9.6 Summary In this chapter, we discussed the following aspects of Verilog: • Procedural continuous assignments can be used to override the assignments on registers and nets. assign and deassign can override assignments on registers. force and release can override assignments on registers and nets. assign and deassign are used in the actual design. force and release are used for debugging. • Parameters defined in a module can be overridden with the defparam statement or by passing a new value during module instantiation. During module instantiation, parameter values can be assigned by ordered list or by name. It is recommended to use parameter assignment by name. • Compilation of parts of the design can be made conditional by using the 'ifdef, 'ifndef, 'elsif, 'else, and 'endif directives. Compilation flags are defined at compile time by using the `define statement. • Execution is made conditional in Verilog simulators by means of the $test$plusargs system task. The execution flags are defined at run time by +<flag_name>. • Up to 30 files can be opened for writing in Verilog. Each file is assigned a bit in the multichannel descriptor. The multichannel descriptor concept can be used to write to multiple files. The IEEE Standard Verilog Hardware Description Language document describes more advanced ways of doing file I/O. • Hierarchy can be displayed with the %m option in any display statement. • Strobing is a way to display values at a certain time or event after all other statements in that time unit have executed. • Random numbers can be generated with the system task $random. They are used for random test vector generation. $random task can generate both positive and negative numbers. • Memory can be initialized from a data file. The data file contains addresses and data. Addresses can also be specified in memory initialization tasks. • Value Change Dump is a popular format used by many designers for debugging with postprocessing tools. Verilog allows all or selected module variables to be dumped to the VCD file. Various system tasks are available for this purpose. [ Team LiB ] [ Team LiB ] 9.7 Exercises 1: Using assign and deassign statements, design a positive edge-triggered D- flipflop with asynchronous clear (q=0) and preset (q=1). 2: Using primitive gates, design a 1-bit full adder FA. Instantiate the full adder inside a stimulus module. Force the sum output to a & b & c_in for the time between 15 and 35 units. 3: A 1-bit full adder FA is defined with gates and with delay parameters as shown below. // Define a 1-bit full adder module fulladd(sum, c_out, a, b, c_in); p arameter d_sum = 0, d_cout = 0; // I/O port declarations output sum, c_out; input a, b, c_in; // Internal nets wire s1, c1, c2; // Instantiate logic gate primitives xor (s1, a, b); and (c1, a, b); xor #(d_sum) (sum, s1, c_in); //delay on output sum is d_sum and (c2, s1, c_in); or #(d_cout) (c_out, c2, c1); //delay on output c_out is d_cout endmodule Define a 4-bit full adder fulladd4 as shown in Example 5-8 on page 77, but p ass the following parameter values to the instances, using the two methods discussed in the book: Instance Delay Values fa0 fa1 d_sum=1, d_cout=1 d_sum=2, d_cout=2 fa2 fa3 d_sum=3, d_cout=3 d_sum=4, d_cout=4 a. Build the fulladd4 module with defparam statements to change instance parameter values. Simulate the 4-bit full adder using the stimulus shown in Example 5-9 on page 77. Explain the effect of the full adder delays on the times when outputs of the adder appear. (Use delays of 20 instead of 5 used in this stimulus.) b. Build the fulladd4 with delay values passed to instances fa0, fa1, fa2, and fa3 during instantiation. Resimulate the 4-bit adder, using the stimulus above. Check if the results are identical. 4: Create a design that uses the full adder example above. Use a conditional compilation (`ifdef). Compile the fulladd4 with defparam statements if the text macro DPARAM is defined by the `define statement; otherwise, compile the fulladd4 with module instance parameter values. 5: Identify the files to which the following display statements will write: //File output with multi-channel descriptor module test; integer handle1,handle2,handle3; //file handles //open files initial begin handle1 = $fopen("f1.out"); handle2 = $fopen("f2.out"); handle3 = $fopen("f3.out"); end //Display statements to files initial begin //File output with multi-channel descriptor #5; $fdisplay(4, "Display Statement # 1"); $fdisplay(15, "Display Statement # 2"); $fdisplay(6, "Display Statement # 3"); $fdisplay(10, "Display Statement # 4"); $fdisplay(0, "Display Statement # 5"); end endmodule 6: What will be the output of the $display statement shown below? module top; A a1(); endmodule module A; B b1(); endmodule module B; initial $display("I am inside instance %m"); endmodule 7: Consider the 4-bit full adder in Example 6-4 on page 108. Write a stimulus file to do random testing of the full adder. Use a random number generator to generate a 32-bit random number. Pick bits 3:0 and apply them to input a; pick bits 7:4 and apply them to input b. Use bit 8 and apply it to c_in. Apply 20 random test vectors and observe the output. 8: Use the 8-byte memory initialization example in Example 9-14 on page 205. Modify the file to read data in hexadecimal. Write a new data file with the following addresses and data values. Unspecified locations are not initialized. Location Address Data 1 2 33 66 4 5 6 z0 0z 01 9: Write an initial block that controls the VCD file. The initial block must do the following: • Set myfile.dmp as the output VCD file. • Dump all variables two levels deep in module instance top.a1.b1.c1. • Stop dumping to VCD at time 200. • Start dumping to VCD at time 400. • Stop dumping to VCD at time 500. • Create a checkpoint. Dump the current value of all VCD variables to the dumpfile. . conditional in Verilog simulators by means of the $test$plusargs system task. The execution flags are defined at run time by +<flag_name>. • Up to 30 files can be opened for writing in Verilog. . ordered list or by name. It is recommended to use parameter assignment by name. • Compilation of parts of the design can be made conditional by using the 'ifdef, 'ifndef, 'elsif,. 9.6 Summary In this chapter, we discussed the following aspects of Verilog: • Procedural continuous assignments can be used to override the assignments on registers