FRDM-K64F freedom develpment board as a guideline for a new PCB. With this design a 50MHz clock is generated by the RMII Ethernet PHY and routed to the EXTAL0 (pin 50) of the K64 cpu. Of interest, the FRDM-K64F Rev E3 schematics show the XTAL0 output (pi

[davidolsen]

We are using the design for the FRDM-K64F freedom development board as a guideline for a new PCB. With this design a 50MHz clock is generated by the RMII Ethernet PHY and routed to the EXTAL0 (pin 50) of the K64 cpu. Of interest, the FRDM-K64F Rev E3 schematics show the XTAL0 output (pin 51) connected to GND thru a 0-ohm resistor (R74) via XTAL_K64_MAIN. When using an external clock, normally you leave the XTAL output unconnected. Why is this output connected to ground?

Thanks

[kerryzhou]

Hi David OLSEN,

    When you use the external reference clock as the clock source, not the external crystal, then you won't use the OSC module, XTAL0 is none used pin, it is better to connect to the ground, just the recommend design.

   The clocking diagram can help you to understand it.

  

Wish it helps you!

If you still have question, please contact me!

Have a great day,

Jingjing

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