Process Statement

Formal Definition

A process statement defines an independent sequential process representing the behavior of some portion of the design.

Simplified Syntax

[process_label:] process [ ( sensitivity_list ) ] [ is ]

    process_declarations

    begin

     sequential_statements

    end process [ process_label ] ;

Description

The process statement represents the behavior of some portion of the design. It consists of the sequential statements whose execution is made in order defined by the user.

Each process can be assigned an optional label.

The process declarative part defines local items for the process and may contain declarations of: subprograms, types, subtypes, constants, variables, files, aliases, attributes, use clauses and group declarations. It is not allowed to declare signals or shared variables inside processes.

The statements, which describe the behavior in a process, are executed sequentially, in the order in which the designer specifies them. The execution of statements, however, does not terminate with the last statement in the process, but is repeated in an infinite loop. The loop can be suspended and resumed with wait statements. When the next statement to be executed is a wait statement, the process suspends its execution until a condition supporting the wait statement is met. See respective topics for details.

A process declaration may contain optional sensitivity list. The list contains identifiers of signals to which the process is sensitive. A change of a value of any of those signals causes the suspended process to resume. A sensitivity list is a full equivalent of a wait on sensitivity_list statement at the end of the process. It is not allowed, however, to use wait statements and sensitivity list in the same process. In addition, if a process with a sensitivity list calls a procedure, then the procedure cannot contain any wait statements.

Examples

Example 1

entity D_FF is
port (D,CLK : in BIT;
      Q : out BIT := '0';
      NQ : out BIT := '1' );
end entity D_FF;
architecture A_RS_FF of D_FF is
begin
  BIN_P_RS_FF: process (CLK)
  begin
    if CLK = '1' and CLK'Event then
      Q <= D;
      NQ <= not D;
    end if;
  end process;
end architecture A_RS_FF;

The flip-flop has two input ports: D, CLK and two output ports: Q and NQ. The value of the input signal D is assigned to the output Q when the value of the CLK changes from '0' to '1'. The change of the signal value initiates the process BIN_P_RS_FF, because the signal CLK is on the sensitivity list of this process. The change of the CLK value from the logical zero to the logical one is sensed by the use of the if statement.

The first part of if statement takes place when the actual value of the signal CLK is checked. The second part confirms that the signal CLK really changes its value (attribute 'Event is responsible for that). In the case when the logical value of this condition is TRUE, then the two assignment signal statement are executed.

Important Notes

  • Sensitivity list and explicit wait statements may not be specified in the same process.

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