When the MMCM or PLL 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 or PLL 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.