描述

Virtex devices no longer have IFD and OFD components.

解决方案

Virtex family devices no longer use IFDs and OFDs to represent IOB FFs. Both UNISIMS and SIMPRIMS libraries do not have simulation models for IFDs and OFDs. Virtex IO FFs are essentially the same as FDCEs with clock enable and clear. There are two ways to utilize IOB FFs in Virtex: 

1. Use IOB=TRUE property in either the UCF file or the synthesis tools.  

UCF syntax: INST name IOB=TRUE; 

2. Use -pr {i | o | b} option in MAP, where i stands for input FF, o stands for  

output FF, and b stands for both input and output.

The schematic libraries still have IFDs and OFDs as valid components. The purpose  

of having the schematic version is for design migration capabilities. The IFD is made  

of an IBUF cell and a FD cell. The OFD is made of a FD cell and an OBUF cell.