How to Terminate Unused Pins of a Clock Buffer Device – KBA229131

Version 2

    Author: PradiptaB_11           Version: **

     

    Question:

    What is the best way to terminate unused pins for EMI are regarding the clock buffer device? Is it okay to let them float? How will it affect other outputs if unused pins are floating?

     

    Answer:

    The two areas of concern with unused outputs are EMI and Clock Skew. Modern packages, styles have geometrically small pins and a few unconnected outputs have no appreciable impact on EMI. When small amounts of EMI or slight variations in skew needs to be avoided, unused outputs should be addressed.

     

    A good practical approach to unused outputs is to place pads for either the capacitor or resistor termination in the design. Keep the trace lengths very short on the buffer output pins to prevent unwanted EMI. During the initial testing of the design, analyze the output skew of the clock buffer to ensure it is within tolerance. If excess skew is observed, populate the termination components. Caps can generally be 5 - 10 pF. The value of the resistor is chosen large enough as to not exceed the IOL of the driver.