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?