Module Declaration
A module is the principal design entity in Verilog. The first line of a module declaration specifies the name and port list (arguments). The next few lines specifies the i/o type (input, output or inout, see Sect. 4.4. ) and width of each port. The default port width is 1 bit.
Then the port variables must be declared wire, wand,. . ., reg (See Sect. 4. ). The default is wire. Typically inputs are wire since their data is latched outside the module. Outputs are type reg if their signals were stored inside an always or initial block (See Sect. 10. ).
| Syntaxmodule module_name (port_list); input [msb:lsb] input_port_list; output [msb:lsb] output_port_list; inout [msb:lsb] inout_port_list; … statements … endmodule |  |
Continuous Assignment
The continuous assignment is used to assign a value onto a wire in a module. It is the normal assignment outside of always or initial blocks (See Sect. 10. ). Continuous assignment is done with an explicit assign statement or by assigning a value to a wire during its declaration. Note that continuous assignment statements are concurrent and are continuously executed during simulation. The order of assign statements does not matter. Any change in any of the right-hand-side inputs will immediately change a left-hand-side output.
| Syntaxwire wire_variable = value; assign wire_variable = expression; |  |
Module Instantiations
Module declarations are templates from which one creates actual objects (instantiations). Modules are instantiated inside other modules, and each instantiation creates a unique object from the template. The exception is the top-level module which is its own instantiation.
The instantiated module’s ports must be matched to those defined in the template. This is specified:
(i) by name, using a dot(.) “ .template_port_name (name_of_wire_connected_to_port )”. or
(ii) by position, placing the ports in exactly the same positions in the port lists of both the template and the instance.
| Syntax for Instantiationmodule_name instance_name_1 (port_connection_list), instance_name_2 (port_connection_list),……. instance_name_n (port_connection_list); |  |
Modules may not be instantiated inside procedural blocks.
Parameterized Modules
You can build modules that are parameterized and specify the value of the parameter at each instantiation of the module. See “Parameter” on page 5 for the use of parameters inside a module. Primitive gates have parameters which have been predefined as delays. See “Basic Gates” on page 3.
| Syntaxmodule_name #(parameter_values)instance_name(port_connection_list); |  |
Synthesis does not support the defparam keyword which is an alternate way of changing parameters.
