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

Tue Aug 22 14:43:15 BST 2017

On Aug 18, 2017, at 19:54, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> 
> On Thu, Aug 17, 2017 at 5:20 PM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
>> 
>> So you have a flood-fill on the bottom layer?
> 
> all layers.
> 
>> Is the flood-fill connected to GND?
> 
> only when it's properly arranged to be so... i.e. when you don't you
> get a warning... short answer: yes.

So it sounds to me like some of the ground vias can connect more than just layers 2 and 5 if they happen to coincide with ground flood-fill on one or more other layers?

>> Can you set the 15mil copper-to-trace separation as a property of the differential traces?
> 
> yyup.  i really like PADS for this reason

Nice.

[…]
>>> there's one place where the diffpairs go past the main power line
>>> (IPSOUT) - that's got a 5 mil copper GND separating it at present: i'd
>>> be nervous about taking that out.
>> 
>> I wouldn't worry because that 5mil copper GND has
>> 5mil spacing on each side, thus ensuring 15mil between the closest
>> differential trace and power.  That should be sufficient.
> 
> ... need to check it.

Those were my understanding of the limits of your board fabricator:
min{spacing} = 5mil
min{trace width} = 5mil

> it's always been very close: in this revision i particularly
> wanted the vias left of the rclamp0524p to be reasonably symmetrical
> and clean, with a straight (diff-paired) path to the rclamp0524p
> instead of taking a turn to get to it (as in previous revisions).
> 
> that required a little bit more space, which meant moving IPSOUT's
> vias a little bit further over.  i could _probably_ move them over a
> bit further...

Sounds fine.

> 
>> The other thing that we can do if we have a little extra space
>> after taking out the intermediary GND shield traces and inter-pair
>> skew compensation wiggles is distribute the intra-pair skew
>> compensation closer to the sources of intra-pair skew--corners.
> 
> aw poop - changing those is quite a task.  there's some bugs due to a
> combination of grid snap and push-and-shove in PADS where removing the
> long straights means i can't add them back in again.  and i need to
> remove them because otherwise i don't know how long the traces are
> from the vias.  what i do is:
> 
> * remove the long sections
> * re-add a *short* diffpair section of only about 1mm
> * those end up being equal length
> * then because the traces aren't complete PADS will tell you exactly
> how long they are
> * therefore i can now measure them both and...
> * therefore i know exactly how much manual "wiggle" to put in the shorter one.
> 
> once the wiggles are done i can re-add the long sections, confident
> that the signals will be matched.
> 
> but it's a pain to do! :)

I'm glad you have a method that works.  I'm sorry it is such a pain.  Too bad it isn't more straightforward.  Is PADS libre software?  I ask because here's an itch.

>> Right now you've done a great job of compensating for intra-pair
>> skew in the first segment:  from CPU lands to first via.
> 
> yehyeh.  they're near-identical.
> 
>> Then there are some very significant wiggles when we first get
>> to the bottom layer
> 
> yes.  intra-pair correction due to wanting to have the 1st layer
> traces all the same length.  it's nearly... 1.5mm  to correct, due to
> not just the offset of the vias but also the turn.  if i tried to
> stagger those first vias the other way (which i tried once) then
> there's not enough room to have those 1st trace segments be equal
> length...
> 
>> and I don't see any other intra-pair skew
>> compensation all the way out to the connector.
> 
> that's because they're all fine... ok i read somewhere that it's ok
> to have some intra-pair skew on short lengths between turns.  sooOo...
> i'm assuming that the critical part is the long straight.  sooOOo i
> arranged for the wiggles to make perfect length-matching just as each
> pair hits the beginning of each long straight.
> 
> now (and i've removed the inter-pair skew in the current revision)
> what i *haven't* done is add in any inter-skew correction at the
> points marked in green (attached).  i'm assuming that those diagonal
> cross-paths (between each green ring) are... within acceptable
> tolerance for intra-skew.
> 
>> If we can do it, the most effective place for intra-pair skew compensation
>> is within 15mil of the skew source--right before or after a bend.
>> If skew originates in a bend and is resolved by a complementary bend within 15mils,
>> then we don't need to add anything specific.
> 
> mmmm *grumble, grumble*.... i think there might be space to add them,
> around where the green rings are, by moving the diagonal pieces to the
> right a bit.

Sounds like a good plan.  How much intra-pair skew do we incur at each of those bends?

>>>> How far are the differential traces from board edge at present?
>>> 
>>> 0.9mm -> 35 mil.
>>> 
>>> to the nearest vias is 0.2mm -> 0.787mil
>> 
>> How far is the board-edge ground shield trace
>> from the edge of the board?
> 
> to the edge of the GND shield trace: 0.46mm -> 18 mil
> 
>> From the closest differential pair trace?
> 
> to the edge of the CK diffpair, 0.93mm -> 36.6 mil
> 
>> How wide is the board-edge ground shield trace?
> 
> pffh :)  peanuts.  very tight.  13 mil (that's to the vias as well,
> which i realise is slightly dodgy).

We'll take what we can fit.

>> I'm guessing you meant the closest vias to the differential pair
>> traces are 0.2mm = 7.87mil?
> 
> yyep.

In order for me to understand better the dimensions you're quoting allow me to resort to a diagram.

>> Are these the ground-to-ground vias for low-impedance connection
>> of reference planes?  (Low-impedance return path close to signal vias?)
> 
> honestly i haven't been thinking in terms so specific: i just add
> them arbitrarily because i heard it was the right thing to do!
> learning fast...
It is good to have a scattering of vias connecting related planes (in this case ground).  With limited space it becomes more important to use what heuristic we can muster to place them strategically.

(Writing from our tent here in John Day, Oregon.  The total solar eclipse yesterday morning was spectacular.  I'm glad we travelled to be in the path of totality.  I've seen partial solar eclipses before but this was well worth the trip.  We're going to visit the John Day Fossil Beds today before we head home tomorrow.)