If all of the load capacitors in the device are turned off, it reduces the capacitance load on the XIN Pin to 12 pF. In such a scenario, a clipped sinewave of +/- 1 Volt peak to peak on the input side or XIN side of an AC coupling capacitor connected back to the output of the TCXO is needed and can be used. The easiest recommendation is to simply purchase a low cost 3 by 5 mm TCXO and drive this into our part as the clock reference.
We have no specific TCXO recommendations however at this time but from an application point of view our clock generators and programming software are set up to handle a driven external reference.
For the external reference, as long as the input reference signal goes through [(VDD/2) +/- 1V)], it should be fine oscillator circuit is biased at VDD/2 and triggers on the rising edge of the input signal at VDD/2 level. So only going through VDD/2 should be sufficient, but the +/-1V requirement is to provide margin for operation across all conditions.