Sequential Assignment Statements

In Sequential Assignment ordering of statements in vhdl code may affect its meaning.

Types of Sequential Assignments :

  All Sequential Assignments should be inside a process . Functions and procedure also execute statements sequential.

1.) if-then
      Example 1. MUX2is1 :-      

 PROCESS (w0,w1,s)  
    BEGIN  
      y <= w0;  
      if ( s = '1' ) then   
          y <= w1;  
      end if;  
  END PROCESS;  

2.) if-elsif
    Example 1. MUX4is1 :-

 PROCESS (w0,w1,s)  
    BEGIN   
      if ( s = "00" ) then   
          y <= w0;  
      elsif ( s = "01" ) then  
          y <= w1;  
      elsif ( s = "10" ) then  
          y <= w2;  
      elsif ( s = "11" ) then  
          y <= w3;  
      else  
          y <= 'X'; -- undefined  
      end if;  
    END PROCESS;  

3.) case statement 
     Example 1. MUX2is1 :-

 PROCESS (w0,w1,s)  
     BEGIN  
      CASE s is  
         WHEN '0' =>   
               y <= w0;  
         WHEN others =>  
               y <= w1;  
      END CASE;   
 END PROCESS;  

      Example 2. Decoder2is4 :-

 PROCESS (I,enb_n)  
     BEGIN  
       if ( enb_n = '0') then  
       CASE I is  
              WHEN "00" =>  
                       y <= "0001";  
              WHEN "01" =>  
                       y <= "0010";  
               WHEN "10" =>  
                       y <= "0100";  
               WHEN "11" =>  
                       y <= "1000";  
               WHEN others =>  
                       y <= "XXXX";       
       END CASE;  
      else  
       y <= (others=>'0');  
      end if;  
      END PROCESS;  

4.) LOOP Statements