The following types of I/O buffers are available for the PCI LogiCORE:
What are the purposes of these three different buffer types?
The IOBUF_PCI33_5 buffer is for 33 MHz 5V PCI designs.
The IOBUF_PCI66_3 and IOBUF_PCI33_3 buffers are for 3.3V 66 MHz and 33 MHz PCI designs.
The 66 MHz buffer (IOBUF_PCI66_3) is more sensitive to noise on the input side and is faster on the output side, which can cause potential SSO concerns. However, the buffer must have these characteristics to be PCI-compliant.
The 33 MHz buffer (IOBUF_PCI33_3) has relaxed characteristics that will not work at 66 MHz, but allows for more gentle output transitions. This minimizes injected noise (e.g., for SSO concerns).
The the I/O should not be any faster than it needs to be for compliance. It is completely acceptable to use a IOBUF_PCI66_3 in a 33 MHz 3.3V design.