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."
Ed WalkerTexas Instruments Online Support - Systems & Tools
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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