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AR# 9973

3.1i Virtex MAP - MAP incorrectly optimizes MUXCYs and XORCYs into LUTs, leading to pack errors.


Keywords: 3.1i MAP, pack:679, Virtex, LUT

Urgency: Standard

General Description:
Cases have been seen where RLOC'dMUXCYs and XORCYs are being reduced to
buffers due to mapping optimization, but are then being incorrectly treated as
RLOC'd LUT components.

This results in packing errors where MAP complains that "there are more than two
function generators" due to conflicts with the real LUTs. The following error appears:

ERROR:Pack:679 - Unable to obey design constraints (MACRONAME =
RLOC = R33C22.S0) which require the combination of the following symbols
into a single slice component:LUT symbol
(Output Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N117890)
LUT symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU
(Output Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N117945)
FLOP symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU5982"
(Output Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N23740)
LUT symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU5983"
(Output Signal =
LUT symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU5984"
(Output Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N117889)
LUT symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU5985"
(Output Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N117944)
FLOP symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU5986" (Output
Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N23739)
LUT symbol "core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/BU5979"
(Output Signal = core_notri/proc_notri/c_norm_pipeline_c_div_result_scale/N118253)
There are more than two function generators. Please correct the design constraints



This problem is fixed in the latest 3.1i Service Pack available at:
http://support.xilinx.com/support/techsup/sw_updates. The first
service pack containing the fix is 3.1i Service Pack 4.

NOTE: Please see (Xilinx Solution 9597) for a similar case that is to be fixed
in Service Pack 8.


A workaround is to override the XORCY RLOC with the
following UCF attribute:


Ideally, the RLOC should only be overridden for the symbols that
are not really LUT symbols. In some cases, it is too difficult to
do this. An alternative is to override the RLOC for every LUT
symbol mentioned in the error.
AR# 9973
Date Created 08/22/2000
Last Updated 08/19/2002
Status Archive
Type General Article