I'm trying to understand the Ethernet Switch product.

On page 54,55, we can see that, the MAC signals, Like for MII and RMII interface, we can understand that the digital MAC port 3, can be configured as either in MAC mode or PHY mode.

But for RGMII interface, there are no 2 separate tables where we can configure as either in MAC or PHY mode?

Why is that?

  • \$\begingroup\$ Does it matter why? \$\endgroup\$
    – user253751
    Apr 25, 2022 at 12:16
  • \$\begingroup\$ I'm unable to understand. Like, for MII and RMII interfaces, the switch has the ability to act in either the PHY mode or MAC mode, but for RGMII interface, why does the switch not have the ability to be in either PHY mode or MAC mode? \$\endgroup\$
    – user220456
    Apr 25, 2022 at 12:17
  • \$\begingroup\$ We can see 2 tables for each interface. Table 4-27 & 4-28 which tells that for the MII interface, the port 3 can be configured as either a PHY or a MAC. Table 4-30 & Table 4-31 tells that for the RMII interface, the port 3 can be configured as either a PHY or a MAC. But we don't see any such tables for RGMII interface. \$\endgroup\$
    – user220456
    Apr 25, 2022 at 12:20

1 Answer 1


The answer is on page 44:


The MII interface is clocked asymmetrically, with the PHY device driving both the RX_CLKx receive clock and the TX_- CLKx transmit clock to the MAC device (...)

The RMII interface uses a single 50MHz clock. This REFCLK may be sourced either from the KSZ9893R or from the connected device (...)

The RGMII interface employs source synchronous clocking, so it is symmetrical and does not require a mode selection. An output clock is generated on the RX_CLK pin, while an input clock is received on the TX_CLK pin (...)

It's all related to who provides the clock(s). In MII or RMII mode one of the two devices provides the single clock used for both TX and RX, so you need to pick which one. In RGMII there's one clock for each direction, so you don't have to device who is providing the clock.

  • \$\begingroup\$ Thank you for the answer. Could you just expand a little more on what is the meaning of asymmetrical clocking and symmetrical clocking? \$\endgroup\$
    – user220456
    Apr 25, 2022 at 12:41
  • 1
    \$\begingroup\$ MII: PHY sends clocks, so you have to device who is PHY. RMII. One of the two sends single clock, so you have to device who. RGMII: each device sends the clock matching the data signals. So you can swap them around as much as you want. \$\endgroup\$
    – jcaron
    Apr 25, 2022 at 12:45
  • \$\begingroup\$ Thank you for the answer. In that case, I've connected 2 switches in back to back fashion. The port 3 of switch 1, is connected to port 3 or switch 2. In that case, which switch will take as PHY mode and which will be MAC mode? \$\endgroup\$
    – user220456
    Apr 25, 2022 at 12:50
  • \$\begingroup\$ Neither, there's no difference between PHY and MAC modes when using RGMII, the interface is symmetrical. \$\endgroup\$
    – jcaron
    Apr 25, 2022 at 12:53
  • \$\begingroup\$ No, I'm asking in case of MII and RMII interface, which would be PHY and which would be MAC? \$\endgroup\$
    – user220456
    Apr 25, 2022 at 13:04

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