Electronic – Drive resistor value for crystal in USB hub

crystaloscillatorusbusb hub

I am designing a one-up two-down USB hub based on a TUSB2036 USB FS hub. I am going to use a 6MHz crystal as recommended in their datasheet: Reccomended configuration for crystal
The usage of the resistor doesn't really make sense to me with the explanation that they give. There is also a FAQ app note on this chip from TI. They say:

FAQ of crystal and Rd selection

This explanation makes much more sense to me; the resistor is used to make sure that the crystal isn't driven with too much current. They have an example schematic that uses a 2k2 resistor value, but they also recommend a 1k5…

They explain how to use the potentiometer to "tune" the value of the resistor using a potentiometer, but they don't say how you are able to see if it is "overdriven".
So, am I safe to use a resistor of around 1k5 – 2k2? How am I to know if the crystal is being overdriven?

I assume I check the "XTAL2" node with a scope and confirm that it is oscillating at 6MHz, but I'm not positive.

Best Answer

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.

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