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Placing values onto nets and variables are called assignments. There are three basic forms:

Legal LHS values

An assignment has two parts - right-hand side (RHS) and left-hand side (LHS) with an equal symbol (=) or a less than-equal symbol (<=) in between.

Assignment type Left-hand side
Procedural
  • Variables (vector/scalar)
  • Bit-select or part-select of a vector reg, integer or time variable
  • Memory word
  • Concatenation of any of the above
Continuous
  • Net (vector/scalar)
  • Bit-select or part-select of a vector net
  • Concatenation of bit-selects and part-selects
Procedural Continous
  • Net or variable (vector/scalar)
  • Bit-select or part-select of a vector net

The RHS can contain any expression that evaluates to a final value while the LHS indicates a net or a variable to which the value in RHS is being assigned.

 
module tb;
  reg clk;
  wire a, b, c, d, e, f;
  reg  z, y;
 
  // clk is on the LHS and the not of clk forms RHS
  always #10 clk = ~clk;
 
  // y is the LHS and the constant 1 is RHS
  assign y = 1; 
 
  // f is the LHS, and the expression of a,b,d,e forms the RHS
  assign f = (a | b) ^ (d & e);
 
  always @ (posedge clk) begin
    // z is the LHS, and the expression of a,b,c,d forms the RHS
    z <= a + b + c + d;
  end
 
  initial begin
    // Variable names on the left form LHS while 0 is RHS
    a <= 0; b <= 0; c <= 0; d <= 0; e <= 0;
    clk <= 0;
  end
endmodule
 

Procedural Assignment

Procedural assignments occur within procedures such as always, initial, task and functions and are used to place values onto variables. The variable will hold the value until the next assignment to the same variable.

The value will be placed onto the variable when the simulation executes this statement at some point during simulation time. This can be controlled and modified the way we want by the use of control flow statements such as if-else-if, case statement and looping mechanisms.

 
  reg [7:0]  data;
  integer    count;
  real       period;
 
  initial begin
    data = 8'h3e;
    period = 4.23;
    count = 0;
  end
 
  always @ (posedge clk) 
    count++;
 

Variable declaration assignment

An initial value can be placed onto a variable at the time of its declaration as shown next. The assignment does not have a duration and holds the value until the next assignment to the same variable happens. Note that variable declaration assignments to an array are not allowed.

 
  module my_block;
    reg [31:0] data = 32'hdead_cafe;
 
    initial begin
      #20  data = 32'h1234_5678;    // data will have dead_cafe from time 0 to time 20
                                    // At time 20, data will get 12345678
    end
  endmodule
 
 
  reg [3:0] a = 4'b4;
 
  // is equivalent to 
 
  reg [3:0] a;
  initial a = 4'b4;
 

If the variable is initialized during declaration and at time 0 in an initial block as shown below, the order of evaluation is not guaranteed, and hence can have either 8'h05 or 8'hee.

 
  module my_block;
    reg [7:0]  addr = 8'h05;
 
    initial 
      addr = 8'hee;
  endmodule
 
 
  reg [3:0] array [3:0] = 0;           // illegal
  integer i = 0, j;                    // declares two integers i,j and i is assigned 0
  real r2 = 4.5, r3 = 8;               // declares two real numbers r2,r3 and are assigned 4.5, 8 resp.
  time startTime = 40;                 // declares time variable with initial value 40
 

Procedural blocks and assignments will be covered in more detail in a later section.

Continuous Assignment

Click here for a step-by-step simulation example !

This is used to assign values onto scalar and vector nets and happens whenever there is a change in the RHS. It provides a way to model combinational logic without specifying an interconnection of gates and makes it easier to drive the net with logical expressions.

 
    // Example model of an AND gate
  wire  a, b, c;
 
  assign a = b & c;
 

Whenever b or c changes its value, then the whole expression in RHS will be evaluated and a will be updated with the new value.

Net declaration assignment

This allows us to place a continuous assignment on the same statement that declares the net. Note that because a net can be declared only once, only one declaration assignment is possible for a net.

 
  wire  penable = 1;
 

Procedural Continuous Assignment

These are procedural statements that allow expressions to be continuously assigned to nets or variables and are of two types.

  • assign ... deassign
  • force ... release

assign deassign

This will override all procedural assignments to a variable and is deactivated by using the same signal with deassign. The value of the variable will remain same until the variable gets a new value through a procedural or procedural continuous assignment. The LHS of an assign statement cannot be a bit-select, part-select or an array reference but can be a variable or a concatenation of variables.

 
  reg q;
 
  initial begin
    assign q = 0;
    #10 deassign q;
  end
 

force release

These are similar to the assign - deassign statements but can also be applied to nets and variables. The LHS can be a bit-select of a net, part-select of a net, variable or a net but cannot be the reference to an array and bit/part select of a variable. The force statment will override all other assignments made to the variable until it is released using the release keyword.

 
  reg o, a, b;
 
  initial begin
    force o = a & b;
    ...
    release o;
  end
 

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