Selected assignment (with-select) in VHDL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a selected assignment with simple case

What is the output value of y when sel = "10"?

VHDL

architecture Behavioral of example is
  signal sel : std_logic_vector(1 downto 0);
  signal y : std_logic_vector(3 downto 0);
begin
  with sel select
    y <= "0001" when "00",
         "0010" when "01",
         "0100" when "10",
         "1000" when others;
end Behavioral;

A "0100"

B "0010"

C "0001"

D "1000"

💡 Hint

Check which value matches the selector "10" in the with-select statement.

The with-select statement assigns "0100" to y when sel is "10". Other values correspond to other selectors.

❓ Predict Output

intermediate

Selected assignment with overlapping conditions

Given this VHDL code, what value does out_sig have when mode = "11"?

VHDL

architecture Behavioral of example2 is
  signal mode : std_logic_vector(1 downto 0);
  signal out_sig : std_logic_vector(7 downto 0);
begin
  with mode select
    out_sig <= x"0F" when "00",
               x"F0" when "01",
               x"AA" when "10",
               x"55" when others;
end Behavioral;

A x"F0"

B x"55"

C x"AA"

D x"0F"

💡 Hint

The selector "11" is not explicitly listed, so it falls under others.

The with-select statement assigns x"55" to out_sig when mode is any value other than "00", "01", or "10". Since "11" is not listed, it matches others.

🔧 Debug

advanced

Identify the syntax error in selected assignment

Which option contains the syntax error in this with-select assignment?

VHDL

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

A Missing semicolon after the last when clause

B Using double quotes instead of single quotes for std_logic_vector

C Missing 'end process;' statement

D Using commas instead of semicolons after each when clause

💡 Hint

Check the punctuation at the end of the with-select statement.

In VHDL, each when clause in a with-select assignment must end with a comma, except the last one which must end with a semicolon. The code ends the last when clause with a comma, causing a syntax error.

🧠 Conceptual

advanced

Behavior of with-select when no matching choice and no others clause

What happens if a with-select statement does not include an 'others' clause and the selector value does not match any explicit choices?

A The signal is assigned a default value of all zeros

B The compiler raises a syntax error during compilation

C The simulation raises a runtime error due to incomplete assignment

D The signal retains its previous value without change

💡 Hint

No assignment occurs if no match and no others, so retains previous value.

If no matching choice is found and there is no others clause, no signal assignment is performed, and the signal retains its previous value.

🚀 Application

expert

Determine the number of unique output values from with-select

Given this with-select statement, how many unique values can out_val take based on code?

VHDL

with code select
  out_val <= "000" when "000",
             "001" when "001",
             "010" when "010",
             "011" when "011",
             "100" when "100",
             "101" when others;

A 5

B 8

C 6

D 7

💡 Hint

Count the distinct output values assigned explicitly and by others.

There are 5 explicit choices with unique outputs plus one others clause with a unique output, totaling 6 unique output values.

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