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AR# 9597

3.1i Virtex MAP - XORCY with RLOC is optimized to LUT, which leads to a pack error.

Description

Keywords: ERROR:Pack:679, 679, pack, LUT, XORCY, RLOC

Urgency: Standard

General Description:
A case has been seen involving an RLOC'd XORCY with a GND'd CI pin. The 3.1i mapper converts the XORCY to a buffer; then, because of the RLOC constraint, it gets treated as a LUT. If there are already two LUTs RLOC'd to the slice, this will result in a pack error with the message "There are more than two function generators."

In the following pack error, "U1/U1/BU2" was originally an XORCY:

ERROR:Pack:679 - Unable to obey design constraints (MACRONAME = U1/U1/hset,
RLOC = R6C0.S1) which require the combination of the following symbols into a
single slice component:

LUT symbol "U1/U1/BU0" (Output Signal = U1/U1/N46)
LUT symbol "U1/U1/BU2" (Output Signal = P75BP<0>)
LUT symbol "U1/U1/BU3" (Output Signal = U1/U1/N66)
MUXCY symbol "U1/U1/BU4" (Output Signal = U1/U1/N82)
XORCY symbol "U1/U1/BU5" (Output Signal = P75BP<1>)
There are more than two function generators. Please correct the design
constraints accordingly.

NOTE: This issue affects the Virtex, Virtex-E and Spartan-II device architectures.

Solution

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 8.

NOTE: This problem is very similar to the one discussed in (Xilinx Answer #9973), which was fixed in SP4. This new fix deals with a different configuration that was not covered by the previous fix.

Meanwhile, a work-around is to override the problem XORCY RLOC with the following UCF attribute:

INST U1/U1/BU2 USE_RLOC=FALSE;

Ideally, the RLOC should only be overridden on the "LUT" symbols listed in the error that are not actually LUTs; however, you can also simply override all LUT symbols listed, and then rely on the packer to pull the LUTs into the correct slice.
AR# 9597
Date Created 06/28/2000
Last Updated 08/19/2002
Status Archive
Type General Article