Low Power VLSI Design: Power Reduction with Clock Gating

In Low Power VLSI Design, minimizing power consumption is a key concern, particularly for devices that need to be energy-efficient and compact. One of the most effective techniques to achieve power savings is clock gating, which selectively disables the clock signal to components that don't need to be active, thereby reducing power consumption. 

What is Clock Gating?

Clock gating is a widely used technique in digital circuit design that reduces dynamic power consumption. By controlling the clock signal, the technique prevents unnecessary toggling of flip-flops or registers, saving energy in areas of the circuit where activity is not required. Since flip-flops are driven by clock signals, disabling the clock when not in use significantly reduces power loss due to switching.

Example Module: DFF Power Reduction Using Clock Gating

The DFF_power_reduction_by_clock_gating module demonstrates the implementation of clock gating to save power in a D Flip-Flop (DFF). This example shows how clock gating can be used to disable the clock when the flip-flop’s state is not changing, thus reducing unnecessary switching activity.

Verilog Code for the Module

module DFF_power_reduction_by_clock_gating(

input d,clk, output reg q

 );

    wire t1,clk1;

    assign t1=q^d;

    assign clk1=t1 & clk ;

    always@(posedge clk1)

       q<=d;

endmodule

Key Components of the Module

• Inputs:

  • d (Data Input): The input data to be stored in the flip-flop.

  • clk (Clock Input): The clock signal driving the flip-flop.

• Output:

  • q (Output Register): The stored value of d when the clock triggers.

• Clock Gating Logic:

  • The XOR gate (t1 = q ^ d) compares the current state (q) with the new input (d) to detect any changes.

  • The clock is gated by the signal t1 & clk, meaning the clock is only passed to the flip-flop when a change in d is detected.

• Power Savings:

  • When d is stable (no change), the clock is disabled, which prevents unnecessary toggling and reduces dynamic power consumption in the flip-flop.

Timing Analysis and Simulation Results

After implementing the clock-gating mechanism, we perform timing analysis and simulation to evaluate the effectiveness of the power-saving technique.

Timing Analysis

The timing analysis provides insight into the critical path and the timing constraints of the design after applying clock gating. By ensuring that the flip-flop is only triggered when needed, we observe that the overall circuit's clocking behavior is optimized, reducing power consumption without compromising the timing performance.

Before Clock Gating: The flip-flop was clocked continuously, leading to unnecessary switching activity and power consumption. 

After Clock Gating: The timing analysis shows that the clock signal is selectively passed only when there is a change in the input d, leading to a significant reduction in dynamic power usage without affecting the timing constraints. 

Timing Analysis Graph

The timing analysis graph compares the delay and performance before and after implementing clock gating. The data shows a reduction in switching activity and an improvement in power efficiency.

Simulation Results

Simulations using standard testbenches show that the clock-gated flip-flop behaves as expected. The simulation diagram provides a clear picture of how the clock signal is gated based on the input d. When there is no change in d, the clock is disabled, and the flip-flop does not toggle, saving power.

Simulation Diagram

In the simulation diagram, you can see:

• Clock (clk) signal transitions at regular intervals.

• Data Input (d) changes state at specific moments.

• Clock Gated Clock (clk1), which is only active when there is a change in d, demonstrating the clock gating effect.

Synthesis Results with Yosys 0.3 and Playground EDA

The next step in evaluating the design is synthesis. We used Yosys 0.3 to synthesize the module and analyzed the results through Playground EDA, an online platform for quick VLSI design and verification.

Yosys Synthesis

Yosys 0.3 performed the synthesis of the DFF with Clock Gating module. The synthesis results show a significant reduction in the resource usage compared to the traditional flip-flop design that doesn't utilize clock gating.

Synthesis Diagram

The synthesis diagram generated by Yosys 0.3 displays the optimized logic and gates used for the design. With clock gating, the design uses fewer flip-flops and logic gates, as the gating mechanism reduces unnecessary clock signal propagation.

• Before Synthesis: The design included additional flip-flops that continuously toggled, leading to higher area and power consumption.

• After Synthesis: The synthesis results show a more compact design, with lower area usage and optimized power consumption due to the clock gating.

Power Reduction and Area Efficiency

From the synthesis results, we can observe the following:

• Power Consumption: Power consumption is significantly lower due to the selective clocking.

• Area Efficiency: Implementing a clock-gated flip-flop generally results in a larger area compared to a non-clock-gated design. This is a trade-off between area and power consumption.