Clock gating for power reduction in Verilog - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a gated clock signal named clk_gated.

Verilog

wire clk_gated = [1] & enable;

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

Aenable

Bclk

Creset

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reset' or 'data' instead of the clock signal.

Using 'enable' alone without the clock.

The gated clock is generated by ANDing the original clock clk with the enable signal.

2fill in blank

medium

Complete the code to instantiate a latch for clock gating control.

Verilog

always @([1] or enable) begin
  if (!enable)
    latch_out <= 0;
  else
    latch_out <= 1;
end

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

Aenable

Bposedge clk

Cnegedge clk

Dposedge reset

Attempts:

3 left

💡 Hint

Common Mistakes

Using positive edge instead of negative edge.

Using enable or reset signals as clock triggers.

The latch is controlled on the negative edge of the clock to hold the enable signal stable for gating.

3fill in blank

hard

Fix the error in the clock gating module declaration.

Verilog

module clock_gating(
  input wire clk,
  input wire enable,
  output wire [1]
);

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Agated_clk

Bclk_enable

Cenable_clk

Dclk_gated

Attempts:

3 left

💡 Hint

Common Mistakes

Using ambiguous or incorrect signal names.

Using input signal names as output.

The output clock signal is commonly named gated_clk to clearly indicate it is the gated version of the clock.

4fill in blank

hard

Fill both blanks to complete the clock gating logic with latch and gated clock output.

Verilog

reg latch_out;

always @([1] or enable) begin
  if (!enable)
    latch_out <= 0;
  else
    latch_out <= 1;
end

assign [2] = clk & latch_out;

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

Anegedge clk

Bposedge clk

Cgated_clk

Dclk_gated

Attempts:

3 left

💡 Hint

Common Mistakes

Using positive edge for the latch.

Confusing output signal names.

The latch triggers on the negative edge of the clock, and the gated clock output is assigned by ANDing the clock with the latch output.

5fill in blank

hard

Fill all three blanks to complete a clock gating module with input clock, enable, and gated clock output.

Verilog

module clock_gating(
  input wire [1],
  input wire [2],
  output wire [3]
);

reg latch_out;

always @(negedge [1] or [2]) begin
  if (![2])
    latch_out <= 0;
  else
    latch_out <= 1;
end

assign [3] = [1] & latch_out;

endmodule

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

Aclk

Benable

Cclk_gated

Dreset

Attempts:

3 left

💡 Hint

Common Mistakes

Using reset instead of enable.

Mixing up input and output signal names.

The module inputs are the clock clk and enable signal enable. The output is the gated clock clk_gated.

Practice

1. What is the main purpose of clock gating in Verilog designs?

easy

A. To increase the clock frequency for faster operation

B. To reduce power consumption by disabling the clock to inactive modules

C. To add delay to the clock signal intentionally

D. To synchronize multiple clock domains

Solution

1. Step 1: Understand clock gating concept

   Clock gating is used to save power by stopping the clock signal to parts of the circuit that are not active.

2. Step 2: Identify the main goal

   The main goal is to reduce power consumption, not to increase speed or add delay.

3. Final Answer:

   To reduce power consumption by disabling the clock to inactive modules -> Option B

4. Quick Check:

   Clock gating = power saving [OK]

Hint: Clock gating stops clock to idle parts to save power [OK]

Common Mistakes:

• Thinking clock gating speeds up the circuit
• Confusing clock gating with clock synchronization
• Assuming clock gating adds intentional delay

2. Which of the following is the correct way to implement clock gating in Verilog?

easy

A. assign gated_clk = ~clk & enable;

B. assign gated_clk = clk | enable;

C. assign gated_clk = clk & enable;

D. assign gated_clk = clk ^ enable;

Solution

1. Step 1: Recall clock gating logic

   Clock gating is done by ANDing the clock with an enable signal to stop the clock when enable is low.

2. Step 2: Check each option

   Only assign gated_clk = clk & enable; uses AND (&) between clock and enable, which is correct for gating.

3. Final Answer:

   assign gated_clk = clk & enable; -> Option C

4. Quick Check:

   Clock gating = clk AND enable [OK]

Hint: Clock gating uses AND between clock and enable [OK]

Common Mistakes:

• Using OR or XOR instead of AND for gating
• Negating clock signal incorrectly
• Confusing enable signal polarity

3. Given the following Verilog snippet, what is the value of gated_clk when clk=1 and enable=0?

assign gated_clk = clk & enable;

medium

A. X (unknown)

B. 1

C. Z (high impedance)

D. 0

Solution

1. Step 1: Evaluate the AND operation

   gated_clk = clk & enable = 1 & 0 = 0.

2. Step 2: Understand output value

   Since enable is 0, gated_clk is forced to 0 regardless of clk.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   1 AND 0 = 0 [OK]

Hint: AND with zero always gives zero [OK]

Common Mistakes:

• Assuming gated_clk equals clk regardless of enable
• Confusing zero with unknown or high impedance
• Thinking gated_clk is always 1 when clk is 1

4. Identify the error in this clock gating code snippet:

module clk_gate(input clk, input enable, output gated_clk);
  always @(posedge clk) begin
    if (enable)
      gated_clk <= clk;
    else
      gated_clk <= 0;
  end
endmodule

medium

A. Using gated_clk as a clock signal inside always block causes timing issues

B. Missing sensitivity to enable signal in always block

C. Incorrect use of nonblocking assignment for gated_clk

D. No error; code is correct

Solution

1. Step 1: Analyze clock gating implementation

   The code tries to assign gated_clk inside an always block triggered by clk, but gated_clk is used as a clock signal, which is unsafe.

2. Step 2: Understand timing issues

   Clock gating should be done with combinational logic (AND gate), not inside sequential always block to avoid glitches and timing problems.

3. Final Answer:

   Using gated_clk as a clock signal inside always block causes timing issues -> Option A

4. Quick Check:

   Clock gating must be combinational, not sequential [OK]

Hint: Clock gating must be combinational logic, not inside always block [OK]

Common Mistakes:

• Trying to gate clock inside always @(posedge clk)
• Ignoring timing hazards from gated clocks
• Assuming nonblocking assignment fixes clock gating issues

5. You want to implement clock gating to reduce power in a design where the enable signal is generated asynchronously. Which approach is safest to avoid timing problems?

hard

A. Synchronize the enable signal to the clock domain before gating

B. Use an AND gate with the clock and enable directly without synchronization

C. Invert the enable signal and AND with clock

D. Use an OR gate between clock and enable for gating

Solution

1. Step 1: Understand asynchronous enable risks

   An asynchronous enable can cause glitches or timing violations if used directly for clock gating.

2. Step 2: Choose safe gating method

   Synchronizing the enable signal to the clock domain ensures stable gating and avoids glitches.

3. Final Answer:

   Synchronize the enable signal to the clock domain before gating -> Option A

4. Quick Check:

   Sync async enable before gating clock [OK]

Hint: Always synchronize async enable before clock gating [OK]

Common Mistakes:

• Using async enable directly causing glitches
• Using OR gate instead of AND for gating
• Inverting enable signal incorrectly