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

[Luke Kenneth Casson Leighton]

On Tue, Sep 19, 2017 at 11:26 PM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:
> On Sat, Sep 16, 2017 at 8:23 AM, Luke Kenneth Casson Leighton
> <lkcl at lkcl.net> wrote:
>> http://rhombus-tech.net/allwinner_a10/news/
>>
>> ok so after the successful DC3 test this is the last final check
>> before sending the gerbers off to the factory for pre-production
>> prototyping.
>
> 3*Cheer!

 :)

>> in the end i used a "keepout" area on both layers 1 and
>> 3, drawn by hand, to ensure that no GND flooding gets near the HDMI
>> traces on layers 1 and 6.
>
> "keepout" on layers 1 and 6, right?

 yehyeh.  only where needed.  not "cut-through to all layers"

> Not a bad idea, especially since
> it allows the situations at both ends of the traces to avoid design
> rule check (DRC) failures because we have copper that has to be closer
> than 15mil there.

 yehyeh

>> l'm including layer 3 as an example of how
>> the group of HDMI vias that come just out of the A20 punch a large
>> hole: GND-flooded layers 2 and 5 as well as 4 (power plane) will also
>> look like that.
>
> Could you put a similar snapshot of layers 2, 4, 5 on hands.com (or
> wherever you think appropriate)?

 they're exactly the same as what you see for layer 3.... except
entirely full.  ok that's not actually true (i just checked) - do a
page-refresh on the URL i just added layer 4 image)

>  I'm interested to see what
> holes/voids and connections the power and ground planes have.

 there are *no* connections on the GND planes.  the power plane (and
GND layers) interestingly have done a full surround on the HDMI vias.
remember i had to separate them by an unusual distance.

> What are the names of the power pins on the A20?  What voltages do you
> supply it?

 1.1, 1.25, 2.5 and 3.3v.

>  (Are any of them Vdiff+/-, e.g?)

 no.

> I'm interested in
> tracking down the power supply pins for the differential HDMI signals
> as that is where our return path for common-mode signal has to go.

 there's no specific power pin for HDMI.  the GND pins are grouped in
with a whole stack of other GND pins, there's absolutely no way it's
practical to get a special GND plane to it: the board is extremely
full already.

> I've read a little (not nearly as much as I'd like, but I lack time)
> about using a taper to match impedance differences while minimizing
> the reflection coefficient.[*]  I'm thinking we can use it at both
> ends of this layout to great advantage.  We taper from 5mil clearance
> around the A20 on layer 1 to 15mil clearance on layer 6.  Later we
> taper from 15mil clearance to whatever the closest copper is at the
> ESD and connector lands.

 that's something that it would be helpful to have a rough diagram,
even if it's hand-drawn [but see below: i think i understand it]

> Is the closest copper on layer 1, around the A20, 5mil from the HDMI
> differential signals?

 yes.  everything's 5 mil design rule.

> What is the distance to the closest copper to the HDMI signals at the ESD lands?

 5 mil

> What is the distance to the closest copper to the HDMI signals at the
> connector lands?

 5 mil

> (I'm guessing in both cases it is likely the neighbouring lands.  Is
> that correct?)
>
> What is the minimum frequency we will be running the HDMI at?  (With
> version 1.4 the highest clock is 340MHz which implies 3.4GHz data rate
> on each data line.  Thus I would expect good edges if we design for
> harmonics of 34GHz.;>)

 :)  1920x1080p60.  honestly though if it works at 1280x720p60 i'll be happy.

> What is the vertical distance from layer to layer in our board stack?

 it's a 6 layer 1.2mm PCB.  if i have actually set the design
parameters right (rather than just telling the factory manually) then
the substrates are 1.35mil and the dielectrics 10mil


> The idea is we can taper the keepouts on our signal vias near the A20
> by the layer and avoid such an abrupt change from layer 1 to layer 6.

 i would very much like to have used layer 3 instead of layer 6 for
the HDMI signals long straightaway but it is too late now

> Likewise, we can change the geometry of the keepout as we approach the
> ESD lands and finally the connector to likewise ease the transition.

 okaaaay i think i understand what you mean.

>> richard if you want to zoom in on those pictures you should be able to
>> click on them in a browser, then expand them: they're actually around
>> 2,500 pixels wide, i just asked them to be displayed in that HTML page
>> as only 1024 otherwise they wouldn't fit :)
>
> Thank you.  I am enjoying the views you posted.

 :)

>> you can see i removed the GND traces in between, and generally kept
>> everything except VIAs away from them.  it's not perfect but thanks to
>> your help i'm pretty happy with it.  if there's nothing major i want
>> to send this off.
>
> There is one place in layer 6 where the space between the CLK pair and
> the adjacent data pair looks like it exceeds 35mil for a non-trivial
> distance.  I think we could safely reintroduce a ground trace
> connecting the 2 or 3 vias in that space and thus keep the environment
> close to 15mil from differential trace to either ground or
> neighbouring signal.

 good call. i know exactly where you mean.  refresh URL and see new image.

  http://rhombus-tech.net/allwinner_a10/news/

 ok i did the taper at the DC3 connector end, and i think i got it
reasonably ok at the A20 end.  haven't run flood-fill.  A20 end is a
bit of a mess, bit unavoidable.  left side is ok.  right side...
because of the immediate turn and the TX2 line...

> I'm not sure which of the gray dots are vias and which are not.  Some
> of the vias might be able to sneak back into the ground-fill (out of
> the 15mil differential line clearance).

 they're all vias with the exception of the 2 we previously identified
as being "centre of component indicator".  which.... i don't believe
are actually in the images because i specifically selected "one and
only one layer" in each.

 you probably saw those dots because previous images included "all
layers".  the dots are on... some special layers, don't know which
ones.

ok.

so.  i really want to wrap this up, and get the gerbers out.

 loootta work... :)

l.