Verilog Assign Statement: The Backbone of Combinational Logic

The assign statement is a fundamental building block in Verilog for defining combinational logic. It continuously assigns a value to a target signal based on an expression involving other signals. This allows you to describe the logic behavior of your design without explicitly specifying the hardware implementation.

Key Features of Assign Statements:

• Continuously update: The assigned value is continuously updated whenever any of the input signals change.
• Combinational logic: The assign statement defines combinational logic, meaning the output depends only on the current values of the input signals, not on any previous states.
• Target signal: The left side of the assign statement specifies the target signal, which is typically a wire or a variable of a wire-like data type.
• Expression: The right side of the assign statement defines an expression that determines the value assigned to the target signal. This expression can involve various operators, including logical operators, arithmetic operators, and functions.

Benefits of Using Assign Statements:

• Conciseness: Assign statements offer a concise and readable way to define combinational logic. They eliminate the need for explicit logic gates and simplify the code.
• Modularization: Assign statements can be combined hierarchically to build complex logic circuits. This modularity helps maintain and reuse code.
• Simulation: Assign statements facilitate efficient simulation of combinational logic circuits. They provide clear points for observing and controlling the behavior of your design.

Examples:

1. Basic AND operation:

assign out = a & b;

This assigns the AND operation of signals a and b to the output signal out.

2. Multiplexer:

assign y = sel ? a : b;

This implements a multiplexer where the output y is assigned a if the select signal sel is high, and b otherwise.

3. Constant assignment:

assign pi = 3.14159;

This assigns the constant value of pi to the signal pi.

4. Vector assignment:

assign bus[3:0] = {a, b, c, d};

This assigns the four-bit vector formed by concatenating signals a, b, c, and d to the four-bit bus bus.

5. Conditional assignment:

assign out = (a > b) ? a : b; // Assign the greater of a and b to out

This assigns the greater of the signals a and b to the output signal out.

Verilog Assign Examples

Here are some examples of Verilog assign statements with different functionalities:

1. Basic AND operation:

<span class="hljs-keyword">assign</span> out = a & b;

This assigns the AND operation of signals a and b to the output signal out.

2. Multi-bit AND operation:

<span class="hljs-keyword">assign</span> [<span class="hljs-number">3</span>:<span class="hljs-number">0</span>] d = a & b;

This assigns the bitwise AND operation of four-bit signals a and b to the four-bit output signal d.

3. OR operation:

<span class="hljs-keyword">assign</span> y = x | sel;

This assigns the OR operation of signals x and sel to the output signal y.

4. NOT operation:

<span class="hljs-keyword">assign</span> enable = !reset;

This assigns the NOT operation of signal reset to the output signal enable.

5. Multiplexer:

<span class="hljs-keyword">assign</span> z = sel ? data1 : data2;

This assigns the value of data1 to the output signal z if the select signal sel is high, and the value of data2 otherwise.

6. Constant assignment:

<span class="hljs-keyword">assign</span> PI = <span class="hljs-number">3</span><span class="hljs-variable">.14159</span>;

This assigns the constant value of pi to the output signal PI.

7. Vector assignment:

<span class="hljs-keyword">assign</span> bus = {a, b, c, d};

This assigns a four-bit vector formed by concatenating signals a, b, c, and d to the four-bit bus bus.

8. Conditional assignment:

<span class="hljs-keyword">assign</span> max = a > b ? a : b;

This assigns the greater of signals a and b to the output signal max.

9. Case statement:

<span class="hljs-keyword">assign</span> y = <span class="hljs-keyword">case</span>(sel)
    <span class="hljs-number">2'b00</span>: data0;
    <span class="hljs-number">2'b01</span>: data1;
    <span class="hljs-number">2'b10</span>: data2;
    <span class="hljs-keyword">default</span>: <span class="hljs-number">4'bx</span>;
<span class="hljs-keyword">endcase</span>;

This assigns different values to the output signal y based on the value of the select signal sel.

10. Procedural continuous assignment:

<span class="hljs-keyword">always_comb</span> <span class="hljs-keyword">begin</span>
  <span class="hljs-keyword">if</span> (a > b) <span class="hljs-keyword">begin</span>
    out = a;
  <span class="hljs-keyword">end</span> <span class="hljs-keyword">else</span> <span class="hljs-keyword">begin</span>
    out = b;
  <span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>

This procedural continuous assignment achieves the same functionality as example 9 but uses an always block with an if-else statement.