AR# 6887

Description

General Description:  

What does "B:" or "S:" represent in the following lines from a UCF file?  

NET "input1" LOC = "S: PIN3";  

NET "input2" LOC = "B: PIN4"; 

This was generated by the Xilinx design tools when I performed a "Lock Device Pins" process. 

The usual syntax is: 

NET "input1" LOC = P3; 

I cannot find a reference to "S: PINnn" or "B: PINnn" (where nn stands for number) in the Libraries Guide.

Solution

These two options are generated by Pin2UCF for Xilinx CPLD families only. 

"S:" is used before the pin locks to ensure that the CPLD fitter optimizes feedback the same way each time, and provides consistent timing results after subsequent re-fits.  

"B:" is used before the pin locks to indicate that this signal is buffered and should remain so to ensure successful fitting for subsequent implementations.