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VHDL Tutorial
Behavioral VHDL 4 to 1 Mux library ieee; use ieee.std_logic_1164.all;
entity MUX41 is port --define inputs and outputs ( S1 : inbit; -- input S S0 : inbit; D3 : in bit; D2 : in bit; D1 : in bit; D0 : in bit; Y : out bit -- output Y, note: NO ‘;’ used on the last line ); end MUX41;
architecture logic of MUX41 is -- Note MUX41 is the same as entity name above begin Y <= (D0 and (not S1) and (not S0)) or (D1 and (not S1) and S0 ) or (D2 and S1 and (not S0)) or (D3 and S1 and S0 ) ; end logic; -- Note matching names ‘logic’
4 to 1 Mux (S1 and S0 active low / Mixed Logic) library ieee; use ieee.std_logic_1164.all;
entity MUX41 is port ( S1_L, S0_L, D3, D2, D1, D0 : in std_logic; --std_logic same as bit, multiple inputs of the same type can be defined on the same line separated by commas
Y : out std_logic ); end MUX41;
architecture logic of MUX41 is
signal S1, S0 : std_logic; --define signals. Signals are like temp variables which are not defined in entity but are needed for the behaviour begin
S1 <= not S1_L; --relating S1 to S1_L; S0 <= not S0_L;
-- No _L in equation below since logic equation does not change
Y <= (D0 and (not S1) and (not S0)) or (D1 and (not S1) and S0 ) or (D2 and S1 and (not S0)) or (D3 and S1 and S0 ) ; end logic;
2 to 1 Mux (using IF/ELSE) library ieee; use ieee.std_logic_1164.all; entity MUX2to1 is port( A, B: in std_logic_vector(7 downto 0); Sel: in std_logic; Y: out std_logic_vector(7 downto 0)); end MUX2to1;
architecture behavior of MUX2to1 is begin process (Sel, A, B) -- rerun process if any changes, sensitivity list, all inputs begin if (Sel = '1') then Y <= B; else Y <= A; end if; -- note that end if is two words end process; end behavior;
Note: Std_logic_vector used to define a signal of more than 1 bit. In this case A, B and Y are all 8 bits and can be referred to as a vector or as individual components such as A(7), A(6),.. etc
Structural VHDL
Structural VHDL uses component description and connection descriptions (i.e. how the components are connected to each other).
For the following example, assume that a VHDL component for an AND gate (called “and”) and a component for the OR gate (called “or”) has already been developed using any of the behavioral methods described above.
ibrary ieee; use ieee.std_logic_1164.all; use work.all;
entity comb_logic is port( a,b,c,d : in std_logic; e: out std_logic ); end comb_logic;
architecture behv of comb_logic is
--Assume you have an "AND" and an "OR" defined first --define similar to other examples
component and is port (a,b : in std_logic; output : out std_logic ); end component;
component or is port (a,b : in std_logic; output : out std_logic ); end component;
--structural definition of e = ab + cd
signal wire1, wire2 : std_logic; begin AND1: and port map ( a, b, wire1 ); AND2: and port map ( c, d, wire2 ); OR1: or port map ( wire1, wire2, e); end behv;
Note: The port statement for entity comb_logic contain a,b,c,d, and e. Components are instantiated by including the entity description as follows: component and is --declaring an “and” component port (a,b : in std_logic; output : out std_logic ); end component; Ports a, b, c, d, and e are defined but we need ‘temp’ signals (which are called wire and wire2 in this case) to connect the components defined above. AND1 is a component of type “and”. The port map maps signals to the port of the “and” component. AND2 is another instance of the “and” component, while OR1 is an instance of the previously defined “or” component. The port map entries have to correspond to the component entity ports. For example component “or” has a port definition Port (a,b : in std_logic; output : out std_logic ); So the port map of OR1 is ( wire1, wire2, e) where wire1, and wire2 correspond to the two inputs of the “or” gate called a and b, and e is mapped onto the output of the “or” component. When doing port maps order matters since there is a one-to-one correspondence between the port declaration and port map. If you do not want to worry about order, you can use explicit definition as follows: OR1: or port map ( a<=wire1, b<=wire2, output<=e);
Some examples were taken from the course website (although they may have been modified a little). For more examples see the course website (Examples VHDL Examples). Follow the tutorial on Creating Graphical Components found in either Examples VHDL Examples or Software/Docs Quartus to include your VHDL components in your design, compile and simulate.
