ELEE 4303 Digital II Introduction to Verilog

ELEE 4303 Digital II Learning Objectives Get familiar with background of HDLs Basic concepts of Verilog languages –Gate level primitives –Module –Ports –Structural and behavioral descriptions –Continuous assignment and procedural statements –Verilog syntax

ELEE 4303 Digital II Hardware Description Languages A hardware description language is a computer language that is used to describe hardware. Two HDLs are widely used –Verilog HDL –VHDL (Very High Speed Integrated Circuit Hardware Description Language) Schematic design entry can be replaced by writing HDL code that CAD tools understand. CAD tools can verify the HDL codes, and create the circuits automatically from HDL codes.

ELEE 4303 Digital II Facts about Verilog We use Verilog, not VHDL, in ELEE –Verilog is more popular in industry than VHDL –They offer similar features History of Verilog –In 1980s, originally developed by Gateway Design Automation. –In 1990, was put in public domain. –In 1995, adopted as an IEEE standard –In 2001, an enhanced version, Verilog 2001

ELEE 4303 Digital II Use of Verilog Functions of Verilog –Design entry, like schematic –Simulation and verification of your design –Synthesis Verilog is a complex, sophisticated language –We focus on a subset of its features that is synthesizable. –For complete Verilog, refer to reference books. –Verilog will be taught in several stages through this course when the language elements are relevant to the design topics. –Appendix A provides a concise summary of Verilog.

ELEE 4303 Digital II Representation of Digital Circuits in Verilog Design entry in several different ways –Truth tables –Schematic captures –Hardware description languages (efficient !) Two types of representations in Verilog –Structural –Behavioral

ELEE 4303 Digital II Structural Representation (1) Gate level primitives –Correspond to commonly used logic gates –Built in CAD tools, so they can be used by user directly. –All available gate primitives are listed in Table A.2 examples –AND gate  and (y, x1, x2); –OR gate  or (y, x1, x2, x3, x4); –NOT gate  not (y, x); Keywords: and, or, not are reserved

ELEE 4303 Digital II Structural Representation (2) Module –A logic circuit  module –Its ports: inputs and outputs –Begins with module, ends with endmodule example

ELEE 4303 Digital II Structural representation (3) One more example

ELEE 4303 Digital II Behavioral Representation (1) Why behavioral representation? –Using primitives can be tedious when circuits are large –It is desired to describe the circuit in more abstract level – behavior Two fundamental types of behavior model –Logic expression –Procedural statements CAD synthesis tools use this representation to construct the actual circuit

ELEE 4303 Digital II Behavioral Representation (2) Logic expression –continuous assignment

ELEE 4303 Digital II Behavioral representation (3) Procedural statements –always block Sensitivity list: statements evaluated only when one or more signals in list changes value. Statements evaluated in order –One or more statements in one always block –One or more always blocks in a module –Declare a signal as a variable reg if the signal is assigned a value by procedural statements

ELEE 4303 Digital II Lexical Conventions White space characters are ignored : SPACE, TAB, new line, blank lines Two forms to introduce comments single line: begin with // multiple lines: /*…..long comments */ Verilog is case sensitive Lower case letters are unique from upper case letter All Verilog keywords are lower case Identifiers begin with an alphabetic character or the underscore character may contain alphabetic characters, numeric characters, _, and $ up to 1024 characters long. The instance of a module must be named while the instance name of a primitive is optional. `include “ *.v”

ELEE 4303 Digital II Examples of lexical conventions Comments Lower case key words identifiers

ELEE 4303 Digital II Quiz Write Verilog code to implement the following circuit using the gate level primitives. Figure 2.25 b