Connect the MMCM feedback loop to compensate for a board
delay instead of an internal clock insertion delay. Specify the delay outside the device
for both best and worst delay cases 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. When using external compensation, adjust the input and output
delay constraint values accordingly. Do not only consider the clock trace delay on the
board as in normal cases.