When to use blocking (combinational) in Verilog

Introduction

Blocking assignments help you write simple, step-by-step logic that runs in order. They are best for describing combinational circuits where outputs change immediately when inputs change.

When you want to model combinational logic like adders or multiplexers.

When you need the assignments to happen one after another in the same block.

When you want to avoid unintended delays in your logic.

When you want to write clear and simple code for combinational circuits.

When you want to ensure outputs update immediately based on inputs.

Syntax

Verilog

always @(*) begin
  a = b + c;
  d = a & e;
end

Use '=' for blocking assignments inside always blocks.

The statements run in order, one after another.

Examples

Simple combinational logic for sum and carry using blocking assignments.

Verilog

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

Output depends immediately on inputs x and y, updated step-by-step.

Verilog

always @(*) begin
  temp = x | y;
  out = ~temp;
end

Sample Program

This example shows a combinational logic block using blocking assignment. The output y updates immediately when inputs change.

Verilog

module comb_logic(input wire a, b, c, output reg y);
  always @(*) begin
    y = (a & b) | c;
  end
endmodule

// Testbench
module test;
  reg a, b, c;
  wire y;
  comb_logic uut(a, b, c, y);
  initial begin
    a = 0; b = 0; c = 0;
    #1 $display("a=%b b=%b c=%b => y=%b", a, b, c, y);
    a = 1; b = 0; c = 0;
    #1 $display("a=%b b=%b c=%b => y=%b", a, b, c, y);
    a = 1; b = 1; c = 0;
    #1 $display("a=%b b=%b c=%b => y=%b", a, b, c, y);
    a = 0; b = 0; c = 1;
    #1 $display("a=%b b=%b c=%b => y=%b", a, b, c, y);
  end
endmodule

Important Notes

Blocking assignments run in order, so the next line sees the updated value immediately.

Use blocking assignments only for combinational logic to avoid simulation mismatches.

Do not mix blocking and non-blocking assignments in the same always block.

Summary

Use blocking assignments (=) for combinational logic inside always @(*) blocks.

They execute statements one by one, updating values immediately.

This helps model circuits where outputs depend instantly on inputs.