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

[Richard Wilbur]

On Aug 10, 2017, at 23:12, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
> 
> On Fri, Aug 11, 2017 at 12:37 AM, Richard Wilbur
> <richard.wilbur at gmail.com> wrote:
> 
>> On Wed, Aug 9, 2017 at 7:23 AM, Luke Kenneth Casson Leighton
>> <lkcl at lkcl.net> wrote:
>>> next set...
>>> 
>>> wiggles.jpg is the layer 6 length-matching area: HX2N/P is the one
>>> that's the longest, it snakes back on itself.  i length-matched all 3
>>> signal pairs to 56.413, and left the CK lines at 57.134 just to give
>>> the tiniest bit of delay (TI recommendations iirc).
>> 
>> Very nicely done!  57.134mm - 56.413mm = 721um
>> => T(delay) = 721um / 150um/ps = 4.8ps
> 
>> That is a very tiny delay!

I would need to do more research to make a meaningful recommendation.  Sorry for bringing up a topic I wasn't prepared to discuss intelligently.  Let's go with what you've done.

According to my calculations you could get away without any inter-pair skew compensation on the board whatsoever and still meet the HDMI specification for the transmitter budget.  What you have done regarding inter-pair skew compensation reserves nearly all of the transmitter inter-pair skew budget from the HDMI standard for the connector and the rest of the system.  This will serve to accommodate less than optimal inter-pair skew imposed by the cable and/or receiver.

>> Now that we have achieved such close
>> synchronization, I'm suggesting we go for the next goal where we
>> design a certain amount of inter-pair skew into the layout for
>> purposes of lowering the strength of our synchronized pulsing data
>> lines to a more diffuse chatter.
> 
> *deep breath*.... aaaaaaaa! :)
> 
> well.... that actually happens for the majority of the length in the
> middle (starting layer 6)
> 
> but.... if i simply *take out* the intermediary wiggles on layer 6....

Ill-founded proposal for which I don't presently have the time to improve.

>>> no - not even enough space to do 5.1mil / 5.0 clearance... just...  too much.
>> 
>> I understand.  We might end up with more room--see discussion below.
> 
> which has probably been truncated...

Turns out we don't have the room to change the trace width or spacing without having a deleterious effect on impedance.