Ethernet Clocking Options on K60_120

[pmt]

Greetings,

I'm trying to figure out my clocking options for the K60_120 and the documentation is somewhat involved.  My application requires an external clock source of which I would like to use a 10, 20, or 40MHz clock.  What are my options to generate a 25MHz Ethernet clock (MII mode)?  What about USB clock?  Can both of these be generated by the MCG, and can I get the 25MHz out to the PHY (what pin on the 144-pin part can I use for CLKOUT)?  Does this place any limit on my core clock frequencies?  

Does Freescale plan some sort of clock planner application to help sort out the clocking options?

[mjbcswitzerland]

Hi

See https://community.freescale.com/thread/110760

The Freescale kits use the Ethernet in RMII mode and an external 50MHz signal since the PHY and CPU need to be synchronised in this mode. If you are using 25MHz I assume that you will be connecting in MII mode instead. The FB_CLKOUT would be suitable as a clock source. Since you will need exactly 25MHz to drive the external PHY with you will be restricting the FlexBus speed (in case you use it for something else too) and possibly also restricting the CPU speed in some situations:

eg. If the CPU clock is set to 100MHz the FlexBus clock can be set to 1/4 this (but 120MHz CPU is not possible).

Free running timer outputs set 25Mhz would presumably be equivalent too.

The K60F120 has 2 crystal circuits and the FB_CLKOUT could be set to a dedicated 25MHz oscillator, although this would probably not have any advantages over using the PHY's 25MHz clock instead.

Regards

Mark