Question: The EZ-USB® family of chips (FX1™/FX2™/FX2LP™) specifies a 24-MHz crystal drive level of 500 μW. What happens when a lesser drive level crystal (say 100 μW) is used?
The drive level is the maximum power you will drive into the crystal when you run the part during worst case conditions. That means 500 μW might be driven into the crystal at times. If the crystal is rated for less than 500 μW, the maximum rating on the crystal could be exceeded. This could result in faster aging of the crystal, putting it out of tolerance, or even in burn out of the crystal. Therefore, it is recommended that you use a crystal with a drive level equal to or greater than the one specified in the datasheet.