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

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Aug 30 02:13:41 BST 2017

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On Wed, Aug 30, 2017 at 1:54 AM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:
> Luke,
> Thank you ever so much for the clarification regarding perspective.
> Comments inline below.
> On Aug 22, 2017, at 12:25, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
>> okaay, wiggle-progress... 1st image is left end, i think CX and TX0 i
>> can shorten even more, there's definitely plenty of space there, big
>> gap.
> Shortening CX and TX0 is fine but I would cramp any
> of the clearances to make it happen.

 there aren't any!!  the GND separation vias see to that (attached).
no there's not enough room to move those GND vias *in between* the
VIAs, they need to be below, unfortunately.  that means the diffpairs
need to curve _round_ them (to the SW slightly).  oops.

 actually it just occurred to me that i could move the HXT?P vias over
by another.... 10-15 mil or so, which would mean that the
corresponding HXT?N traces would be able to go straight (directly
south) instead of SW, S then SE.

  phrrrrddhh :)

> On the other hand, if you were to put them into a 45°
> bend from south to southeast a little sooner,
> we would probably have enough room to adjust the ground shield
> trace on the southwest and west to abide by our 15mil differential
> trace to "anything not in the same pair" clearance (specifically to HTXCN).

 well, unless i add a flood-exclusion zone (thoughts on that?)
anything W or SW of HTXCN is going to get flood-filled with GND.

 or... i _could_ just put in a copper-to-diffpair Design Rule of "15
mil" clearance - that would keep the flood-fill away.

>> anyway those wiggles are done by hand, some of them aren't pretty but
>> in mil those traces are now nearly all to within 0.01 mil.  i cheat a
>> little and have made some of the corners in places a very very tiny
>> arc, where you get better fine-grain control over the amount it takes
>> off.
> Well done.

 ... i'm getting used to it....

>> HTXCN just before the diff-pair vias at the end i'm a little concerned
>> about, it looks a bit too... sharp-angled to make me feel totally
>> comfortable...  not a lot of space... i tried a single wiggle and it
>> went too far away from HTXCP for me to feel happy about it.... put in
>> two much smaller wiggles instead...
>> GND i'll remove as the absolute last thing.
> Sounds great.  Only really want to remove the ground traces
> that can't comply with the 15mil clearance between differential pair and anything not in the same pair.

 marked in this image, i was planning to remove the GND traces that
are marked with red dots.... but only after all's done because
currently they help keep the inter-pair separation.

 does that sound sensible?

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