If statement

Introduction of if statement

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 @(*) begin
    if (condition) begin
        out = x;
    end
    else begin
        out = y;
    end
end

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.

Practice

Problem statement

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.

sel_b1	sel_b2	out_assign out_always
0	0	a
0	1	a
1	0	a
1	1	b

// 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   ); 

endmodule

Solution

// 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   ); 
    assign out_assign=(sel_b1&sel_b2==1) ? b:a;
    always @(*) begin
        if (sel_b1&sel_b2==1) begin
            out_always=b;
        end
        else begin
            out_always=a;
        end
    end
endmodule