1 Brief Introduction to Verilog Weiping Shi

2 What is Verilog? It is a hardware description language Originally designed to model and verify a design Later became an efficient design entry method Allows designers to quickly create and debug large scale designs Similar to C in syntax Verilog will be used in later labs

3 Sample Half Adder module Add_half (sum, c_out, a, b); inputa, b; outputsum, c_out; wire c_out_bar; xor (sum, a, b); nand (c_out_bar, a, b); not (c_out, c_out_bar); endmodule c_out a b sum c_out_bar

4 Module Hierarchy Modules can be instantiated within other modules Allows for simplicity and regularity in the design Example: Use two half adders to create a full adder

5 Module Hierarchy Example module Add_half ( sum, c_out, a, b ); inputa, b; outputsum, c_out; wire c_out_bar; xor (sum, a, b); nand (c_out_bar, a, b); not (c_out, c_out_bar); endmodule Module Add_full ( sum, c_out, a, b, c_in );// parent module input a, b, c_in; output c_out, sum; wire w1, w2, w3; Add_half M1 ( w1, w2, a, b ); Add_half M2 ( sum, w3, w1, c_in );// child module or ( c_out, w2, w3 );// primitive instantiation endmodule

6 Alternative Half Adders module Add_half ( sum, c_out, a, b ); inputa, b; outputsum, c_out; assign { c_out, sum } = a + b; // Continuous assignment endmodule module Add_half (sum, c_out, a, b ); inputa, b; outputsum, c_out; regsum, c_out; ( a or b)// behavior description begin sum = a ^ b; c_out = a & b; end endmodule

7 Structural v.s. Behavioral Verilog can be structural or behavioral Structural definition specifies the gates and their connections explicitly Behavioral definition specifies the functionality of a design Does not contain any structural information such as transistors or gates Logic synthesis software implements the structural

Behavioral Example 2 Bit Comparator module comparator (a_greater, b_greater, equal, a, b); inputa, b; outputa_greater, b_greater, equal; rega_greater, b_greater, equal; or b)// either a or b changes begin if (a > b) begin a_greater = 1; b_greater = 0; equal = 0; end if (a<b) begin a_greater = 0; b_greater = 1; equal = 0; end if (a==b) begin a_greater = 0; b_greater = 0; equal = 1; end endmodule

9 Alternate comparator module comparator (a_greater, b_greater, equal, a, b); inputa, b; outputa_greater, b_greater, equal; assign a_greater = (a > b) ? 1 : 0; assign b_greater = (a < b) ? 1 : 0; assign equal = (a==b) ? 1 : 0; endmodule Uses a conditional continuous assignment to set the outputs.

10 Clarification Registers are used when an output is updated on an event. The value must be held until a new event updates that value. Assign statements are used when the output is continuously being assigned.

11 Testbench Manipulate the module inputs to observe the circuit reaction Uses module hierarchy Introduces the concept of delay

12 Sample Testbench for a Half Adder module tbench; rega,b;// regs connect to module inputs wiresum,cout;// wires connect to module outputs half_adder M1(cout,sum,a,b);// instantiate the half adder initial begin a = 0, b = 0;//time 0 #5a = 1, b = 0;//time 5 #3a = 1, b = 1;//time 8 #4a = 0, b = 1;//time 12 #52a = 0, b = 0;//time 64 #70$finish;//stops the simulation end initial begin $monitor($time,”a = %b, b=%b cout=%b sum=%b”,a,b,cout,sum);//displays the variable values at each //unit of time that an event occurs end endmodule

13 Testbench Results Compiling source file "ha.v" Compiling source file "tbench.v" Highest level modules: tbench 0a = 0, b=0 cout=0 sum=0 5a = 1, b=0 cout=0 sum=1 8a = 1, b=1 cout=1 sum=0 12a = 0, b=1 cout=0 sum=1 64a = 0, b=0 cout=0 sum=0 "tbench.v": $finish at simulation time 134

14 FSM Example: Car speed accelerator brake clock medium lowstopped high a: accelerator b: brake a = 1, b = 0 b = 1 a = 1, b = 0

15 Behavioral Description module car(speed, a, b, clock); inputa, b, clock; output[1:0] speed; reg[1:0] speed; parameterstopped = 2’b00; parameterfast = 2’b11; clock or b) begin if (b == 1 && speed != stopped) speed = speed – 1; else if (b == 0 && a == 1 && speed != fast) speed = speed + 1; end endmodule

16 Where to Get Verilog CD in the back of the book, if you have Verilog version Icarus Verilog