With Select VHDL?

Question

In VHDL, what is with select, and what is the proper syntax?

Answer 1

VHDL With Select

Quick Syntax

with mux_sel select output <=
  input1 when '0',
  input2 when '1',
  (others => '0') when others;

Purpose

The most common use for the with select statement is to a create a multiplexer, or MUX. The with is in reference to a select signal, which then determines the selection of the output signal, as the input options are then laid out given specific selects.

I usually never come across a with select statement. Frankly, the when else statement is so much easier to code, read, and maintain that it's used 99.9% of the time over with select.

Perhaps there is an advantage of with select, but I have never been able to come up with any. My recommendation is to use when else instead.

Best Practices

1. With select statements are great for creating a MUX.

2. Make sure that all of the select signal possibilities are covered. It's a good idea to also cover a when others as a catch all just in case.

3. When else is a better option since it's much easier to read for everyone.

Answer 2

Example :

signal selector : std_logic_vector ( 1 downto 0 ) ;
signal output : std_logic_vector( 3 downto 0 ) ;

with selector select
output <= 
  "0001" when "00" ,
  "0010" when "01" ,
  "0100" when "10" ,
  "1000" when "11" ;

Answer 3

This is a concurrent conditional signal assignment statement. While the concurrent assignment "when-else" is a kind of if-else the assignment "with-select" is like a case statement. I think it has better readability when a choice is more than a few inputs.

with state select result <=
  in0 when state_0,
  in1 when state_1,
  in2 when state_2,
  in3 when others;