Comprehensive Verilog Tutorial for Digital System Design

ENEE 408C LabCapstone Project: Digital System DesignVerilog Tutorial Class Web Site: http://www.ece.umd.edu/class/enee408c

Structure Description • primitive instantiation (AND, NAND, OR, NOR, XOR, XNOR, BUF, NOT, BUFIF, NOTIF) • parameter value assignment

Structural Description Example module weird_logic (a,b,c,d); output a; input b,c,d; wire w; nand g1(w,b,c); nor g2(a,w,d); endmodule don’t forget primitive

Behavioral Description 1 • Boolean-Equation-Based Model module weird_logic (a,b,c,d); output a; input b,c,d; wire w; assign w = ~(b & c); assign a = ~(w | d); endmodule • continuous assignment • level-sensitive • normally used for combinational circuits continuous assignment

Behavioral Description 2 • Cyclic Behavior Model module weird_logic (a,b,c,d); output a; input b,c,d; reg w,a; always@(b or c) w = ~(b & c); always@(d or posedge w) a = ~(w | d); endmodule • always block • can be both level and edge sensitive • do not expire after the last statements

Behavioral Description 2 • Cyclic Behavior Model module weird_logic (a,b,c,d); output a; input b,c,d; wire w; always@(b or c) w = ~(b & c); always@(d or posedge w) a = ~(w | d); endmodule • always block • can be both level and edge sensitive • do not expire after the last statements

Testbench Example module tb_weird_logic; wire A; reg B, C, D; weird_logic instance1(A, C, D, B); initial // two slashes introduce a single line comment begin $monitor ($time,,, "A = %b B = %b C = %b D = %b", A, B, C, D); //waveform for simulating the binaryToESeg driver #10 B = 0; C = 0; D = 0; #10 D = 1; #10 C = 1; D = 0; #10 $finish; end endmodule

Using vectors to represent multi-bit numbers in Verilog • One dimensional: describe data with multiple bits. e.g. reg [3:0] a; a = 4’b1011; a = 4’d11; a = 4’hb; • Two dimension: describe a register array. e.g. reg [3:0] b [0:2]; b[0] = 4’d11; b[1] = 4’d12; b[2] = 4’d13 represents the width of the data (from MSB to LSB) represents the depth of the array (from address 0)

Module Hierarchy and Module Instantiation • A module uses other defined modules to implement a function. • Example: • module fulladder (sum,cout,a,b,cin); • input a, b, cin; • output sum, cout; • assign sum = a ^ b ^ cin; • assign cout = a&b | b&cin | a&cin; • endmodule • module halfadder (sum, cout, a,b); • input a, b; • output sum, cout; • assign sum = a ^ b; • assign cout = a & b; • endmodule • module fulladder (sum,cout,a,b,cin); • input a, b, cin; • output sum, cout; • halfadder A1(.sum(w1), .cout(w2), .a(a), .b(b)); • halfadder A2(.sum(sum), .cout(w3), .a(w1), .b(cin)); • assign cout = w2 | w3; • endmodule

Representing Logical Conditions • if-else statement reg n, m, p; if (n == m) begin p = 1’b0; end else begin p = 1’b1; end • case statement case (n) begin 1’b0: begin m= 1’b0; end 1’b1: begin m= 1’b1; end endcase • ? in continuous assignment wire w1, w2, w3 assign w1 = (w2==1’b1) ? w3: 1’bx;

Loop Structure in Verilog • while while ( i < 4’d10) begin … end • for for ( i = 0; i < 4’d10; i = i+1) • forever All the loops are used ONLY in procedural assignments

Comprehensive Verilog Tutorial for Digital System Design