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Combinational Circuits Using VHDL

Combinational Circuits Using VHDL. Contents. Combinational circuit examples: multiplexer using dataflow multiplexer using behavioral style decoder using structural style. Entity Definition for 4 to 1 Multiplexer. - - Define all necessary libraries library IEEE;

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Combinational Circuits Using VHDL

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  1. Combinational CircuitsUsing VHDL

  2. Contents • Combinational circuit examples: • multiplexer using dataflow • multiplexer using behavioral style • decoder using structural style

  3. EntityDefinition for 4 to 1 Multiplexer -- Define all necessary libraries library IEEE; use IEEE.STD_LOGIC_VECTOR_1164.all; entity mux_4_to_1 is port (S: in STD_LOGIC_VECTOR (1 downto 0); -- select vectorS(1:0) A, B, C, D: in STD_LOGIC; -- inputs Y: out STD_LOGIC); -- outputs end mux_4_to_1;

  4. Architecture for Multiplexer Using Dataflow -- Note: S,A,B,C,D are defined in the entity definition -- and will not be repeated here. architecture mux_4_to_1_arch of mux_4_to_1 is begin-- dataflow style (nocomponents orprocesses) with S select Y <= A when "00", B when "01", C when "10", D when "11", (others => 'U') when others; -- all possibilities are covered end mux_4_to_1_arch;

  5. Architecture for Multiplexer in Behavioral Style (Wakerely, p. 410) -- Note: S,A,B,C,D are defined in the entity definition -- and will not be repeated here. architecture mux4in8p of mux_4_to_1 is begin process (S, A, B, C, D) -- execution if there is a change in S, A, B, C, orD. begin -- All RHS variables and S are in the sensitivity list. case S is when "00" => Y <= A; when "01" => Y <= B; when "10" => Y <= C; when "11" => Y <= D; when others => Y <= (others => 'U'); -- all possibilities covered end case; end process; end mux4in8p;

  6. Decoder 2 to 4 in Structural Style (Ι/ΙΙ) library IEEE; use IEEE.std_logic_1164.all; entity V2to4dec is port (I0, I1, EN: in STD_LOGIC;-- two input selectors Y0, Y1, Y2, Y3: out STD_LOGIC); -- four outputs end V2to4dec; architecture V2to4dec _s of V2to4dec is signal NOTI0, NOTI1: STD_LOGIC; -- internal wires. -- definition of required component (from IEEE) component inv port (I: in STD_LOGIC; O: out STD_LOGIC); end component;

  7. Decoder 2 to 4 in Structural Style (ΙΙ/ΙΙ) component and3 port (I0, I1, I2: in STD_LOGIC; O: out STD_LOGIC); end component; begin --inside the architecture, everything is executed in parallel U1: inv port map (I0, NOTI0); -- can read inputs … U2: inv port map (I1, NOTI1); -- and output to NOT10, NOT1 U3: and3 port map (NOTI0, NOTI1, EN, Y0); -- gate to outputs … U4: and3 port map ( I0, NOTI1, EN, Y1); U5: and3 port map (NOTI0, I1, EN, Y2); U6: and3 port map ( I0, I1, EN, Y3); end Vto4dec_s;

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