I am new with FPGA and timing constraints, so I apologize if the question will sound stupid. I try to understand timing constraints usage with FPGA. I have found the following description (in https://fpgawiki.intel.com/wiki/Timing_Constraints):
In a synchronous design you need to define the clocks used in the design. There are three types of clocks you can define:
Clocks at FPGA clock input PIN
Virtual clocks which are used to describe the clocking of FPGA external registers (external device registers)
Generated clocks which are derived other clocks
The command for creating clocks at FPGA input pins and virtual clocks is the same. The only difference is that you don't specify a FPGA input pin for virtual clocks. Because of this they are both listed under create_clocks.
I would like to ask:
Is it that "virtual clocks" are not really used in design, but just used for the timing analysis ?
I don't understand what it means to put a timing constrain on a real external pin clock. Is it that the timing constraint will change the real clock (I can't believe that it is possible without using a PLL) ?