Always if
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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.
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.
| sel_b1 | sel_b2 | out_assign out_always |
|---|---|---|
| 0 | 0 | a |
| 0 | 1 | a |
| 1 | 0 | a |
| 1 | 1 | b |
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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