Digital Design and Computer Architecture, 2nd Edition

SystemVerilog

module latch(input  logic     clk,

        input  logic  [3:0] d,

        output logic  [3:0] q);

 always_latch

  if (clk) q <= d;

endmodule

always_latch is equivalent to always @(clk, d) and is the preferred idiom for describing a latch in SystemVerilog. It evaluates any time clk or d changes. If clk is HIGH, d flows through to q, so this code describes a positive level sensitive latch. Otherwise, q keeps its old value. SystemVerilog can generate a warning if the always_latch block doesn’t imply a latch.

VHDL

library IEEE; use IEEE.STD_LOGIC_1164.all;

entity latch is

 port(clk: in STD_LOGIC;

    d:  in STD_LOGIC_VECTOR(3 downto 0);

    q:  out STD_LOGIC_VECTOR(3 downto 0));

end;

architecture synth of latch is

begin

 process(clk, ...