ERRORlace:17 - The placement constraints of the IOBs P3D<7> and IDED<3> makes
   this design unroutable due to a physical routing limitation.  This device has
   a shared routing resource connecting the ICLK and OTCLK pins on pairs of
   IOBs.  This restriction means that these pairs of pins must be driven by the
   same signal or one of the signals will be unroutable.  Before continuing
   please remove the placement constraints or move one of these IOBs to a new
   location.

请问哪位大哥知道为什么？谢谢！