Can the MDIO interface be run at 4 MHz?

[joachimwoelk]

Hi,

we plan to use a Kinetis K60 (a MK60FN1M0VMD12, to be precise). It has an Ethernet interface, of which I like to use the MDIO interface only. But I need to run it at frequencies up to 4 MHz, as specifed for CFP MSA modules.

The K60 reference manual K60P144M150SF3RM.pdf only states "The MII_SPEED field must be programmed with a value to provide an MDC frequency of less than or equal to 2.5 MHz to be compliant with the IEEE 802.3 MII specification". The datasheet K60P144M150SF3.pdf does not mention any timing of the MDIO interface.

I presume that the HW would allow higher frequencies, but I don't find anything the maximum MDC clock rate in case IEEE compliance is not required.

Kind Regards

Joachim

[mjbcswitzerland]

Joachim

The MII speed is set by {Ethernet clock speed / (2 * (MSCR + 1))} and so can be set up to 25MHz in RMII mode.
Although the IEEE spec. for the connection limits to 2.5MHz there are various PHYs that specify higher speeds if desired.

Possibly the MDIO is not specified further by NXP because it assumes the 2.5MHz but you can measure to see that this is a long way from the limit of the silicon/interface.

Beware that the data line may benefit from a strong pull-up (eg. 1k) since it is a bi-directional line and the edges may be rounded without this at higher speeds, especially if there are long lines with capacitive loading.

Regards

Mark
Kinetis for professionals: http://www.utasker.com/kinetis.html
http://www.utasker.com/kinetis/TWR-K60N512.html
http://www.utasker.com/kinetis/TWR-K60D100M.html
http://www.utasker.com/kinetis/TWR-K60F120M.html
http://www.utasker.com/kinetis/ELZET80_NET-KBED.html
http://www.utasker.com/kinetis/ELZET80_NET-K60.html