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

Richard Wilbur richard.wilbur at gmail.com
Wed Aug 30 07:18:35 BST 2017

Sent from my iPhone
On Aug 29, 2017, at 19:13, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> On Wed, Aug 30, 2017 at 1:54 AM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
> 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.

Which is an elegant way of doing some intra-pair (within the pair) skew compensation--by avoiding some the sources of skew to begin with.  Sounds like a fine idea!

>> 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.

Flood-exclusion zone is just the ticket if you don't have a more flexible way.  In this case it would increase the maintenance costs (time and effort) of the differential pair clearance to other circuits because it has to be manually checked and moved when needed.

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

I think this is the most maintainable solution.

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

"Practice makes easy."

> 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?

Eminently so!

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