Oscillator and output load

ethernetloadmicrocontrolleroscillator

For the Ethernet application in my design using RMII interface, I need to drive two CMOS loads (two reference clock pins, one of a Freescale Kinetis K60 Microcontroller and one of a TI's PHY chip) through a 50MHz oscillator. The oscillator specifications mentions the "output load" parameter as 15pF. As per the datasheet of microcontroller and PHY chip, the input capacitance of pins is given in general as 7pF.

Will it be ok if I drive these two pins through only one oscillator output?

The reason for using single oscillator is that it will clock both the micoronctroller and the PHY reference clocks together almost skew-less, and it will also save some cost by avoiding the use of an external clock buffer.

Any help would be highly appreciable.

Best Answer

There are at least two popular solutions:

  • two RMII clock slaves (both PHY and MAC)

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  • one RMII clock master (PHY) and one RMII clock slave (MAC)

    enter image description here

Other variants are possible. For example, if your MCU could be clocked by a 50 MHz source, you can use a variant with the MAC as RMII clock master and the PHY as RMII clock slave.

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