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

[Richard Wilbur]

On Sep 26, 2017, at 01:16, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
>
> On Tue, Sep 26, 2017 at 7:00 AM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
>> Then to taper up, we have two options:
>> 1.  spread in both directions from the inner 2 pairs,
>> or
>> 2.  spread to one side or the other.
>
>> If we spread from the middle then the inner two pairs end up shorter than
>> the outer two whereas if we spread from one side the straight pairs will
>> be shorter then those which tapered away.  I'm going to suggest that when
>> we spread we move to the left which would lengthen the CLK lines more than any other.
>
> ok the two diagrams are great, they explain clearly what you're
> suggesting. and still after misunderstanding it i think i finally get
> it... that you need to do the tapers simultaneously on *all* pairs...
> and i don't believe it's possible.

I understand it may not be possible to do the taper in the space we
have available.  I just wanted to note the minimum length in the
signal propagation direction for the two geometries of the taper of
the inter-pair clearance between 5 and 15mil:
1.  spread/contract from/towards the middle:  15mil
2.  spread/contract from/towards one side:  30mil

> ... bear in mind i really don't want to modify these tracks... :)  i
> took out those GND spacings i was using to maintain separation...
>
> basically, it's down to the GND vias in between which are right where
> we want to do the tapering.  up until you get past the GND vias -
> which are there to protect the diff-pair VIAs - all clearances are
> 5mil.  it's the only way to have gotten the 5mil tracks in between the
> A20 BGA pads, for example, it's the only way to squeeze between the
> GND vias and still maintain straight (vertical) tracks of identical
> length and so on.
>
> in order to have the taper just before, it would be necessary to
> *close* the pairs together to a 5mil intra-pair spacing *after* the
> VIAs... and *then* re-open them back up again!   and that's right
> where we want to do the wiggles... which would then have to be
> delayed... which they can't be because there's not enough room to put
> them on the straightaway.... or if they weren't they delayed then the
> reduced space starts interfering with how the wiggles are created....
>
> basically it's massively complicated, and is far more than i would
> like to attempt at this late stage.

We certainly don't have to do any specific explicit taper.  We're
getting one for free at the connector end with the ESD and connector
lands.

I guess I'd be reticent to impose a taper that doesn't either maintain
or improve the symmetry of clearances and impedances.  I don't think
we would necessarily call it progress if we reduced the reflection
coefficient on a few traces and simultaneously increased the impedance
imbalance between the traces of one or more differential pairs
(shifting signal energy from differential to single-ended conduction,
raising the spectre of EMI).

>> Also, the taper at the ESD end should fold in from the bottom (CLK side).  At that end maybe we come from 15mil to 7mil before the ESD lands, if that's the best consistent clearance through the ESD lands, then taper to 5mil before the constraining copper and maintain 5mil to the connector.
>
> again: the GND vias prevent that from being possible, but in this
> case there is also the 45-degree length-correction wiggles to
> consider.

Problematic, I agree.

> now, what *might* work is putting in a very thin triangular wedge
> coming off (and on) each GND via, but in doing so i remember there are
> problems with having sharp points.
>
> in short i don't believe it's possible, and it's getting late in the
> day to try experimenting.

That it is.

By the way, I was reading Toradex, page 23 to review their
recommendations regarding intra-pair skew compensation (the wiggles)
and happened to notice that they actually said that turns within
15mm(millimeters not mils!!!)  are close enough that, if
complementary, can be considered to have cancelled out the intra-pair
skew so that no compensation is needed.  15mm ~= 590mil!  Oops, I
misquoted that value as 15mil in my earlier recommendations!  Sorry
about the mistake, that value is actually a good deal more forgiving
and generous than I made it sound.

That could even obviate the need for all of our intra-pair skew
compensation on the connector side of the long horizontal straight
section!  How far are the turns from each other?  Just eyeballing it
I'd say consecutive turns are no more than 150mil from each other on
the ascending section (heading northeast).

>> I agree.  I was going to suggest tapering down the keepout along with the inter-pair spacing before we get to the ESD lands.  Maybe even before we get to the vias.
>
> there's been some cross-over, i've done an update to the images on
> the website since.

Let's go with the layer 6 that doesn't have an explicit taper.[*]
It's more symmetric for all the pairs and open like layer 1 at the
connector end.

I thank you for all the hard work you've put in on this effort--laying
things out, re-laying things out, adjusting things, and pushing back
on things that sound bad or don't make sense till either I see they
don't make sense or you see that they do.

Reference:

[*]  http://rhombus-tech.net/allwinner_a10/news/eoma68-a20-275-layer6-hdmi.jpg