AR# 1723: XC5200: Use of DI pin to a flipflop in a Logic Cell (LC)
XC5200: Use of DI pin to a flipflop in a Logic Cell (LC)
Keywords: BYPASS, XC5200, SHIFT, RLOC, FFD,
The DI pin on a Logic-Cell offers an efficient bypass for a signal to connect to the D input of a Flipflop without wasting the function generator for feeding through. The benefit of this feature gets important when a design contains large shift registers, Johnson counters or LFSR structures. The DI pin is used whenever possible by ppr automatically. In some designs it may look like the software does not use the DI although it appears that it should use it.
Check which Flipflop elements you are using to build the function (for example the shift register). Macros like FDR have underlying levels of logic and you can step down in hierarchy. This logic prevents the software to use the DI pins. Use FDCE as a flipflop primitive to build the register and ppr will use the direct input DI.