RMII Interface and Reset of KSZ8041

Hi everyone,
actually we encountered some interesting question.

We connected an external Switch KSZ8873 via the RMII Interface which has actuall per default two PHY Adresses (0x01 and 0x02).

Unfortunately the KSZ8041 - PHY on the SOM has the same PHY address strapped (0x01). Because out of different reasons we want to use the external switch, and not just 2 simple ethernet ports switched in software.
We are not sure if the imx6ull has the possibility to hold the KSZ8041 PHY in reset for all the time. This would solve the problem immediately.

The second solution would be to access Global Configuration register of KSZ8873, and set the PHY addresses to 0x02 and 0x03. But how to do this exactly, is not really well documented in the microchip datasheets. I suppose that it might be possible to change by software, but it will require taking a deep look into the driver and trying to figure out how to set this regiseters.

In any case if it is possible to hold the KSZ8041 in reset forever?, this would immediately help and does not cause any further problems by using the external KSZ8873 via RMII for our application.

Thank you very much,
BR Christian

Hi @ChristianM ,

Default PHY address for KSZ8041 set by the manufacturer (Microchip) is 00001 as mentioned in the datasheet, page 14. But on Colibri IMX6ULL module the PHY address has been changed to 00010 using strapping pin 15 (PHYAD2), pin 14 (PHYAD1) and pin 13 (PHYAD0).

Am afraid it would not be possible to keep the ETH PHY in reset state using RST# signal (pin 32), as the reset signal is shared with other interfaces on the Colibri module.


Dear @satyan.tx,
you are right it was a typo (meant 0x02), in any case those PHY addresses are interfering.
Actually we are considering a modification of the actual SOM by isolating the reset pin of KSZ8041 on the SOM and pulling it permanently down manually.
BR Christian

Hi @ChristianM ,

in this document “Management Interfaces for Micrel Switches” http://ww1.microchip.com/downloads/en/AppNotes/ANLAN207-UNG.pdf
on section “Port Numbering”, you can find the registers you need to change on your switch KSZ 8873 for a new PHY address.alt text

There is a mdio command in uboot. mdio.c « cmd - u-boot-toradex.git - U-Boot bootloader for Apalis and Colibri modules

BR Andrija