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AR# 34227

11.x XST - Control signals count can be much larger in XST than in MAP


Some users of XST have reported that the number of control signals reported in XST can be much larger than the number of control signals reported in MAP.  This article explains why these numbers may differ by a large factor.


XST counts the number of different class of FFs that have identical control set. A control set is defined by a set of clock signal, Clock enable, if any, and a set/reset control signal, if any.

XST differs from MAP in that it counts these sets independently on each hierarchical block. That means that if two FF have the same clock/enable/set signal, but are located in two different hierarchical block, then XST will count two different control sets while map will count one.

The main issue here is that XST does not have a forward looking method to determine if these two flops will really be merge in the same slice since there could be different AREA_GROUP on those two hierarchical blocks.

As such, it is entirely possible to have a much larger number of control sets reported in XST than in MAP.

For a discussion on control signals in MAP, refer to (Xilinx Answer 24667).

Linked Answer Records

Associated Answer Records

Answer Number Answer Title Version Found Version Resolved
24667 ISE 14.7 Virtex-5 MAP - What are "unique control sets" in the Design Summary section of the MAP report? N/A N/A
AR# 34227
Date 05/23/2014
Status Archive
Type General Article
  • ISE Design Suite - 11.1
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