D flip-flop with synchronous reset in VHDL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a D flip-flop with synchronous reset?

A D flip-flop with synchronous reset is a memory element that stores the input value (D) on the rising edge of the clock, but if the reset signal is active during the clock edge, it sets the output to zero instead of the input value.

beginner

In VHDL, how do you detect a rising clock edge inside a process?

You use the condition: if rising_edge(clk) then to detect the rising edge of the clock signal inside a process.

beginner

Why is the reset called synchronous in a D flip-flop with synchronous reset?

Because the reset affects the output only at the clock edge, synchronized with the clock signal, not immediately when reset changes.

beginner

What happens if the reset is low during the clock edge in a D flip-flop with synchronous reset?

The flip-flop stores the input value D into the output Q at the clock edge.

intermediate

Show the VHDL code snippet for a D flip-flop with synchronous reset inside a process.

process(clk)
begin
  if rising_edge(clk) then
    if reset = '1' then
      q <= '0';
    else
      q <= d;
    end if;
  end if;
end process;

What does the synchronous reset do in a D flip-flop?

A Resets output only at clock edge when reset is active

B Resets output immediately when reset is active

C Ignores reset signal

D Resets output asynchronously

Synchronous reset affects the output only at the clock edge, not immediately.

Which VHDL statement detects a rising clock edge?

A if clk'event then

B if clk = '1' then

C if rising_edge(clk) then

D if clk = '0' then

The rising_edge(clk) function detects the rising edge of the clock.

In a D flip-flop with synchronous reset, what happens if reset is '0' at clock edge?

A Output Q becomes '1'

B Output Q resets to '0'

C Output Q holds previous value

D Output Q takes the value of D

When reset is '0', the flip-flop stores the input D at the clock edge.

Which signal controls when the D flip-flop updates its output?

A Clock signal

B Reset signal

C Data input D

D Enable signal

The clock signal triggers the flip-flop to update output on its rising edge.

What is the main difference between synchronous and asynchronous reset?

A Both act only on clock edge

B Synchronous reset acts on clock edge; asynchronous acts immediately

C Both act immediately

D Synchronous reset acts immediately; asynchronous on clock edge

Synchronous reset waits for clock edge; asynchronous reset acts as soon as reset is active.

Explain how a D flip-flop with synchronous reset works and why the reset is called synchronous.

Think about when the reset changes the output compared to the clock.

Write the VHDL process code for a D flip-flop with synchronous reset and describe each part.

Focus on the clock edge and reset check inside the process.

Practice

1.

What does a synchronous reset do in a D flip-flop?

easy

A. It disables the clock signal.

B. It clears the stored data immediately when reset is active.

C. It stores data continuously without any reset.

D. It clears the stored data only at the clock's rising edge.

Solution

1. Step 1: Understand synchronous reset behavior

   A synchronous reset affects the flip-flop only during the clock's active edge, not immediately.

2. Step 2: Compare with asynchronous reset

   Asynchronous reset clears data immediately, but synchronous reset waits for the clock edge.

3. Final Answer:

   It clears the stored data only at the clock's rising edge. -> Option D

4. Quick Check:

   Synchronous reset = clears data on clock edge [OK]

Hint: Synchronous reset acts only on clock edge, not instantly. [OK]

Common Mistakes:

• Confusing synchronous with asynchronous reset
• Thinking reset works immediately without clock
• Assuming reset disables clock

2.

Which of the following is the correct VHDL sensitivity list for a D flip-flop with synchronous reset?

process(???)

easy

A. clk, reset

B. clk

C. reset

D. clk, reset, d

Solution

1. Step 1: Identify signals triggering the process

   For synchronous reset, only the clock edge triggers the process, so only 'clk' is in the sensitivity list.

2. Step 2: Understand synchronous reset inside process

   The reset is checked inside the clock edge condition, so it does not go in sensitivity list.

3. Final Answer:

   clk -> Option B

4. Quick Check:

   Synchronous reset uses only clock in sensitivity list [OK]

Hint: Only clock in sensitivity list for synchronous reset. [OK]

Common Mistakes:

• Including reset in sensitivity list for synchronous reset
• Adding data input 'd' to sensitivity list
• Confusing asynchronous reset sensitivity

3.

Given this VHDL code snippet for a D flip-flop with synchronous reset, what is the output q after the rising edge of clk if reset = '1' and d = '0'?

process(clk)
begin
  if rising_edge(clk) then
    if reset = '1' then
      q <= '0';
    else
      q <= d;
    end if;
  end if;
end process;

medium

A. '0'

B. '1'

C. No change

D. Undefined

Solution

1. Step 1: Check reset condition at clock edge

   At rising edge, reset is '1', so q is set to '0'.

2. Step 2: Ignore data input since reset is active

   Because reset is active, q does not take value from d.

3. Final Answer:

   '0' -> Option A

4. Quick Check:

   Reset '1' sets q to '0' on clock edge [OK]

Hint: Reset '1' overrides data input at clock edge. [OK]

Common Mistakes:

• Ignoring reset and taking d value
• Thinking output changes immediately without clock
• Confusing synchronous with asynchronous reset

4.

Identify the error in this VHDL code for a D flip-flop with synchronous reset:

process(clk, reset)
begin
  if rising_edge(clk) then
    if reset = '1' then
      q <= '0';
    else
      q <= d;
    end if;
  end if;
end process;

medium

A. Reset should not be in the sensitivity list for synchronous reset.

B. Missing else statement for reset condition.

C. rising_edge(clk) should be replaced with falling_edge(clk).

D. Data input 'd' should be in sensitivity list.

Solution

1. Step 1: Check sensitivity list for synchronous reset

   Synchronous reset requires only clock in sensitivity list, not reset.

2. Step 2: Understand why reset is excluded

   Reset is checked inside clock edge condition, so including it causes unnecessary triggers.

3. Final Answer:

   Reset should not be in the sensitivity list for synchronous reset. -> Option A

4. Quick Check:

   Synchronous reset sensitivity list = clock only [OK]

Hint: Exclude reset from sensitivity list for synchronous reset. [OK]

Common Mistakes:

• Including reset in sensitivity list
• Changing clock edge without reason
• Adding data input to sensitivity list

5.

You want to design a D flip-flop with synchronous reset that sets output q to '1' when reset is active instead of '0'. Which code modification achieves this?

process(clk)
begin
  if rising_edge(clk) then
    if reset = '1' then
      q <= '0';  -- change this line
    else
      q <= d;
    end if;
  end if;
end process;

hard

A. Replace if reset = '1' with if reset = '0'

B. Add q <= '1'; before the if statement

C. Change q <= '0'; to q <= '1';

D. Move reset check outside the clock edge condition

Solution

1. Step 1: Identify reset output assignment

   The line q <= '0'; sets q to zero on reset.

2. Step 2: Modify reset output to '1'

   Changing this line to q <= '1'; sets q to one when reset is active.

3. Final Answer:

   Change q <= '0'; to q <= '1'; -> Option C

4. Quick Check:

   Reset output changed by modifying assignment line [OK]

Hint: Change reset assignment line to desired output value. [OK]

Common Mistakes:

• Adding assignment outside clock edge
• Reversing reset condition incorrectly
• Adding redundant assignments