Assignment to a read-only object

The target of a signal or variable assignment must be an object that can be written in that context. Sigasi checks the target of every assignment, including each element of an aggregate target, and reports an assignment to an input port, to an input parameter of a subprogram, or to a constant as an error. Such code does not compile, and for ports it usually means the port was declared with the wrong mode or an internal signal was meant instead.

Input port cannot be assigned

A port of mode in can be read but not updated by a signal or variable assignment (LRM 6.5.2). An assignment to it is an error (rule 5), and the message names the port and its mode. A port declared without a mode is an input port too.

VHDL
entity counter is
  port(
    clk    : in  std_logic;
    enable : in  std_logic;
    count  : out unsigned(7 downto 0)
  );
end entity;

architecture rtl of counter is
begin
  enable <= '0';             -- enable is an input port
  count  <= (others => '0'); -- count is an output port
end architecture;

Subprogram parameter cannot be assigned

A signal or variable parameter of mode in cannot be updated either (LRM 6.5.2). Sigasi reports an assignment to such a parameter as an error (rule 6). A parameter declared without a mode is an input parameter.

VHDL
procedure pulse(signal clk : in std_logic; signal strobe : out std_logic) is
begin
  clk    <= '1'; -- clk is an input parameter
  strobe <= '1'; -- strobe is an output parameter
end procedure;

Constant cannot be assigned

A constant has a fixed value once it is elaborated (LRM 6.4.2.2). Only signals or variables can be assigned (rule 7). This covers constants, generics, parameters of class constant, and external names of class constant (LRM 8.7). File parameters cannot be the target of an assignment either.

VHDL
entity fifo is
  generic(DEPTH : natural := 16);
end entity;

architecture rtl of fifo is
  constant WIDTH : natural := 8;
begin
  process is
    variable n : natural;
  begin
    WIDTH := 16;        -- WIDTH is a constant
    DEPTH := 32;        -- DEPTH is a generic
    n     := WIDTH * 2; -- n is a variable
    wait;
  end process;
end architecture;