Introduction
Verilog HDL is one of the two most common Hardware Description Languages (HDL) used by integrated circuit (IC) designers. The other one is VHDL.
HDL’s allows the design to be simulated earlier in the design cycle in order to correct errors or experiment with different architectures. Designs described in HDL are technology-independent, easy to design and debug, and are usually more readable than schematics, particularly for large circuits.
Verilog can be used to describe designs at four levels of abstraction:
(i) Algorithmic level (much like c code with if, case and loop statements).
(ii) Register transfer level (RTL uses registers connected by Boolean equations).
(iii) Gate level (interconnected AND, NOR etc.).
(iv) Switch level (the switches are MOS transistors inside gates).
The language also defines constructs that can be used to control the input and output of simulation.
More recently Verilog is used as an input for synthesis programs which will generate a gate-level description (a netlist) for the circuit. Some Verilog constructs are not synthesizable. Also the way the code is written will greatly effect the size and speed of the synthesized circuit. Most readers will want to synthesize their circuits, so nonsynthe-
sizable constructs should be used only for test benches. These are program modules used to generate I/O needed to simulate the rest of the design. The words “not synthesizable” will be used for examples and constructs as needed that
do not synthesize.
There are two types of code in most HDLs:
Structural, which is a verbal wiring diagram without storage.
assign a=b & c | d; /* “|” is a OR */
assign d = e & (~c);
Here the order of the statements does not matter. Changing e will change a.
Procedural which is used for circuits with storage, or as a convenient way to write conditional logic.
always @(posedge clk) // Execute the next statement on every rising clock edge.
count <= count+1;
Procedural code is written like c code and assumes every assignment is stored in memory until over written. For synthesis, with flip-flop storage, this type of thinking generates too much storage. However people prefer procedural code because it is usually much easier to write, for example, if and case statements are only allowed in procedural code. As a result, the synthesizers have been constructed which can recognize certain styles of procedural code as actually combinational. They generate a flip-flop only for left-hand variables which truly need to be stored. However if you stray from this style, beware. Your synthesis will start to fill with superfluous latches.
This manual introduces the basic and most common Verilog behavioral and gate-level modelling constructs, as well as Verilog compiler directives and system functions. Full description of the language can be found in Cadence Verilog-XL Reference Manual and Synopsys HDL Compiler for Verilog Reference Manual. The latter emphasizes only those Verilog constructs that are supported for synthesis by the Synopsys Design Compiler synthesis tooll.
In all examples, Verilog keyword are shown in boldface. Comments are shown in italics.
