Error in implementation on using LOCK_PINS constraint

[ayan_xilinx]

I am implementing a design in verilog using Xilinx ISE 14.7 using primitive instantiation based approach. An example of instantiating a LUT is as follows:

LUT6_2 #(
.INIT(64'h0000CC5A000000AA) // Specify LUT Contents
) LUT6_2_inst0 (
.O6(sel[0]), // 1-bit LUT6 output
.O5(d0[0]), // 1-bit lower LUT5 output
.I0(ud), // 1-bit LUT input
.I1(ext[0]), // 1-bit LUT input
.I2(ff[0]), // 1-bit LUT input
.I3(ld), // 1-bit LUT input
.I4(1'b0), // 1-bit LUT input
.I5(1'b1) // 1-bit LUT input (fast MUX select only available to O6 output)
);

However, on checking the implemented LUT in Xilinx FPGA editor, it may be seen that Xilinx ISE is not preserving the input ordering to the physical LUT pins. On applying LOCK_PINS constraint as (* LOCK_PINS = “all” *), it is giving design errors. I want to retain the ordering of inputs as specified in the HDL. How should I understand a valid input ordering?

[vemulad]

Hi @ayan_xilinx

What is the error you see when you use LOCK_PINS constraint?

Instead of using ALL attribute use PIN assignment list as mentioned in page-158 of http://www.xilinx.com/support/documentation/sw_manuals/xilinx14_7/cgd.pdf

[ayan_xilinx]

I am synthezing a 4-bit counter using a primitive instantiation based approach along with a constrained placement (placement constraints are mentioned in the .ucf) of the primitives on the FPGA fabric. The .v files and .ucf files are appended for your reference.

When I am providing the (* LOCK_PINS = “all” *) constraints in the .v file without the.ucf file the "Implement Design" is running successfully but is inferring more hardware than expected.

However, when I am providing the .ucf and retaining the LOCK PINS constraint it is giving an error as follows:

Directed packing was unable to obey the user design constraints (LOC=SLICE_X0Y1) which requires the combination of the symbols listed below to be packed into a single SLICEL component.

Retaining the placement constraints is crucial for my design. Also retaining the input ordering of signals to the physical LUT pins must be deterministic, which I am unable to achieve. 

Please help...

Regards,

Ayan..

[ayan_xilinx]

The complete error report is as follows:

The directed pack was not possible because: The function generators were
found to have incompatible input signals.

The symbols involved are:
LUT symbol "LUT6_2_inst1/LUT5" (Output Signal = d0<1>)
LUT symbol "LUT6_2_inst1/LUT6" (Output Signal = sel<1>)
LUT symbol "LUT6_2_inst2/LUT5" (Output Signal = d0<2>)
LUT symbol "LUT6_2_inst2/LUT6" (Output Signal = sel<2>)
Carry symbol "CARRY4_inst0" (Output Signal = cc<0>)
FLOP symbol "FDRE_inst0" (Output Signal = q_0_OBUF)
FLOP symbol "FDRE_inst1" (Output Signal = q_1_OBUF)
LUT symbol "LUT6_2_inst3/LUT5" (Output Signal = d0<3>)
FLOP symbol "FDRE_inst2" (Output Signal = q_2_OBUF)
LUT symbol "LUT6_2_inst3/LUT6" (Output Signal = sel<3>)
FLOP symbol "FDRE_inst3" (Output Signal = q_3_OBUF)
LUT symbol "LUT6_2_inst0/LUT5" (Output Signal = d0<0>)
LUT symbol "LUT6_2_inst0/LUT6" (Output Signal = sel<0>)

[ayan_xilinx]

I am awaiting a reply from your end.