Selected assignment (with-select) in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign output based on input using with-select.

VHDL

with input select
  output <= '1' when '1',
            '0' when [1],
            'Z' when others;

Drag options to blanks, or click blank then click option'

A'1'

B'Z'

C'0'

D'X'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '1' instead of '0' for the when clause.

Forgetting the quotes around the value.

The with-select statement assigns '0' to output when input is '0'.

2fill in blank

medium

Complete the code to select the correct output value for input '2'.

VHDL

with input select
  output <= "00" when '0',
            "01" when '1',
            [1] when '2',
            "11" when others;

Drag options to blanks, or click blank then click option'

A"00"

B"11"

C"01"

D"10"

Attempts:

3 left

💡 Hint

Common Mistakes

Using output "11" for input '2' which is reserved for others.

Forgetting to use double quotes for output values.

When input is '2', output should be "10" as per the selected assignment.

3fill in blank

hard

Fix the error in the with-select statement to assign output correctly.

VHDL

with input select
  output <= '0' when '0',
            '1' when '1',
            'Z' when others
            [1]

Drag options to blanks, or click blank then click option'

A;

B,

C.

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using a comma instead of a semicolon at the end.

Omitting the punctuation entirely.

The with-select statement must end with a semicolon after the last when clause.

4fill in blank

hard

Fill both blanks to complete the with-select statement for a 2-bit output.

VHDL

with sel select
  out <= [1] when '0',
         [2] when '1',
         "11" when others;

Drag options to blanks, or click blank then click option'

A"00"

B"01"

C"10"

D"11"

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up output values for '0' and '1'.

Using single quotes instead of double quotes for output.

Output is "00" when sel is '0' and "10" when sel is '1'.

5fill in blank

hard

Fill all three blanks to complete the with-select statement for a 3-bit output.

VHDL

with address select
  data_out <= [1] when "000",
              [2] when "001",
              [3] when others;

Drag options to blanks, or click blank then click option'

A"101"

B"110"

C"111"

D"100"

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the order of output values.

Using single quotes instead of double quotes for vectors.

data_out is "100" for "000", "101" for "001", and "111" for others.

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