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

[Richard Wilbur]

> On Aug 14, 2017, at 23:39, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> 
> On Mon, Aug 14, 2017 at 10:37 PM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
>> 
>> So if we were to remove the ground shield traces from between differential pairs we could meet the inter-pair spacing recommendations without moving anything else.  This may explain the design by the wits-tech senior engineer you mentioned which worked without ground shield traces between the differential pairs.
> 
> 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.  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.

Are you talking about moving the differential pairs further from the edge of the board?  I'm guessing since there is a ground shield trace along the edge presently, that the ground shield trace would make the distance from the nearest differential trace to board edge at least s + w = 10mil.  If the ground shield trace is 5mil from board edge then we have 15mil from nearest differential trace to board edge.

>> 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.