BCD counter design in VHDL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a signal for the BCD counter.

VHDL

signal counter : std_logic_vector([1] downto 0) := (others => '0');

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

A3

B4

C7

D9

Attempts:

3 left

💡 Hint

Common Mistakes

Using 4 which creates a 5-bit vector (4 downto 0).

Using 7 or 9 bits which is too large for a single BCD digit.

The BCD counter counts from 0 to 9, which requires 4 bits (3 downto 0).

2fill in blank

medium

Complete the process sensitivity list to trigger on clock and reset signals.

VHDL

process([1])

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Aclk, reset

Bclk

Cclk or reset

Dreset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'clk or reset' instead of commas.

Including only clock or only reset.

The process must be sensitive to both clock and reset signals using 'clk, reset'.

3fill in blank

hard

Fix the error in the if statement checking for reset.

VHDL

if [1] = '1' then

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

Aclk

Breset

Ccounter

Denable

Attempts:

3 left

💡 Hint

Common Mistakes

Checking clock instead of reset.

Checking enable or counter signals.

The reset signal is checked to initialize the counter.

4fill in blank

hard

Fill both blanks to correctly increment the BCD counter and reset after 9.

VHDL

if counter = [1] then
    counter <= [2];

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A"1001"

B"0000"

C"1010"

D"1111"

Attempts:

3 left

💡 Hint

Common Mistakes

Resetting at "1010" which is decimal 10, not valid BCD.

Resetting to "1111" which is not zero.

The counter resets to "0000" after reaching "1001" (decimal 9).

5fill in blank

hard

Fill all three blanks to complete the BCD counter process with clock, reset, and increment logic.

VHDL

process([1])
begin
    if [2] = '1' then
        counter <= [3];
    elsif rising_edge(clk) then
        if counter = "1001" then
            counter <= "0000";
        else
            counter <= counter + 1;
        end if;
    end if;
end process;

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

Aclk, reset

Breset

C"0000"

Dclk

Attempts:

3 left

💡 Hint

Common Mistakes

Using only clk in sensitivity list.

Resetting counter to wrong value.

Checking reset incorrectly.

The process is sensitive to clock and reset. It resets counter to "0000" when reset is high.

Practice

1. What is the main function of a BCD counter in digital circuits?

easy

A. To count decimal digits from 0 to 9 in binary form

B. To count binary numbers from 0 to 15

C. To convert analog signals to digital

D. To multiply two binary numbers

Solution

1. Step 1: Understand BCD counting

   A BCD counter counts decimal digits 0 through 9 using binary representation.

2. Step 2: Differentiate from binary counters

   Unlike a binary counter that counts up to 15 (4 bits), BCD resets after 9 to represent decimal digits.

3. Final Answer:

   To count decimal digits from 0 to 9 in binary form -> Option A

4. Quick Check:

   BCD counter counts 0-9 decimal digits [OK]

Hint: BCD counters count decimal digits 0-9 in binary [OK]

Common Mistakes:

• Confusing BCD with binary counters counting 0-15
• Thinking BCD converts analog signals
• Assuming BCD performs arithmetic multiplication

2. Which of the following is the correct VHDL syntax to declare a 4-bit signal named count for a BCD counter?

easy

A. signal count : integer range 0 to 15;

B. signal count : std_logic_vector(3 to 0);

C. signal count : bit_vector(3 to 0);

D. signal count : std_logic_vector(3 downto 0);

Solution

1. Step 1: Identify correct vector type and range

   For a 4-bit signal, std_logic_vector(3 downto 0) is standard and correct.

2. Step 2: Check other options for errors

   Integer range 0 to 15 uses integer type, not vector; bit_vector(3 to 0) uses invalid range direction; std_logic_vector(3 to 0) uses invalid range direction.

3. Final Answer:

   signal count : std_logic_vector(3 downto 0); -> Option D

4. Quick Check:

   4-bit std_logic_vector with downto 0 indexing [OK]

Hint: Use std_logic_vector(3 downto 0) for 4-bit signals [OK]

Common Mistakes:

• Using integer instead of std_logic_vector for bit signals
• Incorrect vector range direction
• Using bit_vector instead of std_logic_vector

3. Given the following VHDL process for a BCD counter, what will be the value of count after the 10th rising clock edge?

process(clk)
begin
  if rising_edge(clk) then
    if count = "1001" then
      count <= "0000";
    else
      count <= count + 1;
    end if;
  end if;
end process;

medium

A. "1010"

B. "0000"

C. "1001"

D. "0001"

Solution

1. Step 1: Understand counting behavior

   The counter increments each clock edge until it reaches "1001" (decimal 9), then resets to "0000".

2. Step 2: Count 10 clock edges

   After 9 increments, count is "1001"; on the 10th clock, it resets to "0000".

3. Final Answer:

   "0000" -> Option B

4. Quick Check:

   BCD resets after 9, so 10th count is 0 [OK]

Hint: BCD resets to 0 after 9 counts [OK]

Common Mistakes:

• Assuming count continues to "1010" which is invalid BCD
• Forgetting reset condition at 9
• Confusing binary and decimal counting

4. Identify the error in this VHDL BCD counter snippet:

process(clk)
begin
  if rising_edge(clk) then
    if count = "1010" then
      count <= "0000";
    else
      count <= count + 1;
    end if;
  end if;
end process;

medium

A. The process is missing a sensitivity list

B. The signal count is not declared

C. The reset condition should check for "1001" instead of "1010"

D. The increment operation is invalid for std_logic_vector

Solution

1. Step 1: Check reset condition value

   BCD counts 0 to 9, so reset should occur at "1001" (decimal 9), not "1010" (decimal 10).

2. Step 2: Verify other parts

   Signal declaration and sensitivity list are correct; increment on std_logic_vector requires numeric conversion but is often handled by libraries.

3. Final Answer:

   The reset condition should check for "1001" instead of "1010" -> Option C

4. Quick Check:

   BCD resets at 9 ("1001"), not 10 ("1010") [OK]

Hint: Reset BCD at "1001" (decimal 9), not "1010" [OK]

Common Mistakes:

• Using "1010" as reset condition instead of "1001"
• Ignoring numeric conversion for increment
• Confusing sensitivity list requirements

5. You want to design a BCD counter in VHDL that counts from 0 to 9 and then resets to 0. Which approach correctly implements this behavior?

hard

A. Use a 4-bit std_logic_vector, increment it each clock, and reset to "0000" when it reaches "1001"

B. Use an integer signal from 0 to 15 and reset at 9

C. Use a 4-bit std_logic_vector and reset at "1111"

D. Use a 3-bit std_logic_vector and reset at "1001"

Solution

1. Step 1: Choose correct signal type and size

   A 4-bit std_logic_vector is needed to represent digits 0 to 9 in BCD.

2. Step 2: Implement correct reset condition

   Reset should occur when count reaches "1001" (decimal 9), then reset to "0000" to repeat counting.

3. Final Answer:

   Use a 4-bit std_logic_vector, increment it each clock, and reset to "0000" when it reaches "1001" -> Option A

4. Quick Check:

   4-bit vector + reset at 9 = correct BCD counter [OK]

Hint: Use 4-bit vector and reset at "1001" for BCD [OK]

Common Mistakes:

• Using integer range 0-15 instead of 0-9
• Resetting at wrong binary value like "1111"
• Using 3-bit vector which can't hold 0-9