COE 202 Introduction to Verilog Computer Engineering Department College of Computer Sciences and Engineering King Fahd University of Petroleum and Minerals

Outline  Introduction  Verilog Syntax  Definition of a Module  Gate Level Modeling  Module Instantiation  Propagation Delay  Boolean Equation-Based Behavioral Models of Combinational Logic  Test Bench Example

Introduction  Verilog is one of the hardware description languages (HDL) available in the industry for hardware modeling, simulation and design.  It allows designers to describe their hardware at different levels of detail (e.g. gate-level, behavioral lavel)  Parallel not serial like programming languages.  Verilog can describe everything from single gate to full computer system.

Verilog  A digital system can be described at several levels of details (more details means more design entry time!): e.g.  Gate-level  Net-list similar to schematic or breadboarding  Behavioral description: programming-like structures (if-then- else, case, loops …etc) to describe what the circuit does (i.e. behavior) rather than how  requires some additional (synthesis) software to actually obtain the logic design  A digital system is described as a set of modules  The module is the basic unit of design

Verilog Syntax  Identifiers:  composed of letters, digits, the underscore character (_), and the dollar sign ($). $ is usually used with a system task or function  The first character of an identifier must be a letter or underscore  Verilog is a case-sensitive language D_BUS is different from D_Bus  Keywords: predefined identifiers that are used to describe language constructs. E.g. module, wire …etc.  Can not be used as user-defined identifiers  White space: space, tab, and newline characters are used to separate identifiers and can be used freely in the Verilog code  Comments: two forms; one-line comment starts with // and multiple-line comment is encapsulated between /* and */

Verilog Data Types  Two groups of Data Types: net and variable.  Net like wire; could be 1-bit or array (e.g. wire a; wire [3:0] sum)  Variable group like reg; The most commonly used data type in this group  Also integer

Module and Ports declaration module [module-name] ( [mode] [ d a t a - t y p e ] [ p o r t - n a m e s ],... [mode] [ d a t a - t y p e ] [ p o r t - n a m e s ] ) ;  Data-type could be wire, reg, integer, real …etc. Ex1.: module eq2 ( input wire [1:0] a, b, output wire aeqb ); Ex2.: module eq1( input i0, il, // no data type declaration output eq // all will be wires );

Gate Level Modeling  Net-list description  built-in primitives gates module my_gate( output OUT1, input IN1, IN2); wire X; // optional and (X, IN1, IN2); not (OUT1, X); endmodule Internal Signal X OUT1 IN2 IN1

Verilog Primitives  Basic logic gates only  and  or  not  buf  xorThese gates are expandable: 1st node  nand is O/P node, followed by 1, 2, 3 …  nor number of input nodes  xnor

Primitive Pins Are Expandable nand (y, in1, in2) ; nand (y, in1, in2, in3) ; nand (y, in1, in2, in3, in4) ;

A Half Adder module Add_half ( output c_out, sum, input a, b); xor (sum, a, b); and (c_out, a, b); endmodule

A Full Adder module fadd ( output co, s, input a, b, c); wire n1, n2, n3; // optional xor (n1, a, b) ; xor (s, n1, c) ; nand (n2, a, b) ; nand (n3,n1, c) ; nand (co, n3,n2) ; endmodule

Module Instantiation  Two ways to connect the ports of the instantiated module to the signals in the instantiating module:  1. By name: [module-name] [instance-name] (. [port-name] ( [signal-name] ), );  2. By order: Add_half M1 (.c_out(Cout),.sum(Sum),.a(A),.b(B)); Add_half M2 (Cout, Sum, A, B); module Add_half ( output c_out, sum, input a, b); xor (sum, a, b); and (c_out, a, b); endmodule

Module Instantiation

Propagation Delay module Add_full_unit_delay(output c_out, sum, input a, b, c_in); wire w1, w2, w3; // optional Add_half_unit_delay M1 (w2, w1, a, b); Add_half_unit_delay M2 (w3, sum, c_in, w1); or #2 (c_out, w2, w3); endmodule module Add_half_unit_delay (output c_out, sum, input a, b); xor #3 (sum, a, b); and #2 (c_out, a, b); endmodule Propagation Delay

Assign Statement  The keyword assign declares a continuous assignment.  It associates the Boolean expression on the RHS (right hand side) with the variable on the LHS (left hand side).  The assignment is sensitive to the variables in the RHS.  Any time an event occurs on any of the variables on the RHS, the RHS expression is revaluated and the result is used to update the LHS.

Boolean Equation-Based Behavioral Models of Combinational Logic  A Boolean equation describes combinational logic by an expression of operations on variables.  In Verilog, this is done by continuous assignment statement.  Example: module AOI_5_CA0 ( input x_in1, x_in2, x_in3, x_in4, x_in5, output y_out); assign y_out = ~( (x_in1 & x_in2) | (x_in3 & x_in4 & x_in5) ); endmodule

Full Adder module fadd (output Cout, S, input A, B, Cin); assign S = A ^(B ^ Cin); assign Cout = (A & B) | (A & Cin) | (B & Cin) ; endmodule

Propagation Delay & Continuous Assignment  Propagation delay can be associated with a continuous assignment so that its implicit logic has same functionality and timing characteristics as its gate level implementation. module fadd (output Cout, S, input A, B, Cin); assign #6 S = A ^(B ^ Cin); assign #5 Cout = (A & B) | (A & Cin) | (B & Cin); endmodule

Testbench Example module t_Add(); wire Sum, Cout; reg a, b, cin; Add_full_unit_delay M1 (Cout, Sum, a, b, cin); initial begin a=0; b=0; cin=0; #10b=1; #10a=1; cin=1; #10b=0; end endmodule

Testbench Example  The keyword initial declares a single-pass behavior that begins executing when the simulator is activated.  Statements within begin and end block keywords are called procedural statements.  Procedural statements execute sequentially  # is a delay control operator  A delay control operator preceding procedural assignment statement suspends its execution and the execution of subsequent statements for specified delay time  reg declaration ensures that variables will keep their value until the next procedural assignment statement