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AR# 16416

6.1i Virtex-II MAP - "ERROR:Pack:679 - Unable to obey design constraints..."

Description

Keywords: ERROR:Pack:679, carry, F

Urgency: Standard

General Description:
A carry chain that is initiated with both MUXCY inputs grounded leads to pack errors similar to the following:

"ERROR:Pack:679 - Unable to obey design constraints
(MACRONAME=USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/hset, RLOC=X0Y-13), which require the combination of the following symbols into a single SLICE component:
LUT symbol "USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.0_U1" (Output Signal =
USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/Z0_L)
FLOP symbol "USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.0_U4" (Output Signal =
USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RESULT_POINTER<0>)
LUT symbol "USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.1_U1" (Output Signal =
USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/Z1_L)
MUXCY symbol "USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.1_U2" (Output Signal =
USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/CY<2>)
XORCY symbol "USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.1_U3" (Output Signal =
USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/Z1_S)
FLOP symbol "USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.1_U4" (Output Signal =
USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RESULT_POINTER<1>)
The function generator USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/L.L.0_U1 is unable to be placed in the F position because the output signal doesn't match other symbols' use of the F signal. The signal USER/G_NORMAL_G_NORMAL_PART/HOSTIO/RSLT/Z1_L already uses F. Please correct the design constraints accordingly."

(NOTE: This Answer Record is only a good match for your case if the pack error contains the phrase "The function generator ABC is unable to be placed in the F position because the output signal doesn't match other symbols' use of the F signal. The signal XYZ already uses F".)

Solution

This error occurs because the MUXCY is optimized to GND and uses a slice resource to source the GND that conflicts with a packing constraint.

You can work around the issue by applying a no-clip property to the output net of the first MUXCY in the carry chain. The following is a sample UCF constraint:

NET "net_name" S ;

This problem is fixed in the latest 6.1i Service Pack, available at:
http://support.xilinx.com/support/techsup/sw_updates
The first service pack containing the fix is 6.1i Service Pack 1.
AR# 16416
Date Created 12/19/2002
Last Updated 03/02/2006
Status Archive
Type General Article