Drive resistor value for crystal in USB hub
http://ww1.microchip.com/downloads/en/Appnotes/00849a.pdf


  RS is a series resistor that is selected to prevent overdriving the crystal. It is not often needed if gain (Clock mode), C1 and C2 are selected properly. If the circuit is being overdriven and a lower gain Clock mode cannot be selected, then adding impedance with RS can decrease gain.
  This overdrive can be visually demonstrated by looking at the Osc-Out pin, which is the driven pin, with an oscilloscope. Connecting the probe to the Osc-In pin will load the pin too much and negatively affect performance.
  The output signal should not be clipped or squashed. Overdriving the crystal can also lead to the circuit jumping to a higher harmonic.