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

[Luke Kenneth Casson Leighton]

On Sat, Sep 23, 2017 at 8:26 AM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:


> After spending a couple minutes studying layers 3 and 4, here's what I see:
> 1.  It looks like there may be a difference in the signal via antipads on layers 3 and 4 and that would be a way for us to give just that handful of vias special properties--if need be--although in this case it interestingly looks like the antipads are larger on layer 4 than layer 3.  (optical illusion?)
> 2.  The (minimum?) polygon size or line width of the fill looks larger on layer 3 than layer 4.

 layer 4 HDMI vias are actually covered by VCC 3v3 flood-filled plane.
it's a 5mil clearance to that.  i decided i didn't like that, so i
made a cut-back in the 3V3 plane so that GND covers it instead.  it's
*still* 5mil even on that flood fill.

 so it's just something weird about the flood-fill on layer 3,
possibly due to it being a copper pour not a "plane area".  don't
know.  if absolutely necessary i can put in some tracks that split the
pairs.


> I think the second point, or something along those lines, likely explains the void on layer 3 and lack of void on layer 4.  It looks like if we were to find and adjust that fill parameter on layer 3, some of your explicit guard traces might become redundant. (I can see why you added them because the ground fill wasn't working as expected.)
>
> It would be nice to change layer 3 to make the signal path more uniform on the way through the vias but it's not the end of the world if we can't, as long as layers 2 and 5 resemble layer 4 in the vicinity of the HDMI differential signal vias adjacent to the A20.

 i have to use it as a signal layer, so there's tracks running round
the back of some of the diff-pair VIAs.

> The remaining questions are where do we impose 5mil clearance (by bringing in the fill), where do we start tapering, and what does the taper look like?  Likewise, but in reverse order, at the other end.

 yehyeh

>>>>>>> Is the closest copper on layer 1, around the A20, 5mil from the HDMI
>>>>>>> differential signals?
>>>>>>
>>>>>> yes.  everything's 5 mil design rule.
>>>
>>> I agree that 5mil is the design rule.  The question is, "How close did
>>> we actually get?"  What I'm referring to as foreign copper is any
>>> trace, via, component land/pad, or fill that is not part of the
>>> differential pair under consideration.  In other words, did we make it
>>> from A20 land to via without getting closer than 10mil?  7mil?  We can
>>> adjust the proximity of ground fill with a manual keepout if we need
>>> more space so I'm not too worried about that.  I'm more curious about
>>> distance to other traces, lands/pads, or vias.
>>
>> ok - let me re-run the flood fill and do a quick review, starting from the A20.
>>
>> so.  layer 1.  surrounded, all 5mil.  tracks are only 60mil or so to
>> the VIAs.  didn't do a keepout.  all 5mil.
>>
>> layer 3 (the VIAs) - some sort of curve on the flood-fill, it's 5mil
>> but there's a void in the middle.
>
> Are layers 2, 4, and 5 also 5mil away from the differential signal at the vias?

 yes.  layer 3 is the only exception.

>> layer 6, starts @ 5mil, expands out to 15mil (mostly).  exceptions:
>> distance to TX2 "long wiggle" is 7mil,  distance from bottom VIAs
>> along board edge (to TXC), 11.2mil, distance to track *between* the
>> VIAs 15mil.  distance to GND vias ABOVE the hdmi tracks (TX2), 19mil.
>>
>> in theory then i could move the entire set of horizontal tracks up
>> by... 4 mil... i reeaallly don't want to though as it means redoing
>> the whole f*****g lot of wiggles.... argh :)
>
> Can't select and move?

 you can... but there are special rules which ensure that 45 degree
angles on two adjacent segments are "respected".  it gets extremely
weird and extremely frustrating.

> Since it is such a long section it would be beneficial to move the traces.

 argh.  i kinda reached that conclusion :)

 what i can do to some degree is manually enter values (adding 4mil up
and 4 mil left/right) so that there's less to redo by hand.

> So if we bring in the keepout at 5mil on layer 6 and taper it slowly to 7mil by the point we get to the TX2 wiggle which exhibits 7mil clearance.  To make this work we have to start the pairs off around 5mil inter-pair spacing and then spread them as we taper the keepout.  I realize this is more complicated than what I first described.

 it's too much.  i can just about manage adding 4mil manually to every
single one of those long straights, moving them up from the 11mil
clearance to the bottom board-line VIAs to 15mil, thus taking 4mil off
that 19mil clearance and resulting in 15mil there as well.

 we don't have *room* for 7 mil inter-pair spacing.

 if i've misunderstood, do let me know.

>> yehhh there are so many GND vias at the ESD end i'd question its
>> effectiveness...
>
> You'd question the effectiveness of what?  The ESD component?  The taper?

 putting in a taper at the end is significantly disrupted by the
presence of non-removable VIAs.  you can *add* a taper... but then the
VIAs (which cannot be moved) are *already* within about 5mil or 7mil
of the tracks.

 what *would* work is bringing the taper in *BEFORE* the ESD
components.  it also coincides with the double 45-degree bending of
the group of tracks, so is still a bit... dodgy.

 see this picture for reference:
http://rhombus-tech.net/allwinner_a10/news/eoma68-a20-275-layer6-hdmi.jpg

 basically there's no point in tapering *after* the ESD components
because the GND vias are already closer than the taper would bring GND
in.

 l.