AR# 41013


13.1 XST - DSP48E not inferred correctly when using MAC with subtraction


This situation involves a MAC thatis configured to do a multiplication operation followed by a subtraction operation. It is definitely possible to performall of this calculationusing aDSP48E slicefor Virtex-5 FPGA. However, it has been found that XST is not inferring the DSP48E correctly. It is setting up the CARRYIN incorrectly that leads to the result of the calculation being out by 1.

For example,263 * 2 = 526 - 24 = 502

Post XST netlist calculates to 501


This is a known issue and it has been decided not to fix this problemsince a workaround exists which would be more ideal.

The workaround at this time is to use the new XST parser.

In order to use the new XST parser, add the following switch in the XST script:
-use_new_parser yes

The same switch can alsobe addedto the Other XST Command Line Options field present in the XST Synthesis Options Category within the ISE GUI.

AR# 41013
Date 12/15/2012
Status Active
Type General Article
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