D flip-flop with synchronous reset in Verilog - 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 Verilog, how do you describe a synchronous reset inside an always block?

You check the reset signal inside the clock edge condition, usually with an if statement before assigning the D input to the output. For example: if (reset) q <= 0; else q <= d;

beginner

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

Because the reset only affects the output at the clock edge, meaning the reset signal is synchronized with the clock and does not change the output asynchronously.

beginner

Write the sensitivity list for a D flip-flop with synchronous reset in Verilog.

The sensitivity list includes only the clock signal, for example: always @(posedge clk) begin ... end

beginner

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

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

In a D flip-flop with synchronous reset, when does the reset affect the output?

A Immediately when reset is activated

B Only at the clock edge

C Only when the clock is low

D At any time asynchronously

The reset is synchronous, so it only affects the output at the clock edge.

Which signal is included in the sensitivity list of a synchronous reset D flip-flop in Verilog?

A clk only

B reset only

C clk and reset

D d and reset

Only the clock signal is in the sensitivity list because reset is synchronous and checked inside the clock edge.

What is the output Q when reset is active during the clock edge in a synchronous reset D flip-flop?

A Undefined

B 1

C Same as D input

D 0

When reset is active, output Q is set to 0 synchronously at the clock edge.

How do you write the reset condition inside the always block for synchronous reset?

A if (reset) q = 0; else q = d;

B if (reset) q <= d; else q <= 0;

C if (reset) q <= 0; else q <= d;

D if (reset) q = d; else q = 0;

The reset sets output to 0; otherwise, output follows input D.

What type of flip-flop behavior does a synchronous reset NOT have?

A Immediate reset without clock

B Reset at clock edge

C Data storage on clock edge

D Reset checked inside always block

Synchronous reset does not reset output immediately; it waits for the clock edge.

Explain how a D flip-flop with synchronous reset works and how it differs from an asynchronous reset flip-flop.

Think about when the reset changes the output.

Write a simple Verilog code snippet for a D flip-flop with synchronous reset and explain each part.

Focus on the clock and reset handling inside the always block.

Practice

1. What does a D flip-flop with synchronous reset do when the reset signal is active?

easy

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

B. It stores the inverse of the input data.

C. It ignores the reset signal and keeps the data.

D. It clears the stored data only on the clock edge.

Solution

1. Step 1: Understand synchronous reset behavior

   A synchronous reset affects the flip-flop only at the clock edge, not immediately.

2. Step 2: Compare reset types

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

3. Final Answer:

   It clears the stored data only on the clock edge. -> Option D

4. Quick Check:

   Synchronous reset = clear on clock edge [OK]

Hint: Synchronous reset acts only at clock edges [OK]

Common Mistakes:

• Confusing synchronous with asynchronous reset
• Thinking reset clears data immediately
• Assuming reset stores input data

2. Which of the following is the correct Verilog syntax for a D flip-flop with synchronous reset inside an always block?

easy

A. always @(posedge clk or posedge reset) begin if (reset) q <= 0; else q <= d; end

B. always @(negedge clk) begin if (!reset) q <= d; else q <= 0; end

C. always @(posedge clk) begin if (reset) q <= 0; else q <= d; end

D. always @(reset) begin if (reset) q <= 0; else q <= d; end

Solution

1. Step 1: Identify synchronous reset sensitivity list

   Synchronous reset triggers only on clock edge, so only posedge clk is in sensitivity list.

2. Step 2: Check reset condition inside always block

   Reset is checked inside the block, not in sensitivity list, matching synchronous reset behavior.

3. Final Answer:

   always @(posedge clk) begin if (reset) q <= 0; else q <= d; end -> Option C

4. Quick Check:

   Synchronous reset inside posedge clk block [OK]

Hint: Synchronous reset inside posedge clk always block [OK]

Common Mistakes:

• Including reset in sensitivity list (asynchronous reset)
• Using reset in sensitivity list but coding synchronous reset
• Using negedge clk incorrectly

3. Given the following Verilog code, what is the value of q after the second rising edge of clk if reset is 1 at the first clock and 0 at the second?

reg q;
always @(posedge clk) begin
  if (reset) q <= 0;
  else q <= d;
end

medium

A. 1

B. d (input value)

C. 0

D. Undefined

Solution

1. Step 1: Analyze first clock edge with reset=1

   At first posedge clk, reset=1, so q is set to 0.

2. Step 2: Analyze second clock edge with reset=0

   At second posedge clk, reset=0, so q takes value of d input.

3. Final Answer:

   d (input value) -> Option B

4. Quick Check:

   Reset active clears q, then q follows d [OK]

Hint: Reset clears q only at clock edge, then q follows d [OK]

Common Mistakes:

• Assuming reset clears q immediately
• Thinking q stays 0 after reset deactivates
• Confusing asynchronous reset behavior

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

always @(posedge clk or posedge reset) begin
  if (reset) q <= 0;
  else q <= d;
end

medium

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

B. The assignment operator should be '=' instead of '<='.

C. The clock edge should be negedge, not posedge.

D. The reset condition should be checked after the else statement.

Solution

1. Step 1: Check sensitivity list for synchronous reset

   Synchronous reset triggers only on clock edge, so reset must NOT be in sensitivity list.

2. Step 2: Confirm correct sensitivity list

   Only posedge clk should be in sensitivity list for synchronous reset.

3. Final Answer:

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

4. Quick Check:

   Synchronous reset excludes reset from sensitivity list [OK]

Hint: Synchronous reset excludes reset from sensitivity list [OK]

Common Mistakes:

• Including reset in sensitivity list for synchronous reset
• Using blocking assignment instead of non-blocking
• Checking reset condition incorrectly

5. You want to design a D flip-flop with synchronous reset that sets output q to 0 on reset and otherwise stores input d. Which Verilog code correctly implements this with minimal logic and proper timing?

hard

A. always @(posedge clk) begin if (reset) q <= 0; else q <= d; end

B. always @(posedge clk or posedge reset) begin if (reset) q <= 0; else q <= d; end

C. always @(posedge clk) begin if (reset) q <= d; else q <= 0; end

D. always @(posedge reset) begin if (reset) q <= 0; else q <= d; end

Solution

1. Step 1: Confirm synchronous reset sensitivity list

   For synchronous reset, only posedge clk is in sensitivity list.

2. Step 2: Check reset logic inside always block

   Reset condition sets q to 0 when active; else q follows d input.

3. Step 3: Verify minimal and correct logic

   always @(posedge clk) begin if (reset) q <= 0; else q <= d; end matches synchronous reset behavior with minimal logic and correct timing.

4. Final Answer:

   always @(posedge clk) begin if (reset) q <= 0; else q <= d; end -> Option A

5. Quick Check:

   Synchronous reset inside posedge clk block with correct logic [OK]

Hint: Use posedge clk only and check reset inside block [OK]

Common Mistakes:

• Including reset in sensitivity list (asynchronous reset)
• Using negedge clk incorrectly
• Inverting reset logic