When placing a clock on a N side of the GCLK pin pair in a Spartan 6 FPGA, PlanAhead issues the following DRC error:
I/Os placed on disallowed sites -
Single ended global clock terminal mii_tx_clk drives a global clock buffer. For proper functioning, this terminal needs to be placed on the P side of a differential package pin. This terminal, placed at C9 violates this requirement