Writing testbenches with Verilog

Q: How to colorize Verilog keywords in an editor?

Emacs users please see: http://www.verilog.com/emacs_install.html

Q:When declaring a vector in Verilog, can we use [LSB:MSB] to represent the big-endian convention?

Vectors can be declared at [high#,low#] or [low#,high#], but the left number in the squared brackets is always the MSB of the vector. [MSB:LSB]

Both MSB constant expression and LSB constant expression shall be constant expressions of any integer value - positive, negative, or zero. LSB constant expression can be a greater, equal, or lesser value than MSB constant expression.

To use the little-endian convention, set the LSB to the smallest bit number.

output [7:0] result;

To use the big-endian convention, set the LSB to the largest bit number.

inout [0:15] data_bus;

Q: Is it possible to use a variable to specify the range of a vector

The range referencing in an expression must be a constant. Single bit can be referenced with a variable, but a vector cannot.

reg [0:7] A, B;
B = 3;
A[0:B] = 3'b111;       // illegal
A[B] = 1'b1;

The following example is, however, legal. The parameter statement allows the programmer to give constants a name and serves a similar role that the const keyword serves in C.

parameter size = 8;
reg [size - 1:0] A;       // defines an 8-bit register

Q: Why my signed number calculation returned an incorrect number?

Integer numbers are signed (two’s complement) or unsigned. Verilog only “keeps track” of the sign of a negative constant if it is

Assigned to an integer,
Assigned to a parameter without using a base

In other cases, even though the bit representations may be identical to the signed number (hexadecimal fffffff4 in the following example), a negative constant is treated as an unsigned number. Once Verilog “loses” the sign, keeping track of signed numbers becomes your responsibility.

module negative_numbers;
   parameter PA = -12, PB = -'d12, PC = -32'd12, PD = -4'd12;
   integer IA , IB , IC , ID ;
   reg [31:0] RA , RB , RC , RD ;
   initial begin #1;
      IA = -12; IB = -'d12; IC = -32'd12; ID = -4'd12;
      RA = -12; RB = -'d12; RC = -32'd12; RD = -4'd12; #1;
      $display(" parameter integer reg[31:0]");
      $display ("-12 =",PA,IA,,,RA);
      $displayh(" ",,,,PA,,,,IA,,,,,RA);
      $display ("-'d12 =",,PB,IB,,,RB);
      $displayh(" ",,,,PB,,,,IB,,,,,RB);
      $display ("-32'd12 =",,PC,IC,,,RC);
      $displayh(" ",,,,PC,,,,IC,,,,,RC);
      $display ("-4'd12 =",,,,,,,,,,PD,ID,,,RD);
      $displayh(" ",,,,,,,,,,,PD,,,,ID,,,,,RD);
   end
endmodule

Output:

verilog	parameter	integer	reg[31:0]
-12	-12	-12	4294967284
	fffffff4	fffffff4	fffffff4
-‘d12	4294967284	-12	4294967284
	fffffff4	fffffff4	fffffff4
-32’d12	4294967284	-12	4294967284
	fffffff4	fffffff4	fffffff4
-4’d12	4	-12	4294967284
	4	fffffff4	fffffff4

Q: Which loop functions are available in Verilog? How to use them?

Repetition in Verilog can be implemented via for loop, while loop, and repeat statement.

for(i=0; i < 10; i = i+1)        // No ++ or -- in Verilog
   begin
   $display("i = %0d", i);
end

i = 0;
while(i < 0)
   begin
   $display("i = %0d", i);
   i = i + 1;
end

i = 0;
repeat(10)
   begin
   $display("i = %0d", i);
   i = i + 1;
end

The above three examples all produce the same result.

Q: Is it possible to specify tasks in one module and then invoke it from other modules?

Just like the Procedure and Sub-routine in the other programming language, in Verilog, many lines of code can be grouped together and called anywhere from within that module. This is called task in Verilog. Tasks are local to modules. It must be declared outside any begin..end statements of the module, but within the module declarations. Task must be defined in the module in which they are used. It is possible to define task in a separate file - only if you do not put it in another module!

// file name: a_task.v
task add;

   input [7:0] a;
   output [7:0] b;

   begin
      b = a + 100;
   end

endtask

module add_100 (temp_a, temp_b);

   input [7:0] temp_a;
   output [7:0] temp_b;
   reg [7:0] temp_b;
   `include "a_task.v"      //

   always @ (temp_a)
      add (temp_a, temp_b);

endmodule

Q: How to call a task inside a task?

If you want to call a task inside a task, the declarations can be anywhere except being nested within a procedural block.

module nested;

   task foo;                  //declare foo
      begin
         $display("foo");
      end
   endtask

   task bar;
      begin
         $display("bar");
         foo;                  //call task foo from within another task
      end
   endtask

   initial
      bar;

endmodule

Design Verification