Hello Gary,
Please take a look at section 5.4 Clock injection from the QN908x Hardware Design Considerations document.
There's an update pending for the app note though if using a square wave input. A 1nF capacitor should be added in series with the XTAL_IN input. This is to reduce the current draw that might otherwise occur.
ANA_CTRL0[XTAL_MODE] must be set to the correct value as well for the clock input type.
Regards,
Gerardo