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AR# 21252

7.1i XST - Pipelined MULT_STYLE attribute does not work when use_dsp48 is set to no

Description

Keywords: 1947, hdl, advisor

I want to use a pipelined, slice-based multiplier scheme in my design. This is typically done by setting the MULT_STYLE attribute to "pipe_lut" and setting the use_DSP48 attribute to "no" (as specified in the User Guide). However, when I review my XST report, there are specific warnings that indicate the attributes were not picked up. The message is similar to the following:

"INFO:Xst:1947 - HDL ADVISOR - Multiplier <_n0059> was identified in your design with 4 potential pipelined stages on the output. You might improve the performance by using the pipeline feature available with mult_style attribute. Found registered multiplier on the signal <_n0058> with 2 register level(s)."

How can I get XST to recognize the attributes?

Solution

Under normal operation, obtaining a slice-based pipelined multiplier is done by setting the use_DSP48 attribute to "no" and the MULT_STYLE attribute to "pipe_lut".

To work around this issue (allowing XST to pipeline the slice-based multipliers), set the use_DSP48 attribute to "auto" and the MULT_STYLE attribute to "pipe_lut".

Normally, the MULT_STYLE attribute is ignored when the use_dsp48 attribute is set to auto.

This issue is fixed in ISE 8.1i.
AR# 21252
Date Created 09/04/2007
Last Updated 01/07/2009
Status Archive
Type General Article