I generated Verilog Post-Route simulation model of my original Verilog module, using Xilinx ISE.
It will generate a Verilog module using LUT and fpga level primitives such as IBUF,X_LUT4, ...

When trying to compile this code directly and synthesis it inside ISE itself, it cannot find Xilinx primtivies and assert compilation error such as ERROR:HDLCompilers:87 - "test.v" line 26 Could not find module/primitive 'X_OPAD'.

I want to know how can I include related modules/libraries inside ISE verilog code to avoid compilation errors.
Part of code is brought below:

module tripler (
  wire GATE3_OUT_0;
  wire GATE1_OUT_0;
  wire GATE2_OUT_0;
  wire GATE4_OUT_0;
  wire GATE5_OUT_0;
  wire GATE6_OUT_0;
  wire GATE3_OUT;
  wire GATE1_OUT;
  wire GATE2_OUT;
  wire GATE4_OUT;
  wire GATE5_OUT;
  wire GATE6_OUT;
  wire TRIPLE_OUT;
  wire VCC;

  X_OPAD #(
    .LOC ( "PAD1" ))
  X_OBUF #(
    .LOC ( "PAD1" ))
  X_IPAD #(
    .LOC ( "PAD2" ))
  X_BUF #(
    .LOC ( "PAD2" ))
  X_BUF #(
    .LOC ( "SLICE_X0Y13" ))
  X_BUF #(
    .LOC ( "SLICE_X0Y13" ))
  • \$\begingroup\$ Why are you trying to synthesize a simulation model? X_ modules are from the Xilinx simulation library, they are not device primitives. \$\endgroup\$
    – mng
    Commented Jan 21, 2015 at 19:25
  • \$\begingroup\$ What are you trying to accomplish by using X_ macros directly? Are you looking to do something that can't be done with the 'normal' primitive set (IBUF/OBUF/etc) ? \$\endgroup\$ Commented Aug 29, 2015 at 1:16

1 Answer 1


Primitives with prefix "X_" don't look like normal primitives.

I tried once to hide pins from top-level. So I instantiated IPAD and OPAD primitives in VHDL by hand.


  • Synthesis XST -> complained about black-boxes
  • Translate -> complained about black-boxes
  • Map -> was very happy
  • P&R -> run as normal
  • BitGen -> run as normal
  • test on FPGA -> all as expected

Xilinx has several primitive libraries:

  • ...

I don't know if there is also a documentation for XST/iSim, but I found this for Synth/xSim (Vivado): Vivado Design Suite User Guide - Logic Simulation (UG900). Have a look at pages 14 ff. It lists all libraries and when they are bound in simulation (post synth. / post impl.)

  • \$\begingroup\$ I did include SIMPRIM modules (modules to simulate post place & route delays) to my design which include those X_BUF and ... modules. However Xilinx is now compalining about missing item which is glbl module. I added it as well but it seems that ISE ignore it. glbl module is a module which is intended to drive reset pins of all elements inside FPGA. However I don't know what is missing. Currently I added Name glbl module to my code however here is Xilinx compiler complaint: 'glbl.GTS' could not be resolved ! \$\endgroup\$
    – VSB
    Commented Jan 21, 2015 at 19:29
  • \$\begingroup\$ There is a -insert_glbl switch in the Simulation Process Properties. Parameter description: Automatically Insert glbl Module in the Netlist - I never change such things, maybe it helps. \$\endgroup\$
    – Paebbels
    Commented Jan 21, 2015 at 20:17

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