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

Recall & Review

beginner

What is a D flip-flop with asynchronous reset?

A D flip-flop with asynchronous reset is a memory element that stores the input value (D) on the rising edge of the clock, but can be reset to 0 immediately when the reset signal is active, regardless of the clock.

beginner

In VHDL, how do you detect an asynchronous reset in a process sensitivity list?

You include the reset signal in the sensitivity list along with the clock, so the process triggers on changes to either the clock or the reset signal.

beginner

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

Because the reset affects the output immediately when activated, without waiting for the clock edge.

beginner

What happens to the output Q when the asynchronous reset is active in a D flip-flop?

The output Q is forced to 0 immediately, regardless of the clock or input D.

intermediate

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

process(clk, reset) is begin if reset = '1' then Q <= '0'; -- asynchronous reset elsif rising_edge(clk) then Q <= D; -- store input on clock edge end if; end process;

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

A Immediately sets output to 0 regardless of clock

B Sets output to 0 only on clock rising edge

C Stores input D on falling clock edge

D Disables the flip-flop

The asynchronous reset forces the output to 0 immediately, without waiting for the clock.

Which signals should be in the sensitivity list for a D flip-flop with asynchronous reset in VHDL?

A D and clk

B clk only

C reset only

D clk and reset

Both clk and reset must be in the sensitivity list to detect changes for asynchronous reset.

In VHDL, what keyword checks for the rising edge of the clock?

A clk = '1'

B posedge(clk)

C rising_edge(clk)

D edge(clk)

The function rising_edge(clk) is the standard way to detect a rising clock edge in VHDL.

If reset is '0' and clock rises, what happens in a D flip-flop with asynchronous reset?

A Q takes the value of D

B Q resets to 0

C Q holds previous value

D Q toggles

When reset is inactive ('0'), the flip-flop stores the input D on the rising clock edge.

What is the output Q after reset is set to '1' in a D flip-flop with asynchronous reset?

A Same as D

B '0'

C '1'

D Undefined

The asynchronous reset forces Q to '0' immediately when reset is '1'.

Explain how a D flip-flop with asynchronous reset works and how it is implemented in VHDL.

Think about what happens when reset is active versus when clock rises.

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

Start with the process header, then handle reset, then clock edge.

Practice

1. What is the main function of the asynchronous reset in a D flip-flop?

easy

A. It stores data only on the rising edge of the clock.

B. It immediately clears the output regardless of the clock.

C. It delays the output change until the next clock edge.

D. It toggles the output on every clock cycle.

Solution

1. Step 1: Understand asynchronous reset behavior

   Asynchronous reset clears the output immediately, without waiting for the clock.

2. Step 2: Compare with clocked behavior

   Unlike clocked data storage, asynchronous reset acts instantly when active.

3. Final Answer:

   It immediately clears the output regardless of the clock. -> Option B

4. Quick Check:

   Asynchronous reset = immediate clear [OK]

Hint: Asynchronous reset acts instantly, ignoring clock [OK]

Common Mistakes:

• Confusing asynchronous reset with clocked data storage
• Thinking reset waits for clock edge
• Assuming reset toggles output

2. Which of the following VHDL code snippets correctly declares a D flip-flop with asynchronous reset?

easy

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

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

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

D. process(reset) begin if reset = '1' then q <= '0'; end if; end process;

Solution

1. Step 1: Check sensitivity list for asynchronous reset

   Asynchronous reset requires reset signal in sensitivity list along with clock.

2. Step 2: Verify reset condition before clock edge

   Reset condition must be checked first, then clock edge for data update.

3. Final Answer:

   process(clk, reset) begin if reset = '1' then q <= '0'; elsif rising_edge(clk) then q <= d; end if; end process; -> Option C

4. Quick Check:

   Reset in sensitivity list + reset checked first = correct async reset [OK]

Hint: Async reset needs reset in sensitivity list and checked first [OK]

Common Mistakes:

• Omitting reset from sensitivity list
• Checking reset inside clock edge condition
• Using only reset process without clock

3. Given this VHDL process for a D flip-flop with asynchronous reset:

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

If reset is '0', d is '1', and a rising edge on clk occurs, what will be the value of q?

medium

A. Undefined

B. '0'

C. No change

D. '1'

Solution

1. Step 1: Check reset signal

   Reset is '0', so asynchronous reset does not clear output.

2. Step 2: Check clock edge and data input

   Rising edge on clk triggers q to take value of d, which is '1'.

3. Final Answer:

   '1' -> Option D

4. Quick Check:

   Reset '0' + rising clk + d='1' means q='1' [OK]

Hint: Reset '0' means normal clocked data update [OK]

Common Mistakes:

• Assuming reset clears output when '0'
• Ignoring clock edge
• Thinking output stays unchanged

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

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

medium

A. Reset signal is not in the sensitivity list for asynchronous reset.

B. The reset condition should be checked after the clock edge.

C. The process should not use rising_edge(clk) for a flip-flop.

D. The output q should be assigned using <= only inside clock edge.

Solution

1. Step 1: Check sensitivity list for asynchronous reset

   Asynchronous reset requires reset signal in sensitivity list to respond immediately.

2. Step 2: Analyze reset checking position

   Reset is checked before clock edge, which is correct, but missing reset in sensitivity list causes error.

3. Final Answer:

   Reset signal is not in the sensitivity list for asynchronous reset. -> Option A

4. Quick Check:

   Async reset needs reset in sensitivity list [OK]

Hint: Async reset must be in sensitivity list [OK]

Common Mistakes:

• Omitting reset from sensitivity list
• Checking reset after clock edge
• Misusing rising_edge for flip-flop

5. You want to design a D flip-flop with asynchronous reset that sets output q to '0' immediately when reset is '1', and otherwise updates q with d on the rising edge of clk. Which VHDL process correctly implements this behavior and avoids unintended latches?

hard

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

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

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

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

Solution

1. Step 1: Check sensitivity list for asynchronous reset

   Reset and clk must be in sensitivity list for asynchronous reset to work properly.

2. Step 2: Verify reset checked before clock edge

   Reset condition checked first ensures immediate clearing without waiting for clock.

3. Step 3: Confirm no latches created

   All outputs assigned in all branches, so no unintended latches occur.

4. Final Answer:

   process(clk, reset) begin if reset = '1' then q <= '0'; elsif rising_edge(clk) then q <= d; end if; end process; -> Option A

5. Quick Check:

   Reset in sensitivity + reset first + all assignments = correct async reset flip-flop [OK]

Hint: Reset in sensitivity list and checked first avoids latches [OK]

Common Mistakes:

• Checking reset inside clock edge (not async)
• Missing reset in sensitivity list
• Creating latches by incomplete assignments