For the 48MHz clock input X1 the datasheets say it is a CMOS level. However it does not say which xVDD source the CMOS level is referenced to.
In the cases where DVDD/AVDD and IOVDD are different levels, and XI is an input, which VDD is the CMOS level referred to?
"It is an analog I/O when used as a crystal input. Is it the same when used as a digital external clock input? Or does it refer to IOVDD in that case? In our application IOVDD=1.8V as needed by logic which interfaces to it. AVDD and DVDD can be 2.5V to 3.3V, controlled by an external CPU."
The preferred way (injects the least amount of noise) to input a clock is to have a 600-1000 mVpp sine input AC-coupled through an external capacitor to XI. However a full swing CMOS input (between GND and AVDD) connected directly to XI also works, although the signal must come from a strong driver as it has to override the internally generated bias voltage on XI. You can of course also input a square wave signal with less than full swing (but more than 600 mVpp) but then again, it has to be AC-coupled into XI.
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