Some problem about CPLD EPM570

Hello,

I have some problems about EPM570. What's the GCLK maximum value that could be accepted? I searched datasheet and found it can input higher than 100 MHz. But I input a 100 MHz XTAL and CPLD cannot output any signal.

I also input 100 MHz to normal I/O pin and it can't be detected. I am wondering about the maximum frequency that can be accepted? Or there the I/O impedance should also be take into consideration?

I searched internet and found most of people use 50 MHz to GCLK, I dont know why. Is there any other reference files I missed?

Thanks
jacklee