How to Use With Select Statement in VHDL: Syntax and Example

In VHDL, the with select statement assigns values to a signal based on the value of a selector expression. It works like a switch-case, where you specify the selector and list possible values with their corresponding outputs, ending with others for default.

Syntax

The with select statement assigns a value to a signal depending on the value of a selector expression. It has these parts:

• Selector: The expression you check (like a variable or signal).
• Choices: Possible values the selector can have.
• Assignments: The value assigned to the target signal for each choice.
• Others: A default assignment if none of the choices match.

vhdl

with selector_signal select
    target_signal <= value1 when choice1,
                     value2 when choice2,
                     -- more choices
                     default_value when others;

Example

This example shows how to use with select to assign a 2-bit output based on a 2-bit input selector.

vhdl

library ieee;
use ieee.std_logic_1164.all;

entity with_select_example is
    port(
        sel : in std_logic_vector(1 downto 0);
        out_signal : out std_logic_vector(1 downto 0)
    );
end with_select_example;

architecture Behavioral of with_select_example is
begin
    with sel select
        out_signal <= "00" when "00",
                      "01" when "01",
                      "10" when "10",
                      "11" when others;
end Behavioral;

Common Pitfalls

Common mistakes when using with select include:

• Not covering all possible selector values, which can cause simulation warnings or mismatches.
• Forgetting the others clause, which acts as a default case.
• Using incompatible types between selector and choices.
• Trying to use with select inside a process (it must be outside processes as a concurrent statement).

vhdl

wrong_way: 
with sel select
    out_signal <= "00" when "00",
                  "01" when "01";  -- Missing others clause

right_way:
with sel select
    out_signal <= "00" when "00",
                  "01" when "01",
                  "10" when "10",
                  "11" when others;

Quick Reference

Part	Description
with selector_signal select	Start statement with selector expression
target_signal <=	Signal to assign value to
value when choice	Assign value when selector equals choice
default_value when others	Default assignment for unmatched choices

Key Takeaways

Use with select for simple conditional signal assignments based on a selector.

Always include the others clause to cover all possible selector values.

with select is a concurrent statement and cannot be used inside processes.

Ensure selector and choice values have compatible types.

It works like a clean and readable switch-case for signals in VHDL.