Description of syntax for IF, CASE, WITH and WHEN

  1 -- =============================================================================
  2 -- file name is:  mux4_1.vhd       (mux=multiplexer)
  3 -- Author:        Kim Petersen
  4 -- Created:       00.04.10      last modified: 00.04.13
  5 -- =============================================================================
  6 --  It is a 4 input multiplexer with the function as:
  7 --      sel            Input => output     comments
  8 --     MSB  LSB
  9 --      0    0          in0  => output
 10 --      0    1          in1  => output
 11 --      1    0          in2  => output
 12 --      1    1          in3  => output
 13 --
 14 --------
 15 --    IF   and CASE can only be used inside a process.
 16 --    WHEN and WITH can only be used outside a process.
 17 --
 18 --    IF   corresponds to WHEN
 19 --    CASE correpsonds to WITH
 20 -- =============================================================================
 21 
 22 library ieee;
 23 use ieee.std_logic_1164.all;      -- can be different dependent on tool used.
 24 use ieee.std_logic_unsigned.all;  -- can be different dependent on tool used.
 25 
 26 entity  mux4_1 is
 27   port (s0               : in  STD_LOGIC;
 28     s1     : in  STD_LOGIC;
 29     in0    : in  STD_LOGIC;
 30     in1    : in  STD_LOGIC;
 31     in2    : in  STD_LOGIC;
 32     in3    : in  STD_LOGIC;
 33     output : out STD_LOGIC
 34   );
 35 end mux4_1;
 36 -- =============================================================================
 37 -- =============================================================================
 38 architecture if_example of mux4_1 is
 39 
 40 -- =============================================================================
 41 begin
 42 
 43   mux : process(s0, s1, in0, in1, in2, in3)
 44   begin
 45 
 46     if    (s0 = '0' and s1 = '0') then
 47       output <= in0;
 48     elsif (s0 = '1' and s1 = '0') then
 49       output <= in1;
 50     elsif (s0 = '0' and s1 = '1') then
 51       output <= in2;
 52     elsif (s0 = '1' and s1 = '1') then
 53       output <= in3;
 54     else          -- (s0 or s1 are not 0 or 1)
 55       output <= 'X';
 56     end if;
 57 
 58   end process mux;
 59 
 60 end if_example;
 61 
 62 -- =============================================================================
 63 -- =============================================================================
 64 architecture case_example of mux4_1 is
 65 
 66 -- =============================================================================
 67 begin
 68 
 69   mux : process(s0, s1, in0, in1, in2, in3)
 70     variable  sel : STD_LOGIC_VECTOR(1 downto 0);
 71   begin
 72     sel := s1 & s0;   -- concatenate s1 and s0
 73 
 74     case sel is
 75       when  "00"  => output <= in0;
 76       when  "01"  => output <= in1;
 77       when  "10"  => output <= in2;
 78       when  "11"  => output <= in3;
 79       when others => output <= 'X';
 80     end case;
 81 
 82   end process mux;
 83 
 84 end case_example;
 85 
 86 -- =============================================================================
 87 -- =============================================================================
 88 architecture with_example of mux4_1 is
 89 
 90   signal  sel : STD_LOGIC_VECTOR(1 downto 0);
 91 -- =============================================================================
 92 begin
 93   sel <= s1 & s0;  -- concatenate s1 and s0
 94   with sel  select
 95     output <= in0 when "00",
 96     in1 when "01",
 97     in2 when "10",
 98     in3 when "11",
 99     'X' when others;
100 
101 end with_example;
102 
103 -- =============================================================================
104 -- =============================================================================
105 architecture when_example of mux4_1 is
106 
107 -- =============================================================================
108 begin
109 
110   output <= in0 when (s1 & s0) = "00" else
111               in1 when (s1 & s0) = "01" else
112               in2 when (s1 & s0) = "10" else
113               in3 when (s1 & s0) = "11" else
114               'X';
115 
116 end when_example;