The datasheet for the LPC55S6x states that an external clock can be used on the XTAL32M_P pin if the clock is a square wave from 0V to 850mV. Can someone elaborate on this? What is the tolerance of "0" and "850mV"? Is a DC offset acceptable as long as the amplitude is 850mVpp? Is 850mV the absolute maximum for the the pin?
Previous LPC parts allowed an external oscillator to be capacitor divided and driven into the XTALIN pin, but that process will not guarantee a 0-850mV signal from a 3.3V oscillator (there will be some DC offset). Without a little more information (or a recommended circuit) it is near impossible to design a robust system.