LVDS Assignment in Quartus

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Does ALT_INBUF_DIF have a VHDL implementation available for use as a component?

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Your question is answered on the same Help page I copied from before. More from that Help page:

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VHDL Component Declaration:

The following VHDL component declaration is located in the VHDL Design File (.vhd) ALTERA_PRIMITIVES_COMPONENTS.VHD located in the <Quartus II installation directory>\libraries\vhdl\altera directory.

component alt_inbuf_diff
    generic(
        io_standard           : string := "NONE"; 
        location              : string := "NONE";
        enable_bus_hold       : string := "NONE";
        weak_pull_up_resistor : string := "NONE"; 
        termination           : string := "NONE";
    port(
        i    : in std_logic; 
        ibar : in std_logic;
        o    : out std_logic); 
end component;

VHDL LIBRARY-USE Declaration:

The VHDL LIBRARY-USE declaration is not required if you use the VHDL Component Declaration.

LIBRARY altera; 
USE altera.altera_primitives_components.all;

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