The D2000 data says the chip may be driven with an external clock but it does not give any voltage levels for this.
Is the oscillator input on the 1.8V or PVDD/IOVDD/AVDD domain?
It would be good to know if I should be feeding with 3v3 or 1.8V signal.
According to the datasheet of the D2000, HYB_XTALI is the Crystal/oscillator input for System Clock.
And according to table 3 of page 30, HYB_XTALI is powered with DVDD. DVDD uses 1.8V nominal and in deep sleep mode it can be configured to go to 1.35V. (Look at Table 1 page 22 for details)