Advanced Low Power Techniques  

 
Power consumption can be divided into two aspects:
Dynamic power – the power that is consumed by a device when it is actively switching from one state to another. Dynamic power consists of switching power, consumed while charging and discharging the loads on a device, and internal power (also referred to as short circuit power), consumed internal to the device while it is changing state.

Leakage power – the power consumed by a device not related to state changes (also referred to as static power). Leakage power is actually consumed when a device is both static and switching, but generally the main concern with leakage power is when the device is in its inactive state, as all the power consumed in this state is considered “wasted” power.
Various techniques have been developed to reduce both dynamic and leakage power. The two most common traditional, mainstream techniques are:
Clock gating – the disconnecting of the clock from a device it drives when the data going into the device is not changing. This technique is used to minimize dynamic power.

Multi-Vth optimization – the replacement of faster Low-Vth cells, which consume more leakage power, with slower High-Vth cells, which consume less leakage power. Since the High-Vth cells are slower, this swapping only occurs on timing paths that have positive slack and thus can be allowed to slow down.

As technologies have shrunk, leakage power consumption has grown exponentially, thus requiring more aggressive power reduction techniques to be used. Similarly, clock frequency increases have caused dynamic power consumption of the devices to outstrip the capacity of the power networks that supply them, and this becomes especially acute when high power consumption occurs in very small geometries, as this is a power density issue as well as a power consumption issue.

Several advanced low power techniques have been developed to address these needs. The most commonly adopted techniques today are:

Multi-voltage (MV) – the operation of different areas of a design at different voltage levels. Only specific areas that require a higher voltage to meet performance targets are connected to the higher voltage supplies. Other portions of the design operate at a lower voltage, allowing for significant power savings. Multi-voltage is generally a technique used to reduce dynamic power, but the lower voltage values also cause leakage power to be reduced.

Power gating – the complete shut off of supply nets to different areas of a design when they are not needed (also known as MTCMOS or power shutdown). Since the power has been completely removed from these shutdown areas, the power for these areas is reduced essentially to zero. This technique is used to reduce leakage power.

It is very common to see multi-voltage and power gating used together on the same design, whereby different regions operate at different voltages, and one or more of those regions can also be shutdown.

For more information on advanced low power techniques, see the glossary.