 A test bench is an HDL program used for applying stimulus to an HDL design in order to test it and observe its response during simulation.  In addition to the always statement, test benches use the initial statement to provide a stimulus to the circuit under test.  The always statement executes repeatedly in a loop. The initial statement executes only once starting from simulation time=0 and may continue with any operations that are delayed by a given number of units as specified by the symbol #.

 A stimulus module is an HDL program that has the following form. module testname Declare local reg and wire identifiers Instantiate the design module under test. Generate stimulus using initial and always statements Display the output response. endmodule  A test module typically has no inputs or outputs.  The signals that are applied as inputs to the design module for simulation are declared in the stimulus module as local reg data type.  The outputs of the design module that are displayed for testing are declared in the stimulus model as local wire data type.  The module under test is then instantiated using the local identifiers.

The stimulus model generates inputs for the design module by declaring identifiers TA and TB as reg data type, and checks the output of the design unit with the wire identifier TC. The local identifiers are then used to instantiate the design module under test.

 The response to the stimulus generated by the initial and always blocks will appear at the output of the simulator as timing diagrams.  It is also possible to display numerical outputs using Verilog system tasks. ◦ $display – display one-time value of variables or strings with end-of-line return, ◦ $write – same $display but without going to next line. ◦ $monitor – display variables whenever a value changes during simulation run. ◦ $time – displays simulation time ◦ $finish – terminates the simulation  The syntax for $display,$write and $monitor is of the form Task-name (format-specification, argument list); E.g. $display(%d %b %b, C,A,B); $display(“time = %0d A = %b B=%b”,$time,A,B);

//Dataflow description of 2-to-1-line multiplexer module mux2x1_df (A,B,select,OUT); input A,B,select; output OUT; assign OUT = select ? A : B; endmodule

//Stimulus for mux2x1_df module testmux; reg TA,TB,TS; //inputs for mux wire Y; //output from mux mux2x1_bh mx (TA,TB,TS,Y); // instantiate mux initial begin $monitor(”select=%b A=%b B=%b OUT=%b",TS,TA,TB,Y); TS = 1; TA = 0; TB = 1; #10 TA = 1; TB = 0; #10 TS = 0; #10 TA = 0; TB = 1; end endmodule

Design Module ABOut Truth Table

Design Module Test Bench A in Y

module and_2in (A, B, Out); input A, B; output Out; wire Out; assign Out = A & B; endmodule

`include “and_2in.v” `timescale 1ns/10ps module and_2in_tb; reg A, in; wire Y; and_2in a1(.A(A),.B(in),.Out(Y)); initial begin A=0; in=0; #20; A=0; in=1; #20; A=1; in=0; #20; A=1; in=1; #40; $stop; $finish; end endmodule

D clk Q Q_n reset_n Design Module Test Bench DClkreset_nQQ_n

D clk Q Q_n reset_n Design Module TestBench in out outbar clock reset

module dff (D, reset_n, clk, Q, Q_n); input D, clk, reset_n; output Q, Q_n; reg Q, Q_n; (posedge clk or negedge reset_n) begin end if(reset_n == 0) begin Q <= 0; Q_n<=1; end else begin Q <= D; Q_n <= !D; end

`include “dff.v” `timescale 1ns/10ps module dff_tb; reg in, clock, reset; wire out, outbar; dff d1(.D(in),.reset_n(reset),.clk(clock),.Q(out),.Q_n(outbar));

initial begin in =0; reset =0; clock =0; #10; reset =1; #5; in=1; #10; in=0; #40; $stop; end always #10 clock = ~clock; endmodule

 Reset the counter asynchronously through active low global reset reset_n  reset the count when the count has reached value max_count  Advance the counter through active high enable signal reset_n max_count[7:0] enable clk count[7:0]

assign reset_count = (count == max_count); /* Generates the signal reset_count which resets the counter when the count reaches value of input max_count */ assign cnt_next = count +1; // Preincrement the value of count

(posedge clk or negedge reset_n) begin if(reset_n ==0) count <= 8’d0; else if(reset_count == 1) count <= 8’d0; else if(enable ==1) count <= cnt_next; else count <= count; end

`include “counter.v” `timescale 1ns/10ps module counter_tb; reg reset_n, clk, enable; reg[7:0] max_count; wire[7:0] count; counter c1(.reset_n(reset_n),.clk(clk),.enable(enable),.max_count(max_count),.count(count) );

initial begin reset_n =0; clk =0; enable =0; max_count =100; #25; reset_n =1; enable =1; #5000; $stop; end always #10 clock =~ clock; endmodule

 Loads data set_data into the register when load signal is high  Shifts data when shift signal is high  Can perform both right and left shift operations  rl_n signal indicates whether to shift right or left. A high value shifts right and a low value shifts left.  data_valid is high whenever data is valid and eoc is asserted when transfer is complete

reset_n clk rl_n shift load set_data[15:0] data_out data_valid eoc

(posedge clk or negedge reset_n) begin if(reset_n == 0) begin data_out<=0; data_valid<=0; eoc <=0; sr <=0; cnt_reg <=0; end else if(load ==1) begin sr<= set_data; cnt_reg <=16; data_out<=0; data_valid<=0; eoc <=0; end

else if ((shift==1) && (cnt_reg !=0)) begin sr > 1) : (sr <<1); data_out <= rl_n ? sr[0] : sr[15]; cnt_reg <= cnt_reg -1; data_valid <=1; eoc<=0; end

else if ((shift==1) && (cnt_reg ==0)) begin sr <=sr; cnt_reg <=cnt_reg; data_out<=0; data_valid<=0; eoc <=1; end

else begin sr <=sr; cnt_reg <=cnt_reg; data_out<=data_out; data_valid<=data_valid; eoc <=eoc; end

module parallel_serial_tb ; reg reset_n, load, shift, rl_n, clk; reg[15:0] set_data; wire data_out, data_valid, eoc; parallel_serial p1(.reset_n(reset_n),.load(load),.shift(shift),.rl_n(rl_n),.clk(clk),. set_data(set_data),.data_out(data_out),.data_valid(data_valid),.eoc(eoc) );

initial begin reset_n =0;load=0;shift=0;rl_n=0;clk=0; set_data=0; #25; reset_n =1; set_data = 16’hAAAA; load =1; #20; load =0; shift =1; #400; shift =0; #20; rl_n =1; load=1; #20; shift=1; #500; $stop; end always #10 clk = ~clk; endmodule