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I need to develop a custom board that integrates both a system on module (e.g. raspberry CM4 or a jetson nano) and a network switch. I found some network switch IC from microchip (e.g. KSZ8795) that support a variable number of ports and the application notes / schematics of the eval boards shows how to make an independent network switch. Can I simply connect the pins of the SoM related to the ethernet direclty to one port of the network switch IC? Because usually they are connected to ethernet magnetics. If it is not possible how can I connect them together on the same PCB?

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You can use capacitive coupling. Take a look at Micrel Application Note 120, the exact circuit depends on which switch you are using, but typically look something like this:

enter image description here

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  • \$\begingroup\$ It is not so sure if you can, because it depends. Capacitive coupling can be used only if both PHYs are compatible with it. I have used a a chip with a PHY which did support capacitive coupling, but the output driver mode had to be configured over the MDIO registers in a non-default mode to be able to support capacitive coupling. It wasn't directly selecting between current mode output or voltage mode output, but something along the lines how the output drivers drive the output. Some PHYs therefore are not guaranteed to support it. \$\endgroup\$
    – Justme
    Nov 30, 2023 at 18:45
  • \$\begingroup\$ Gathering info about the PHY that use my SoM is not easy. And it looks like that capacitive coupling depends by the PHY. To my understanding the best solution would be to wire the (R)MII pins of the SoM to the (R)MII pins of the ethernet switch (but unfortunately I can't). As a general solution (even if more expensive), what do you think about using two magnetics chip wired together on the same board? The connection would be: SoM CPU -- SoM PHY --- magnetics 1 -- magnetics 2 -- ethernet switch \$\endgroup\$
    – Damien
    Dec 1, 2023 at 8:12
  • \$\begingroup\$ The CM4 uses a Broadcom BCM54210 PHY and data on this PHY is hard to come by. Any solution (magnetic or capacitive) will be somewhat specific to the PHY on the SOM and the switch. You will have to understand the biasing requirements for both and design accordingly. Since you are looking at a 10/100 switch it shouldn't be too difficult to get something working. Also - layout will be important, even at 100Mbps. \$\endgroup\$ Dec 2, 2023 at 9:02

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