On Sep 20, 2017, at 13:27, Richard Wilbur <[email protected]> wrote: > On Wed, Sep 20, 2017 at 1:57 AM, Luke Kenneth Casson Leighton > <[email protected]> wrote: >> On Tue, Sep 19, 2017 at 11:26 PM, Richard Wilbur >> <[email protected]> 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. _______________________________________________ arm-netbook mailing list [email protected] http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook Send large attachments to [email protected]
