When the MMCM feedback loop is connected for compensating
a board delay instead of an internal clock insertion delay, you must specify the delay
outside the device for both best and worst delay cases by using the set_external_delay command. Failure to specify this delay
makes I/O timing analysis associated with the MMCM irrelevant and can potentially lead
to an impossible timing closure situation. Also, when using external compensation, you
must adjust the input and output delay constraint values accordingly instead of just
considering the clock trace delay on the board like in normal cases.