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

[Luke Kenneth Casson Leighton]

On Tue, Aug 15, 2017 at 12:08 AM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:

> 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.

 hey this is all extremely worthwhile.  it's me who is barely able to
follow along.

> 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.

 ah ha!!  that would be better, it's quite a mess to be honest.

> 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.

 .... i'm translating this to mean "lose the large middle set
 of wiggles on TX0, TX1 and TX2".  they're bugging me anyway ("beauty" criteria)

 plus, we know that the very first design.. i should open that up
shouldn't i... never had large wiggles and it worked fine.  looking at
it now, the guy who designed it had all the vias coming out from the
CPU in a straight line, no diff-pair via considerations *at all*, ran
the CK lines right past *all* those vias, but, butbutbut, he put CK on
layer 6, TX0-2 on layer 3

 i'm amazed it worked.

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

 blech :)

l.