Hierarchical Identifiers

Hierarchical path names are based on the top module identifier followed by module instant identifiers, separated by periods.

This is useful basically when we want to see the signal inside a lower module, or want to force a value inside an internal module. The example below shows how to monitor the value of an internal module signal.
Example
1 //-----------------------------------------------------

2 // This is simple adder Program

3 // Design Name : adder_hier

4 // File Name : adder_hier.v

5 // Function : This program shows verilog hier path works

6 // Coder : Deepak

7 //-----------------------------------------------------

8 `include "addbit.v"

9 module adder_hier (

10 result , // Output of the adder

11 carry , // Carry output of adder

12 r1 , // first input

13 r2 , // second input

14 ci // carry input

15 );

17 // Input Port Declarations

18 input [3:0] r1 ;

19 input [3:0] r2 ;

20 input ci ;

22 // Output Port Declarations

23 output [3:0] result ;

24 output carry ;

26 // Port Wires

27 wire [3:0] r1 ;

28 wire [3:0] r2 ;

29 wire ci ;

30 wire [3:0] result ;

31 wire carry ;

33 // Internal variables

34 wire c1 ;

35 wire c2 ;

36 wire c3 ;

38 // Code Starts Here

39 addbit u0 (r1[0],r2[0],ci,result[0],c1);

40 addbit u1 (r1[1],r2[1],c1,result[1],c2);

41 addbit u2 (r1[2],r2[2],c2,result[2],c3);

42 addbit u3 (r1[3],r2[3],c3,result[3],carry);

44 endmodule // End Of Module adder

46 module tb();

48 reg [3:0] r1,r2;

49 reg ci;

50 wire [3:0] result;

51 wire carry;

53 // Drive the inputs

54 initial begin

55 r1 = 0;

56 r2 = 0;

57 ci = 0;

58 #10 r1 = 10;

59 #10 r2 = 2;

60 #10 ci = 1;

61 #10 $display("+--------------------------------------------------------+");

62 $finish;

63 end

65 // Connect the lower module

66 adder_hier U (result,carry,r1,r2,ci);

68 // Hier demo here

69 initial begin

70 $display("+--------------------------------------------------------+");

71 $display("| r1 | r2 | ci | u0.sum | u1.sum | u2.sum | u3.sum |");

72 $display("+--------------------------------------------------------+");

73 $monitor("| %h | %h | %h | %h | %h | %h | %h |",

74 r1,r2,ci, tb.U.u0.sum, tb.U.u1.sum, tb.U.u2.sum, tb.U.u3.sum);

75 end

77 endmodule
+--------------------------------------------------------+

| r1 | r2 | ci | u0.sum | u1.sum | u2.sum | u3.sum |

+--------------------------------------------------------+

| 0 | 0 | 0 | 0 | 0 | 0 | 0 |

| a | 0 | 0 | 0 | 1 | 0 | 1 |

| a | 2 | 0 | 0 | 0 | 1 | 1 |

| a | 2 | 1 | 1 | 0 | 1 | 1 |

+--------------------------------------------------------+
Data Types
Verilog Language has two primary data types:

Nets - represent structural connections between components.
Registers - represent variables used to store data.

Every signal has a data type associated with it:

Explicitly declared with a declaration in your Verilog code.
Implicitly declared with no declaration when used to connect structural building blocks in your code. Implicit declaration is always a net type "wire" and is one bit wide.

Types of Nets

Each net type has a functionality that is used to model different types of hardware (such as PMOS, NMOS, CMOS, etc)

Net Data Type	Functionality
wire, tri	Interconnecting wire - no special resolution function
wor, trior	Wired outputs OR together (models ECL)
wand, triand	Wired outputs AND together (models open-collector)
tri0, tri1	Net pulls-down or pulls-up when not driven
supply0, supply1	Net has a constant logic 0 or logic 1 (supply strength)
trireg	Retains last value, when driven by z (tristate).