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I am new at ethernet and it comes complex. Trying to use STM32's RMII interface with switch. I just want to see at IT's switch gigabit port. I don't want to transfer data gigabit, it doesn't matter. Some gigabit switches doesn't see megabit ports. If i choose RMII, RGMII supporting PHY will switch or computer see as gigabit ethernet?

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  • \$\begingroup\$ Some gigabit switches doesn't see megabit ports: replace these switches. There's little excuse for that; there is valid reasons why your hardware needs to support Gigabit-only (namely, large frames can be crucial for some data streaming applications, even if the net rate stays significantly < 1Gb/s), but on the switch side, that's just strange. \$\endgroup\$ Sep 8, 2022 at 15:06

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If your MCU has no gigabit interface (GMII/RGMII) then you don't have gigabit interface to the PHY and then the PHY can't form a gigabit link either.

If you have only MII/RMII then only 100Mbps link can be formed with MCU.

But the picture in your question has a switch chip with two gigabit PHYs and separate MCU interface so the gigabit ports can form gigabit links even if RMII/MII port is used with MCU, because they are completely separate.

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The gigabit PHY would have to say it supports RMII in the datasheet. If it's not explicit you might ask the company but just because it supports RGMII does not mean it's backwards compatible.

I've also seen network devices support RGMII and RMII but not MII so it's important to check.

In the datasheet of the KSZ9563RNXC it says:

One port with 10/100/1000 Ethernet MAC with RMII/MII/RGMII interface.

So the STM32 could connect via RGMII and 10/100. It won't make sense for the STM32 to connect faster than that, because it can't handle the bandwidth of a 1000TX link.

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