Reset strategy (sync vs async) in Verilog

Introduction

Resetting a digital circuit makes sure it starts in a known state. There are two main ways to reset: synchronous and asynchronous. Each way controls when and how the reset happens.

When you want the circuit to reset only at the clock edge to avoid timing issues.

When you need the circuit to reset immediately, no matter the clock.

When designing a system that must recover quickly from errors.

When working with FPGA or ASIC designs that have specific reset requirements.

When you want to avoid glitches caused by reset signals.

Syntax

Verilog

module example_sync_reset(input clk, input reset, input d, output reg q);
  always @(posedge clk) begin
    if (reset) q <= 0;  // synchronous reset
    else q <= d;
  end
endmodule

module example_async_reset(input clk, input reset, input d, output reg q);
  always @(posedge clk or posedge reset) begin
    if (reset) q <= 0;  // asynchronous reset
    else q <= d;
  end
endmodule

Synchronous reset happens only on the clock edge.

Asynchronous reset can happen anytime, independent of the clock.

Examples

This is a synchronous reset example. Reset affects the output only at the clock's rising edge.

Verilog

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

This is an asynchronous reset example. Reset can happen immediately when the reset signal goes high.

Verilog

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

Sample Program

This testbench shows how synchronous and asynchronous resets affect outputs differently. The clock toggles every 5 time units. Reset is asserted and deasserted at different times to see the effect.

Verilog

module testbench();
  reg clk = 0;
  reg reset = 0;
  reg d = 0;
  wire q_sync, q_async;

  // Clock generation
  always #5 clk = ~clk;

  // Synchronous reset module
  example_sync_reset sync_reset(.clk(clk), .reset(reset), .d(d), .q(q_sync));

  // Asynchronous reset module
  example_async_reset async_reset(.clk(clk), .reset(reset), .d(d), .q(q_async));

  initial begin
    $monitor($time, " clk=%b reset=%b d=%b q_sync=%b q_async=%b", clk, reset, d, q_sync, q_async);

    // Initial values
    d = 1;
    reset = 1; // Assert reset
    #10;
    reset = 0; // Deassert reset
    #20;
    d = 0;
    #20;
    reset = 1; // Assert reset again
    #10;
    reset = 0;
    #20;
    $finish;
  end
endmodule

Important Notes

Asynchronous reset can cause glitches if not handled carefully.

Synchronous reset is safer for timing but may delay reset response.

Check your FPGA or ASIC documentation for recommended reset style.

Summary

Reset makes sure your circuit starts correctly.

Synchronous reset happens with the clock; asynchronous reset happens anytime.

Choose reset style based on your design needs and hardware rules.

Practice

1. What is the main difference between a synchronous reset and an asynchronous reset in Verilog?

easy

A. Synchronous reset is triggered by the clock edge, asynchronous reset works immediately.

B. Synchronous reset works immediately, asynchronous reset waits for the clock edge.

C. Both resets work only on the clock's rising edge.

D. Asynchronous reset requires a clock, synchronous reset does not.

Solution

1. Step 1: Understand synchronous reset behavior

   Synchronous reset activates only on the clock edge, so the reset signal is sampled with the clock.

2. Step 2: Understand asynchronous reset behavior

   Asynchronous reset activates immediately when asserted, independent of the clock.

3. Final Answer:

   Synchronous reset is triggered by the clock edge, asynchronous reset works immediately. -> Option A

4. Quick Check:

   Reset timing difference = synchronous on clock, async immediate [OK]

Hint: Sync reset waits clock, async reset acts immediately [OK]

Common Mistakes:

• Confusing which reset depends on clock
• Thinking async reset waits for clock edge
• Assuming both resets behave the same

2. Which of the following is the correct Verilog syntax for a synchronous reset inside an always block?

easy

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

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

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

D. always @(negedge rst) begin q <= 0; end

Solution

1. Step 1: Identify synchronous reset syntax

   Synchronous reset is checked inside a clock-only sensitivity list, typically @(posedge clk).

2. Step 2: Check reset condition inside the block

   The reset condition is inside the always block, using if (rst) to reset synchronously.

3. Final Answer:

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

4. Quick Check:

   Sync reset inside clock-only always block [OK]

Hint: Sync reset inside clock-only always block [OK]

Common Mistakes:

• Including reset in sensitivity list for sync reset
• Using asynchronous sensitivity list for sync reset
• Missing clock edge in always block

3. Given the following Verilog code, what will be the value of q after the asynchronous reset rst is deasserted?

reg q;
always @(posedge clk or posedge rst) begin
  if (rst) q <= 1'b0;
  else q <= d;
end

medium

A. q will immediately reset to 0 when rst is asserted, then follow d on clock edges.

B. q will reset to 0 only on the next clock edge after rst is asserted.

C. q will never reset because rst is asynchronous.

D. q will hold its previous value regardless of rst.

Solution

1. Step 1: Analyze asynchronous reset sensitivity

   The always block triggers on posedge clk or posedge rst, so when rst is asserted, q resets immediately.

2. Step 2: Behavior after reset deassertion

   After rst is deasserted, q updates on clock edges following the value of d.

3. Final Answer:

   q will immediately reset to 0 when rst is asserted, then follow d on clock edges. -> Option A

4. Quick Check:

   Async reset immediate, then sync update [OK]

Hint: Async reset triggers immediately, sync updates after [OK]

Common Mistakes:

• Thinking async reset waits for clock edge
• Assuming q holds value during reset
• Confusing reset assertion and deassertion timing

4. Identify the error in this Verilog code snippet implementing a synchronous reset:

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

medium

A. The assignment operator should be = instead of <=.

B. The reset condition should be checked outside the always block.

C. The clock edge should be negedge instead of posedge.

D. The reset should not be in the sensitivity list for synchronous reset.

Solution

1. Step 1: Check sensitivity list for synchronous reset

   Synchronous reset should only be sensitive to the clock edge, not the reset signal.

2. Step 2: Identify the error

   Including posedge rst makes the reset asynchronous, which contradicts the synchronous reset intent.

3. Final Answer:

   The reset should not be in the sensitivity list for synchronous reset. -> Option D

4. Quick Check:

   Sync reset sensitivity = clock only [OK]

Hint: Sync reset sensitivity list excludes reset signal [OK]

Common Mistakes:

• Including reset in sensitivity list for sync reset
• Misunderstanding sensitivity list purpose
• Using blocking assignment instead of non-blocking

5. You need to design a flip-flop with a reset that must clear the output immediately when asserted, but also synchronize with the clock to avoid metastability issues. Which reset strategy is best and how should it be implemented?

hard

A. Use an asynchronous reset without synchronization.

B. Use a synchronous reset only, ignoring immediate reset requirement.

C. Use an asynchronous reset with a synchronizer circuit on the reset signal.

D. Use two separate resets: one async and one sync, both controlling the flip-flop.

Solution

1. Step 1: Understand immediate reset need

   Immediate clearing requires asynchronous reset to react without waiting for clock.

2. Step 2: Avoid metastability by synchronizing reset

   To prevent metastability, the asynchronous reset signal should be passed through synchronizer flip-flops before use.

3. Step 3: Combine both requirements

   Using async reset with synchronizer achieves immediate reset and safe clock domain crossing.

4. Final Answer:

   Use an asynchronous reset with a synchronizer circuit on the reset signal. -> Option C

5. Quick Check:

   Async reset + synchronizer = immediate and safe reset [OK]

Hint: Async reset plus synchronizer avoids metastability [OK]

Common Mistakes:

• Using only synchronous reset ignoring immediate reset
• Using async reset without synchronization causing metastability
• Trying to use two resets on same flip-flop incorrectly