Power consumption

Here are some tips to predict the power consumption early in the design cycle i.e at the RTL Level.

Determine your design components power consumption

Find which components power consumption in your design is fixed by the specification and which components will be affected through power reduction techniques. For instance, input output power can be fixed at the specification level and memory power can also be fixed in the specification level. But memory can be powered down when not in use. If large amount of power is consumed by the clock in your design, then you need to have a clock gating techniques.

The designers can do an RTL power analysis before the design is synthesized. This analysis can't be as accurate as Gate level synthesis. But it gives an overall idea of potential power saving. For example, how much power could be saved if block "A" could be powered down 65 percent of the time, or if block "A" operated at 0.8 volt rather than 1 V?

Accurate switching activity from your simulation

To ensure an accurate power estimate, you need to use the most accurate data you have available at any given point in the design flow and revise your estimate as new data becomes available. But getting accurate switching activity is a huge challenge.

If switching activity data is not available from simulation, designers should estimate the switching activity on the chip's primary inputs and apply that estimate within the power analysis tool. Most power analysis tools can propagate the switching activity data through both the combinatorial and sequential logic.

Low power design

In today's chip industry, power consumption is the primary concern of the hardware designers who implement wireless and mobile applications. Though, EDA tools are emerging, many design decisions - which are influencing the power consumptions - are made at the system and architectural level prior to writing RTL. The power consumption of a circuit can be classified as either dynamic or static. Dynamic power consumption is a function of switched capacitance and supply voltages. Static power consumption comprises the circuit-activity-independent leakage power consumption. Leakage power depends on process technology parameters such as threshold voltages, supply voltages, circuit state, and temperature.

Methodology

Cells that do not perform a required function are turned off using sleep transistors. But instead of disabling just the clock signal, sleep transistors also disconnect cells from their power supply. Therefore, power gating reduces both dynamic and static power consumption. Power gating can be implemented in two different ways: fine or coarse grain.

FINE GRAIN power gating requires that each cell come with its own sleep transistor.

Advantage

• Good timing control

Disadvantages

• Increased area overhead
• Less leakage control
• Required a standard cell library with sleep transistor

COARSE-GRAIN power gating methodology is implemented using special sleep transistor cells. One sleep transistor cell is used to turn on and off a set of standard cells. The coarse-grained approach requires less area than fine-grain power gating due to the lower number of sleep transistors and less routing of enable signals for power gating. Fewer sleep transistors result in better leakage control.

Unlike fine grained power gating, when the power is switched off in coarse grain power gating, the power is disconnected from all the registers resulting in loss of data. If the data is to be preserved when the power is disconnected, then we need to store the data somewhere, where there is no power gating - This is done by a special register called "Retention" register. The key advantage of retention register is that they are simple to use and are very quick to save and restore the state.

State Retention Power Gating

This technique allows the voltage supply is reduced to zero for the majority of the SOC block's logic gates while maintaining the voltage supply for the state element of that block. The state of the SOC is always saved in the sequential components. Using the SRPG technique, when in the inactive mode, power to the combination logic is turned off and the sequential stays on thereby reducing the power consumption greatly when the application is in stop mode.