Blocking vs non-blocking assignment in Verilog

Introduction

Blocking and non-blocking assignments control how values update in Verilog code. They help decide if statements run one after another or all at once.

When writing combinational logic where order matters, use blocking assignments.

When writing sequential logic like flip-flops, use non-blocking assignments.

When you want to avoid race conditions in hardware simulation.

When modeling circuits that update all signals simultaneously.

When debugging timing issues in your design.

Syntax

Verilog

blocking assignment: variable = expression;
non-blocking assignment: variable <= expression;

Blocking assignments use the '=' sign and execute statements in order.

Non-blocking assignments use the '<=' sign and update variables after all right-hand sides are evaluated.

Examples

Shows the difference in syntax between blocking and non-blocking assignments.

Verilog

a = b + c;  // blocking assignment
x <= y + z; // non-blocking assignment

Blocking assignments used in combinational logic inside an always block.

Verilog

always @(*) begin
  sum = a + b;  // blocking
  carry = (a & b);
end

Non-blocking assignment used in sequential logic triggered by clock edge.

Verilog

always @(posedge clk) begin
  q <= d;  // non-blocking
end

Sample Program

This program shows how blocking assignments update variables immediately and in order, while non-blocking assignments update all variables together after the block finishes.

Verilog

module test;
  reg a, b, c;
  reg x, y, z;

  initial begin
    a = 1; b = 0; c = 1;
    x = 0; y = 1; z = 0;

    // Blocking assignments
    a = b;
    b = c;
    c = a;

    $display("After blocking: a=%b b=%b c=%b", a, b, c);

    // Reset values
    a = 1; b = 0; c = 1;

    // Non-blocking assignments
    a <= b;
    b <= c;
    c <= a;

    #1; // wait for non-blocking updates
    $display("After non-blocking: a=%b b=%b c=%b", a, b, c);
  end
endmodule

Important Notes

Blocking assignments run one after another, so the next line sees the updated value.

Non-blocking assignments schedule updates to happen later, so all right sides use old values.

Use blocking for combinational logic and non-blocking for sequential logic to avoid bugs.

Summary

Blocking assignments use '=' and run statements in order, updating variables immediately.

Non-blocking assignments use '<=' and update variables after evaluating all right sides.

Choose blocking for combinational logic and non-blocking for sequential logic to model hardware correctly.

Practice

1. Which of the following statements correctly describes the difference between blocking and non-blocking assignments in Verilog?

easy

A. Blocking assignments are used only in testbenches, non-blocking only in synthesis.

B. Blocking assignments use '<=' and update variables after all right sides are evaluated, while non-blocking assignments use '=' and update variables immediately.

C. Blocking assignments use '=' and update variables immediately, while non-blocking assignments use '<=' and update variables after all right sides are evaluated.

D. Both blocking and non-blocking assignments use '=' but differ in execution speed.

Solution

1. Step 1: Understand blocking assignment behavior

   Blocking assignments use '=' and execute statements in order, updating variables immediately.

2. Step 2: Understand non-blocking assignment behavior

   Non-blocking assignments use '<=' and update variables only after all right-hand sides are evaluated, allowing parallel updates.

3. Final Answer:

   Blocking assignments use '=' and update variables immediately, while non-blocking assignments use '<=' and update variables after all right sides are evaluated. -> Option C

4. Quick Check:

   Blocking = immediate, Non-blocking = delayed update [OK]

Hint: Remember '=' blocks, '<=' schedules update later [OK]

Common Mistakes:

• Confusing '=' and '<=' usage
• Thinking both update variables immediately
• Assuming non-blocking is only for testbenches

2. Which of the following is the correct syntax for a non-blocking assignment in Verilog?

easy

A. reg_var = value;

B. reg_var <= value;

C. reg_var := value;

D. reg_var == value;

Solution

1. Step 1: Identify non-blocking assignment operator

   Non-blocking assignments use the '<=' operator in Verilog.

2. Step 2: Check syntax correctness

   reg_var <= value; uses '<=' correctly, while others use invalid or different operators.

3. Final Answer:

   reg_var <= value; -> Option B

4. Quick Check:

   Non-blocking uses '<=' [OK]

Hint: Non-blocking always uses '<=' operator [OK]

Common Mistakes:

• Using '=' instead of '<=' for non-blocking
• Confusing ':=' or '==' as assignment
• Syntax errors with missing semicolons

3. Consider the following Verilog code snippet inside an always block triggered on the positive edge of clk:

reg a, b, c;
always @(posedge clk) begin
  a = b;
  b <= c;
end

What will be the value of 'a' and 'b' after the clock edge if initially b=0 and c=1?

medium

A. a = 0, b = 1

B. a = 1, b = 1

C. a = 1, b = 0

D. a = 0, b = 0

Solution

1. Step 1: Evaluate blocking assignment 'a = b;'

   Blocking assignment updates 'a' immediately with current 'b' value, which is 0.

2. Step 2: Evaluate non-blocking assignment 'b <= c;'

   Non-blocking assignment schedules 'b' to update after the block with 'c' value, which is 1.

3. Final Answer:

   a = 0, b = 1 -> Option A

4. Quick Check:

   Blocking immediate, non-blocking delayed update [OK]

Hint: Blocking updates now; non-blocking updates after block ends [OK]

Common Mistakes:

• Assuming both update simultaneously
• Confusing initial values of variables
• Ignoring order of assignments

4. Identify the error in the following Verilog code snippet:

always @(posedge clk) begin
  reg1 <= reg2;
  reg2 = reg1;
end

medium

A. Using blocking assignment for reg2 causes incorrect sequential behavior.

B. Non-blocking assignment cannot be used inside always blocks.

C. Missing semicolon after reg2 assignment.

D. reg1 and reg2 must be wires, not regs.

Solution

1. Step 1: Analyze assignment types in sequential block

   In sequential always blocks, non-blocking assignments should be used for all registers to avoid race conditions.

2. Step 2: Identify blocking assignment misuse

   Using blocking assignment '=' for reg2 causes immediate update, which can cause incorrect behavior in sequential logic.

3. Final Answer:

   Using blocking assignment for reg2 causes incorrect sequential behavior. -> Option A

4. Quick Check:

   Sequential logic needs non-blocking assignments [OK]

Hint: Use non-blocking for all regs in sequential blocks [OK]

Common Mistakes:

• Thinking blocking is allowed in sequential always blocks
• Ignoring race conditions caused by mixed assignments
• Assuming wires can be assigned in always blocks

5. You want to model a flip-flop with asynchronous reset in Verilog. Which assignment style correctly models the sequential behavior inside the always block?

always @(posedge clk or posedge reset) begin
  if (reset)
    q = 0;
  else
    q <= d;
end

What is the problem with this code?

hard

A. Reset should use non-blocking '<=' and data should use blocking '=' assignments.

B. The sensitivity list is incorrect; it should not include reset.

C. Both assignments should be blocking '=' for correct flip-flop behavior.

D. Mixing blocking '=' for reset and non-blocking '<=' for data causes simulation mismatches.

Solution

1. Step 1: Understand assignment consistency in sequential blocks

   Sequential logic should use non-blocking assignments consistently to avoid simulation and synthesis mismatches.

2. Step 2: Identify mixed assignment problem

   Using blocking '=' for reset and non-blocking '<=' for data causes timing mismatches and incorrect behavior.

3. Final Answer:

   Mixing blocking '=' for reset and non-blocking '<=' for data causes simulation mismatches. -> Option D

4. Quick Check:

   Use non-blocking consistently in sequential logic [OK]

Hint: Use non-blocking for all sequential assignments including reset [OK]

Common Mistakes:

• Mixing blocking and non-blocking in same always block
• Ignoring asynchronous reset in sensitivity list
• Assuming blocking '=' is fine for reset