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

[Richard Wilbur]

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.  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).

> 2nd image, right end, this is where i've added intra-pair wiggles at
> the 45 degree bends.  i can't get away with a proper 4-turn using 45
> degrees, PADS goes "nope that's too short a trace, i'm gonna assume
> you want a straight line instead" - there's probably an option
> somewhere for that but i've not found it.  alternatively i can add in
> a (curvy) accordion....

I agree.  If you don't have enough room to make a trace facet length >= 1.5 * trace width then I would also resort to an arc (curve).  (Which for 5mil wide traces suggests minimum facet length of 7.5mil.)

> 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.

> 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.