Assign statement for continuous logic in Verilog - Step-by-Step Execution

Concept Flow - Assign statement for continuous logic

Start

↓

Declare wire

↓

Assign expression

↓

Continuous update

↓

Output reflects input changes

↓

End

The assign statement continuously drives a wire with the value of an expression, updating output whenever inputs change.

Execution Sample

Verilog

wire out;
assign out = a & b;
// out updates whenever a or b changes

This code continuously assigns the AND of inputs a and b to wire out.

Execution Table

Step	Inputs (a,b)	Assign Expression	out Value	Explanation
1	a=0, b=0	a & b	0	Both inputs 0, AND is 0
2	a=0, b=1	a & b	0	One input 0, AND is 0
3	a=1, b=0	a & b	0	One input 0, AND is 0
4	a=1, b=1	a & b	1	Both inputs 1, AND is 1
5	a=0, b=1	a & b	0	Input changed, output updates to 0

💡 Simulation ends after input changes; assign continuously updates output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
a	0	0	0	1	1	0
b	0	0	1	0	1	1
out	x	0	0	0	1	0

Key Moments - 2 Insights

Why does the output 'out' change immediately when inputs 'a' or 'b' change?

Because the assign statement continuously drives 'out' with the expression 'a & b', so any input change updates 'out' instantly, as shown in execution_table steps 4 and 5.

Is 'out' a variable or a wire, and why?

'out' must be declared as a wire because assign statements drive wires continuously, not variables. This is shown in the code sample where 'wire out;' is declared.

Visual Quiz - 3 Questions

Look at the execution_table at Step 4, what is the value of 'out'?

A x (unknown)

B 0

C 1

D Depends on previous value

💡 Hint

Check the 'out Value' column at Step 4 in the execution_table.

At which step does the output 'out' first become 1?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'out Value' column in the execution_table to find when it changes to 1.

If input 'a' stayed 0 and 'b' changed from 0 to 1, what would 'out' be?

A 0

B 1

C x (unknown)

D Depends on previous output

💡 Hint

Refer to Step 2 in execution_table where a=0 and b=1, and see the 'out Value'.

Concept Snapshot

assign statement syntax:
assign <wire> = <expression>;
- Continuously drives the wire with expression value
- Output updates immediately when inputs change
- Used only with wires, not variables
- Enables combinational logic in Verilog

Full Transcript

The assign statement in Verilog continuously drives a wire with the value of an expression. Whenever any input in the expression changes, the output wire updates immediately to reflect the new value. For example, if we assign 'out' as the AND of inputs 'a' and 'b', 'out' will be 1 only when both 'a' and 'b' are 1. The execution table shows step-by-step how 'out' changes as inputs change. This continuous assignment is fundamental for modeling combinational logic in Verilog.

Practice

1.

What is the main purpose of the assign statement in Verilog?

easy

A. To declare a register variable

B. To continuously drive a wire with a logic expression

C. To create a clock signal

D. To define a module interface

Solution

1. Step 1: Understand the role of assign

   The assign statement is used to drive wires continuously with logic expressions.

2. Step 2: Differentiate from other statements

   It is not used for declaring registers, clocks, or module interfaces.

3. Final Answer:

   To continuously drive a wire with a logic expression -> Option B

4. Quick Check:

   assign drives wires continuously = B [OK]

Hint: Assign always drives wires with logic expressions [OK]

Common Mistakes:

• Confusing assign with register declaration
• Thinking assign creates clock signals
• Using assign inside always blocks

2.

Which of the following is the correct syntax for a continuous assignment in Verilog?

?

easy

A. assign out = a & b;

B. assign out := a & b;

C. assign out == a & b;

D. assign out => a & b;

Solution

1. Step 1: Recall correct assign syntax

   The correct syntax uses a single equals sign: assign out = expression;

2. Step 2: Check each option

   assign out = a & b; uses = correctly. Options B, C, and D use invalid operators for assignment.

3. Final Answer:

   assign out = a & b; -> Option A

4. Quick Check:

   Assign uses = for continuous assignment = A [OK]

Hint: Assign uses single =, not := or == [OK]

Common Mistakes:

• Using := like in other languages
• Using == which is comparison, not assignment
• Using => which is invalid here

3.

Given the code below, what is the value of out when a=1 and b=0?

wire a, b, out;
assign out = a | b;

medium

A. Z (high impedance)

B. 0

C. 1

D. X (unknown)

Solution

1. Step 1: Understand the assign expression

   The output out is assigned the OR of a and b.

2. Step 2: Calculate OR for given inputs

   With a=1 and b=0, 1 | 0 = 1.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   1 OR 0 = 1 [OK]

Hint: OR of 1 and 0 is 1 [OK]

Common Mistakes:

• Confusing OR with AND operation
• Assuming output is unknown or high impedance
• Forgetting assign updates continuously

4.

Identify the error in the following Verilog code snippet:

wire a, b, out;
assign out = a & b

medium

A. Missing semicolon at the end of the assign statement

B. Incorrect use of wire keyword

C. Assign statement cannot use & operator

D. Output out should be declared as reg

Solution

1. Step 1: Check syntax of assign statement

   Every assign statement must end with a semicolon.

2. Step 2: Identify missing semicolon

   The code misses a semicolon after a & b, causing syntax error.

3. Final Answer:

   Missing semicolon at the end of the assign statement -> Option A

4. Quick Check:

   Assign statements require semicolon = C [OK]

Hint: Always end assign statements with semicolon [OK]

Common Mistakes:

• Omitting semicolon at end of assign
• Thinking out must be reg for assign
• Believing & operator is invalid in assign

5.

How would you use the assign statement to create a 2-input XOR gate output out from inputs a and b?

hard

A. assign out = ~(a ^ b);

B. assign out = a & b;

C. assign out = a | b;

D. assign out = a ^ b;

Solution

1. Step 1: Recall XOR operator in Verilog

   The XOR operator is ^ and produces 1 when inputs differ.

2. Step 2: Write assign statement for XOR

   Use assign out = a ^ b; to continuously assign XOR result.

3. Final Answer:

   assign out = a ^ b; -> Option D

4. Quick Check:

   XOR uses ^ operator = D [OK]

Hint: XOR operator is ^ in assign statements [OK]

Common Mistakes:

• Using & or | instead of ^ for XOR
• Negating XOR result mistakenly
• Trying to assign XOR inside always block only