The PCI™ MegaCore® implements a multiplexed high-width address and data (ad) bus. Simultaneous switching noise (SSN) can occur if the bus changes state from 0xFFFF_FFFF to 0x0000_0000 or 0x0000_0000 to 0xFFFF_FFFF. You should assign the pins for address and data (ad) bus farther away from the analog power supply pins (VCCA and VCCA_PLL) to avoid any probable instability caused by simultaneous switching noise.
Altera® recommends using the recommended PCI pin locations specified in the PCI constraint file (if applicable). By default, the add_pci_constraints command makes the Altera-recommended PCI pin location assignments.
Refer to the PCI Compiler User Guide for more information.