Answers Database

XC5200: Use of DI pin to a flipflop in a Logic Cell (LC)


Record #1723

Problem Title:
XC5200: Use of DI pin to a flipflop in a Logic Cell (LC)


Problem Description:
Urgency: Standard

Keywords: BYPASS, XC5200, SHIFT, RLOC, FFD,

Description

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.


Solution 1:

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.



End of Record #1723