and (if not properly tuned) reduce the impedance.
> So it seems they are working with the stratagem that it is
>  better to treat each component of the signal the same.

 indeed.  however i don't want to change the BOM, apart from anything
 that's a TI part not a "Well Known Easily Sourceable Part In The Shenzhen
 Huaqiang Road Eco-System".

 dual rclamp0524's, one each side, it is.


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