11.4.2 Internal Tri-State to Multiplexor Mapping

All internal tri-states must be mapped to multiplexors. The antifuse technology only supports tri-states as in/out ports, but not internal tri-states. The following examples show an internal tri-state followed by a multiplexor that the internal tri-state should change to as in figure below.

Note: Some synthesis tools automatically map internal tri-states to multiplexors.

Figure 11-34. Internal Tri-State and the Related Multiplexor

VHDL Tri-State

library IEEE;
use IEEE.std_logic_1164.all;
entity tribus is
port (A, B, C, D : in std_logic_vector(7 downto 0);
E0, E1, E2, E3 : in std_logic;
Q : out std_logic_vector(7 downto 0));
end tribus;
architecture rtl of tribus is
begin
Q <= A when(E0 = '1') else "ZZZZZZZZ";
Q <= B when(E1 = '1') else "ZZZZZZZZ";
Q <= C when(E2 = '1') else "ZZZZZZZZ";
Q <= D when(E3 = '1') else "ZZZZZZZZ";
end rtl;

VHDL Multiplexor

library IEEE;
use IEEE.std_logic_1164.all;
entity muxbus is
port (A, B, C, D : in std_logic_vector(7 downto 0);
E0, E1, E2, E3 : in std_logic;
Q : out std_logic_vector(7 downto 0));
end muxbus;
architecture rtl of muxbus is
signal E_int : std_logic_vector(1 downto 0);
begin
process (E0, E1, E2, E3)
variable E : std_logic_vector(3 downto 0);
begin
E := E0 & E1 & E2 & E3;
case E is
when "0001" => E_int <= "00";
when "0010" => E_int <= "01";
when "0100" => E_int <= "10";
when "1000" => E_int <= "11";
when others => E_int <= "--";
end case;
end process;
process (E_int, A, B, C, D)
begin
case E_int is
when "00" => Q <= D;
when "01" => Q <= C;
when "10" => Q <= B;
when "11" => Q <= A;
when others => Q <= (others => '-');
end case;
end process;
end rtl;

Verilog Tri-State

module tribus (A, B, C, D, E0, E1, E2, E3, Q);
input [7:0]A, B, C, D;
output [7:0]Q;
input E0, E1, E2, E3;
assign Q[7:0] = E0 ? A[7:0] : 8'bzzzzzzzz;
assign Q[7:0] = E1 ? B[7:0] : 8'bzzzzzzzz;
assign Q[7:0] = E2 ? C[7:0] : 8'bzzzzzzzz;
assign Q[7:0] = E3 ? D[7:0] : 8'bzzzzzzzz;
endmodule

Verilog Multiplexor

module muxbus (A, B, C, D, E0, E1, E2, E3, Q);
input [7:0]A, B, C, D;
output [7:0]Q;
input E0, E1, E2, E3;
wire [3:0] select4;
reg [1:0] select2;
reg [7:0]Q;
assign select4 = {E0, E1, E2, E3};
always @ (select4)
begin
case(select4)
4'b0001 : select2 = 2'b00;
4'b0010 : select2 = 2'b01;
4'b0100 : select2 = 2'b10;
4'b1000 : select2 = 2'b11;
default : select2 = 2'bxx;
endcase
end
always @ (select2 or A or B or C or D)
begin
case(select2)
2'b00 : Q = D;
2'b01 : Q = C;
2'b10 : Q = B;
2'b11 : Q = A;
endcase
end
endmodule