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AR# 1723

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

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.

解决方案

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.
AR# 1723
创建日期 08/31/2007
Last Updated 10/01/2008
状态 Archive
Type 综合文章