H7-ETH-USB bridge dilemma - electrical connections

[LCE]

Hello circuit designers and PCB guys,

I have a problem of how to connect the H7 ethernet via a PHY and a ethernet USB bridge.

• to the outside world:
  • ethernet connection via RJ45 to host
  • USB connection to host (e.g. laptop without ethernet)
• I must NOT use the H7's USB interface, another USB driver is out of the question (says the SW department)
• with a ethernet-USB bridge the H7-device looks like any other ethernet device, no USB driver needed

So I tested Microchip's LAN9512 ethernet-USB bridge on their dev board, that works great, see attached picture.

Problem:

I have to place the LAN9512 on the same PCB, and I must get rid of that extra RJ45/magnetics + cable (the red parts in the attached pic).

I must use as few components as possible, but how should I wire these parts without making it too dirty (it must work reliably and has to pass a EMI test someday...).

• Possibly the cleanest solution might be using a multi-port PHY like the LAN9303 (anybody used that?), and switching ports when USB is attached.

• The not so clean version would be to place the USB's ethernet transformer very close to the regular ethernet transformer at the RJ45 (see green line with the "?"). But as we're having 125 MHz signals there and driving 2 transformers all of a sudden, this sounds like a stupid idea...

Any thoughts on that or other ideas?

Or experience with multiport PHYs like the LAN9303?

[Bob S]

Hmmm, maybe I mis-understood your requirement. Do you need to keep the LAN8742->Magnetics->RJ45 path, and ADD the ability to connect LAN8742->LAN9512?

If so, can BOTH interfaces be active at the same time? That is not allowed without putting a hub/switch on the board (i.e. LAN9303 as you suggest).

[LCE]

The 2 interfaces would not be active at the same time.

Imagine a PoE supplied ethernet device (main use case, USB inactive), which can also be used with an ethernet-less laptop via USB.

So as soon as someone plugs in USB, the main ethernet path will be deactivated.

So, the only "clean" solution would be a dual PHY like LAN9303, no matter how I turn things around.

But your advice concerning the capacitive coupling, then between one port of the LAN9303 and the LAN9512, would be used.