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.