This application note is intended for use with all SMSC LAN products incorporating a parallel crystal circuit. The information contained in this application note provides one option to control the voltage across the input of the crystal circuitry. Link here
The designer should incorporate the series resistor shown below in his design to guarantee that the specified device maximum input voltage levels are not exceeded. It is the responsibility of the designer to ensure that all input voltages in his entire design do not exceed the recommended voltage levels in the applicable data sheet. It is strongly recommended to utilize all design suggestions from SMSC and then verify the operation of the circuit in a lab environment. Verification includes measuring all input waveforms with an oscilloscope to ensure proper voltage levels.