Selected assignment (with-select) in VHDL - Mini Project: Build & Apply

Selected Assignment Using with-select in VHDL

📖 Scenario: You are designing a simple digital circuit that outputs a specific 4-bit value based on a 2-bit input selector. This is a common task in digital design where you want to choose one of several values depending on a control signal.

🎯 Goal: Build a VHDL code snippet that uses a with-select statement to assign a 4-bit output signal based on a 2-bit input selector signal.

📋 What You'll Learn

Create a 2-bit input signal called sel.

Create a 4-bit output signal called out_val.

Use a with sel select statement to assign out_val based on sel.

Assign these exact values for out_val: "0001" for sel = "00", "0010" for sel = "01", "0100" for sel = "10", and "1000" for sel = "11".

💡 Why This Matters

🌍 Real World

Selected assignment with <code>with-select</code> is used in digital circuits to choose one output from many inputs based on control signals, like multiplexers.

💼 Career

Understanding <code>with-select</code> statements is essential for hardware engineers and FPGA designers to write clear and efficient VHDL code.

1

Create input and output signals

Declare a 2-bit input signal called sel and a 4-bit output signal called out_val inside an architecture.

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity selector is
    Port (
        sel : in std_logic_vector(1 downto 0);
        out_val : out std_logic_vector(3 downto 0)
    );
end selector;

architecture Behavioral of selector is
begin
    -- Declare signals sel and out_val are already declared in entity
    -- Your code here
end Behavioral;

Hint

Remember, input and output signals are declared in the entity port list.

2

Add with-select statement

Inside the architecture body, write a with sel select statement to assign out_val based on sel.

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity selector is
    Port (
        sel : in std_logic_vector(1 downto 0);
        out_val : out std_logic_vector(3 downto 0)
    );
end selector;

architecture Behavioral of selector is
begin
    -- Add with-select statement to assign out_val based on sel
    -- Your code here
end Behavioral;

Hint

Use the syntax: with sel select out_val <= value when condition, ...;

3

Assign exact values for each selector case

Complete the with sel select statement by assigning out_val the exact values: "0001" for sel = "00", "0010" for sel = "01", "0100" for sel = "10", and "1000" for sel = "11".

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity selector is
    Port (
        sel : in std_logic_vector(1 downto 0);
        out_val : out std_logic_vector(3 downto 0)
    );
end selector;

architecture Behavioral of selector is
begin
    -- Assign exact values for out_val using with-select
    -- Your code here
end Behavioral;

Hint

Make sure each when condition matches the exact selector value and output.

4

Test output with a print statement (simulation)

Add a simple process to print the value of out_val for each sel value in a testbench style process. Print the output using report statements for sel values "00", "01", "10", and "11".

VHDL

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity selector is
    Port (
        sel : in std_logic_vector(1 downto 0);
        out_val : out std_logic_vector(3 downto 0)
    );
end selector;

architecture Behavioral of selector is
begin
    with sel select
        out_val <= "0001" when "00",
                   "0010" when "01",
                   "0100" when "10",
                   "1000" when "11";

    -- Add a process to simulate and print out_val for each sel
    -- Your code here
end Behavioral;

Hint

Use a process with a loop to assign sel values and print out_val using report.

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