On Sep 20, 2017, at 13:27, Richard Wilbur richard.wilbur@gmail.com wrote:
On Wed, Sep 20, 2017 at 1:57 AM, Luke Kenneth Casson Leighton lkcl@lkcl.net wrote:
On Tue, Sep 19, 2017 at 11:26 PM, Richard Wilbur richard.wilbur@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:
- ground plane (layer 2) via to SoC ground pin land (layer 1)
- 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.