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

[Richard Wilbur]

On Sep 20, 2017, at 13:27, Richard Wilbur <richard.wilbur at gmail.com> wrote:
> On Wed, Sep 20, 2017 at 1:57 AM, Luke Kenneth Casson Leighton
> <lkcl at lkcl.net> wrote:
>> On Tue, Sep 19, 2017 at 11:26 PM, Richard Wilbur
>> <richard.wilbur at gmail.com> wrote:
>>> 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'm not looking to provide any special connection to the power or
> ground pins.  I just want to make sure we don't obstruct the return
> current path any more than necessary on its way from bottom reference
> ground plane (layer 5) to top reference ground plane (layer 2) to the
> power supply pins of the differential drivers:
> 1.  ground plane (layer 2) via to SoC ground pin land (layer 1)
> 2.  ground plane (layer 2) via to power supply decoupling capacitor
> ground land (layer 1), through decoupling capacitor to land on power
> supply trace (layer 1), through trace to SoC power supply pin land
> (layer 1).
>
> The goal is to avoid unnecessarily impeding this return current path.
> I'm trying to avoid making the path >~200mil and putting any major
> obstruction (like a huge layer void) in the way.

While browsing the A20 datasheet, I found that the HDMI section does
have a power pin but not labelled the way I was asking you about.
Page 18 mentions that VCC-HDMI is a power pin on ball #T13.  On the
schematic it is connected to net HVP which has power decoupling
capacitor C108 which looks like "104" => 0.1uF.

Looks like the path from the copper on layer 2 below the HDMI pins
(balls #TUVW 22,23) on the SoC (processor) to the VCC-HDMI power pin
(#T13) is ~300mil.  It doesn't look overly impeded.