Always if

From HDLBits


An if statement usually creates a 2-to-1 multiplexer, selecting one input if the condition is true, and the other input if the condition is false.

Always if mux.png
always @(*) begin
    if (condition) begin
        out = x;
    else begin
        out = y;

This is equivalent to using a continuous assignment with a conditional operator:

assign out = (condition) ? x : y;

However, the procedural if statement provides a new way to make mistakes. The circuit is combinational only if out is always assigned a value.

A bit of practice

Build a 2-to-1 mux that chooses between a and b. Choose b if both sel_b1 and sel_b2 are true. Otherwise, choose a. Do the same twice, once using assign statements and once using a procedural if statement.

Module Declaration

// synthesis verilog_input_version verilog_2001
module top_module(
    input a,
    input b,
    input sel_b1,
    input sel_b2,
    output wire out_assign,
    output reg out_always   ); 

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