Selected assignment (with-select) in VHDL

Introduction

Selected assignment lets you choose a value for a signal based on different conditions in a clear and simple way.

When you want to set a signal to different values depending on another signal's value.

When you have multiple options and want to pick one cleanly without many if-else statements.

When designing hardware logic that depends on a control signal to select outputs.

When you want your code to be easy to read and maintain by clearly showing choices.

Syntax

VHDL

with <selector_signal> select
    <target_signal> <= <value1> when <choice1>,
                      <value2> when <choice2>,
                      ...
                      <default_value> when others;

The selector_signal is the signal you check to decide the value.

The target_signal is the signal you assign a value to based on the selector.

Examples

This example sets out to different 4-bit values depending on the 2-bit sel signal.

VHDL

with sel select
  out <= "0001" when "00",
         "0010" when "01",
         "0100" when "10",
         "1000" when others;

Here, led turns on only if mode is "on"; otherwise, it is off.

VHDL

with mode select
  led <= '1' when "on",
         '0' when others;

Sample Program

This VHDL code uses selected assignment to set the 4-bit output out based on the 2-bit input sel. It picks one of four values depending on sel.

VHDL

library ieee;
use ieee.std_logic_1164.all;

entity selected_assign is
  port(
    sel : in std_logic_vector(1 downto 0);
    out : out std_logic_vector(3 downto 0)
  );
end selected_assign;

architecture behavior of selected_assign is
begin
  with sel select
    out <= "0001" when "00",
           "0010" when "01",
           "0100" when "10",
           "1000" when others;
end behavior;

Important Notes

Always include a when others clause to cover unexpected values.

Selected assignment is combinational logic; it updates output immediately when selector changes.

Summary

Selected assignment chooses a value based on a selector signal.

It is cleaner than many if-else statements for multiple choices.

Always use when others to handle all cases.

Practice

1. What is the main purpose of a with-select statement in VHDL?

easy

A. To create a loop that repeats a set of statements

B. To declare a new signal in the architecture

C. To assign a value to a signal based on the value of another signal

D. To define a new data type

Solution

1. Step 1: Understand the role of with-select

   The with-select statement chooses a value for a signal depending on another signal's value.

2. Step 2: Compare with other options

   Loops, declarations, and data types are unrelated to with-select.

3. Final Answer:

   To assign a value to a signal based on the value of another signal -> Option C

4. Quick Check:

   Selected assignment = conditional signal assignment [OK]

Hint: Remember: with-select picks signal values based on another signal [OK]

Common Mistakes:

• Confusing with loops or process statements
• Thinking it declares signals
• Mixing with data type definitions

2. Which of the following is the correct syntax for a with-select assignment in VHDL?

easy

A. with sel select out = "00" when "00", "01" when "01", others => "11";

B. with sel select out <= "00" when "00", "01" when "01", others => "11";

C. with sel select out <= "00" when => "00", "01" when => "01", others => "11";

D. with sel select out <= "00" when "00" "01" when "01", others => "11";

Solution

1. Step 1: Check the syntax format

   The correct syntax uses commas between choices and when others => for default.

2. Step 2: Identify the correct arrow symbol

   The assignment uses <= and the choices use => after when.

3. Final Answer:

   with sel select out <= "00" when "00", "01" when "01", others => "11"; -> Option B

4. Quick Check:

   Correct syntax uses commas and 'others =>' [OK]

Hint: Look for commas between choices and 'others =>' for default [OK]

Common Mistakes:

• Using wrong arrow symbols
• Missing commas between choices
• Omitting 'others =>' clause

3. Given the following VHDL code:

with sel select
  out <= "00" when "00",
         "01" when "01",
         "10" when "10",
         others => "11";

sel = "10";

What will be the value of out?

medium

A. "10"

B. "01"

C. "00"

D. "11"

Solution

1. Step 1: Match sel value with choices

   The value of sel is "10", which matches the third choice.

2. Step 2: Determine assigned value

   When sel is "10", out is assigned "10".

3. Final Answer:

   "10" -> Option A

4. Quick Check:

   sel = "10" means out = "10" [OK]

Hint: Match sel value exactly to get out value [OK]

Common Mistakes:

• Choosing default 'others' instead of exact match
• Confusing when clauses
• Ignoring quotes around values

4. Identify the error in this with-select statement:

with sel select
  out <= "00" when "00",
         "01" when "01",
         "10" when "10";

medium

A. Missing semicolon at the end

B. Incorrect use of commas between choices

C. Wrong assignment operator used

D. Missing when others => clause

Solution

1. Step 1: Check completeness of choices

   The with-select must cover all possible values, including unexpected ones.

2. Step 2: Identify missing default case

   There is no when others => clause to cover unmatched values.

3. Final Answer:

   Missing when others => clause -> Option D

4. Quick Check:

   Always include 'when others =>' in with-select [OK]

Hint: Always add 'when others =>' to cover all cases [OK]

Common Mistakes:

• Forgetting the default 'others' clause
• Thinking semicolon is missing
• Misusing commas or arrows

5. You want to assign a 2-bit output signal out based on a 2-bit input sel using with-select. The mapping is:
- "00" -> "11"
- "01" -> "10"
- "10" -> "01"
For any other value, out should be "00".

Which of the following with-select statements correctly implements this?

hard

A. with sel select
out <= "11" when "00",
"10" when "01",
"01" when "10",
others => "00";

B. with sel select
out <= "00" when "00",
"01" when "01",
"10" when "10",
others => "11";

C. with sel select
out <= "11" when "00",
"10" when "01",
"01" when "10";

D. with sel select
out <= "11" when "00",
"10" when "01",
"01" when "10",
others => "11";

Solution

1. Step 1: Match each sel value to correct out

   "00" maps to "11", "01" to "10", "10" to "01" as given.

2. Step 2: Check the default others value

   For any other sel, out should be "00".

3. Step 3: Verify options

   with sel select out <= "11" when "00", "10" when "01", "01" when "10", others => "00"; matches all mappings and default correctly.

4. Final Answer:

   with sel select out <= "11" when "00", "10" when "01", "01" when "10", others => "00"; -> Option A

5. Quick Check:

   Correct mapping + others = "00" [OK]

Hint: Check each mapping and default carefully in with-select [OK]

Common Mistakes:

• Missing 'others' clause
• Swapping output values
• Assigning wrong default value