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

[Luke Kenneth Casson Leighton]

On Tue, Sep 26, 2017 at 7:00 AM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:
> On Sep 23, 2017, at 03:47, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
>>
>> On Sat, Sep 23, 2017 at 8:26 AM, Richard Wilbur
>> <richard.wilbur at gmail.com> wrote:
>> […]
>> 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 guess we don't have to worry about layer 3 if it's going to be that uncooperative and we don't gain that much by making the desired changes.

 wheww :)

> 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.

 ... 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.

> 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.

 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.

> 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.

l.
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