VHDL Reference Guide - Variable Assignment

**Variable Assignment**

Sequential Statement	---- used in ---->	Process Procedure Function

Syntax

variable_name := expression;

See LRM section 8.4

Rules and Examples

Assignments may be made from signals to variables and vice-versa, providing the types match:

process (A, B, C, SEL)
  variable X : integer range 0 to 7;
begin
  if SEL = '1' then
    X := B;
  else
    X := C;
  end if;
    Z <= A + X;
end process;

A variable assignment takes effect immediately:

architecture EX1 of V is
  signal A,B,Y,Z : integer;
begin
  process (A, B) 
    variable M, N : integer;
  begin
    M := A;
    N := B;
    Z <= M + N;
    M := 2*A;
    Y <= M + N;
  end process;
end EX1;

An equivalent architecture with concurrent signal assignments

architecture EX2 of V is
  signal A,B,Y,Z:integer;
begin
  Z <= A + B;
  Y <= 2*A + B;
end EX2;

A variable assignment may not be given a delay.

A variable in a process can act as a register, if it is read before it has been written to, since it retains its value between sucessive process activations.

process (CLK)
   variable Q : std_ulogic;
begin
  if CLK'event and CLK='1' then
    PULSE <= D and not(Q);
    Q := D;
    -- PULSE and Q act as registers
  end if;
end process;

Synthesis Issues

Variable assignments are generally synthesisable, providing they use types and operators acceptable to the synthesis tool.

In a "clocked process", each variable which has its value read before it has had an assignment to it will be synthesised as the output of a register.

In a "combinational process", reading a variable before it has had an assignment may cause a latch to be synthesised.

Variables declared in a subprogram are synthesised as combinational logic.

Whats New in '93

In VHDL-93, a variable assignment may have a label:

label: variable_name := expression;

VHDL-93 supports shared variables which may be accessed by more than one process.However, the language does not define what happens if two or more processes make conflicting accesses to a shared variable at the same time.