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AR# 37204

XST - The "buffer_type = IBUF" constraint does not work when "Add I/O Buffer" is disabled

Description

If "Add I/O Buffer" (-iobuf) is disabled in Synthesis,"buffer_type = IBUF" constraint (see following examples)does notinsert IBUF to the constrained net. What is the problem?

For example:

attribute buffer_type: string;
attribute buffer_type of reset: signal is "ibuf";

OR

(*buffer_type = "ibuf"*) input reset;

Solution


This is a known issue of XST. When"Add I/O Buffer" (-iobuf) is disabled, XST does not honor the"buffer_type = IBUF" constraint.
To work around this problem, perform the following
  1. Don't disable "ADD I/O Buffer" but add Buffer_type constraints on port that doesn't need buffers and set the value of the constraint to none
  2. instantiate IBUF manually in the HDL code where needed.
  • VHDL example:

    IBUF_inst : IBUF
    generic map (
    IBUF_LOW_PWR => TRUE, -- Low power (TRUE) vs. performance (FALSE) setting for refernced I/O standards
    IOSTANDARD => "DEFAULT")
    port map (
    O => reset_IBUF, -- Buffer output
    I =>reset -- Buffer input (connect directly to top-level port)
    );

  • Verilog example:

    IBUF #(
    .IBUF_LOW_PWR("TRUE"), // Low power (TRUE) vs. performance (FALSE) setting for referenced I/O standards
    .IOSTANDARD("DEFAULT") // Specify the input I/O standard
    )IBUF_inst (
    .O(reset_IBUF), // Buffer output
    .I(reset) // Buffer input (connect directly to top-level port)
    );
AR# 37204
Date Created 08/03/2010
Last Updated 12/15/2012
Status Active
Type General Article
Tools
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