For test my fpga clock, I input a clock throuth one of the dedicated clock pad, and output it from a general I/O. From the oscillograph, I found some clock pads can give the correct signal, and the same clock input the other clock pads only have the '1' output.
p.s. I use the same vhdl code as follows: clkin-->ibufg-->clkout. Just differences in assign clock pins.
Any comments would be appreciated! thank you in advance!