|Summary |Design Units |Sequential Statements |Concurrent Statements |Predefined Types |Declarations |

|Resolution and Signatures |Reserved Words |Operators |Predefined Attributes |Standard Packages |

VHDL Sequential Statements

These statements are for use in Processes, Procedures and Functions.
The signal assignment statement has unique properties when used sequentially.

Sequential Statements

  • wait statement
  • assertion statement
  • report statement
  • signal assignment statement
  • variable assignment statement
  • procedure call statement
  • if statement
  • case statement
  • loop statement
  • next statement
  • exit statement
  • return statement
  • null statement
  • wait statement

    Cause execution of sequential statements to wait.
     [ label: ] wait [ sensitivity clause ] [ condition clause ] ;
     wait for 10 ns;              -- timeout clause, specific time delay.
     wait until clk='1';          -- condition clause, Boolean condition
     wait until A>B and S1 or S2; -- condition clause, Boolean condition
     wait on sig1, sig2;          -- sensitivity clause, any event on any
                                  -- signal terminates wait

    assertion statement

    Used for internal consistency check or error message generation.
     [ label: ] assert boolean_condition [ report string ] [ severity name ] ;
     assert a=(b or c);
     assert j<i report "internal error, tell someone";
     assert clk='1' report "clock not up" severity WARNING;
     predefined severity names are: NOTE, WARNING, ERROR, FAILURE
     default severity for assert is ERROR

    report statement

    Used to output messages.
     [ label: ] report string [ severity name ] ;
     report  "finished pass1";  -- default severity name is NOTE
     report  "Inconsistent data." severity FAILURE;

    signal assignment statement

    The signal assignment statement is typically considered a concurrent
    statement rather than a sequential statement. It can be used as
    a sequential statement but has the side effect of obeying the
    general rules for when the target actually gets updated.
    In particular, a signal can not be declared within a process or
    subprogram but must be declared is some other appropriate scope.
    Thus the target is updated in the scope where the target is declared
    when the sequential code reaches its end or encounters a 'wait'
    or other event that triggers the update.
     [ label: ] target <= [ delay_mechanism ] waveform ;
        reject time_expression
        waveform_element [, waveform_element]
        value_expression [ after time_expression ]
        null [ after time_expression ]
      sig1 <= sig2;
      Sig <= Sa and Sb or Sc nand Sd nor Se xor Sf xnor Sg;
      sig1 <= sig2 after 10 ns;
      clk <= '1' , '0' after TimePeriod/2 ; 
      sig3 <= transport sig4 after 3 ns;
      sig4 <= reject 2 ns sig5 after 3 ns; -- increasing time order
      sig6 <= inertial '1' after 2 ns, '0' after 3 ns , '1' after 7 ns;
      Note: omitting  [ after time_expression ] is equivalent
            to after 0 fs;
     More information in Concurrent Statements signal assignment statement.

    variable assignment statement

    Assign the value of an expression to a target variable.
     [ label: ] target := expression ;
     A := -B + C * D / E mod F rem G abs H;
     Sig := Sa and Sb or Sc nand Sd nor Se xor Sf xnor Sg;

    procedure call statement

    Call a procedure.
     [ label: ] procedure-name [ ( actual parameters ) ] ;
     do_it;  -- no actual parameters
     compute(stuff, A=>a, B=>c+d); -- positional association first,
                                         -- then named association of
                                   -- formal parameters to actual parameters

    if statement

    Conditional structure.
     [ label: ] if  condition1  then
                elsif  condition2  then      \_ optional
                     sequence-of-statements  /
                elsif  condition3  then      \_ optional
                     sequence-of-statements  /
                else                         \_ optional
                     sequence-of-statements  /
                end if [ label ] ;
     if a=b then
     elsif b<c then
     end if;

    case statement

    Execute one specific case of an expression equal to a choice.
    The choices must be constants of the same discrete type as the expression.
     [ label: ] case  expression  is
                  when choice1 =>
                  when choice2 =>            \_ optional
                     sequence-of-statements  /
                  when others =>             \_ optional if all choices covered
                     sequence-of-statements  /
                end case [ label ] ;
      case  my_val  is
        when 1 =>
        when 3 =>
        when others =>
      end case;

    loop statement

    Three kinds of iteration statements.
     [ label: ] loop
                     sequence-of-statements -- use exit statement to get out
                end loop [ label ] ;
     [ label: ] for variable in range loop
                end loop [ label ] ;
     [ label: ] while  condition  loop
                end loop [ label ] ;
        exit when end_file;
      end loop;
      for I in 1 to 10 loop
        AA(I) := 0;
      end loop;
      while not end_file loop
      end loop;
    all kinds of the loops may contain the 'next' and 'exit' statements.

    next statement

    A statement that may be used in a loop to cause the next iteration.
     [ label: ] next [ label2 ] [ when condition ] ;
     next outer_loop;
     next when A>B;
     next this_loop when C=D or done; -- done is a Boolean variable

    exit statement

    A statement that may be used in a loop to immediately exit the loop.
     [ label: ] exit [ label2 ] [ when condition ] ;
     exit outer_loop;
     exit when A>B;
     exit this_loop when C=D or done; -- done is a Boolean variable

    return statement

    Required statement in a function, optional in a procedure.
     [ label: ] return [ expression ] ;
     return; -- from somewhere in a procedure
     return a+b; -- returned value in a function

    null statement

    Used when a statement is needed but there is nothing to do.
     [ label: ] null ;

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