Managing Static (Leakage) Power - Computer Architecture Techniques for Power-Efficiency - page 173

FIGURE 5.18: Relative contribution of dynamic and leakage power in an embedded processor. Repro-
duced from [ 231 ]. Copyright 2005 IEEE.

Increasing V T exponentially decreases the subthreshold leakage current and conse-
quently subthreshold leakage power, in accordance to the formulas given in Section
5.1.1. Assuming V gs = 0and V ds = V dd , the formula for the subthreshold leakage

power becomes:

V dd I s0 1

V t e − V T − V off

e − V dd P sub = V dd I Dsub = − .

n v

t

Considering dynamic or leakage power independently, the performance can be traded
for power by scaling either V dd or V T . Because in both cases performance degradation is linear
to the scaling of the V dd or V T , whereas power savings are either quadratic or exponential, the
resulting improvement in EDP is substantial.
Considering, however, total power, the sum of the dynamic and leakage powers, it is not
obvious which quantity is more profitable to scale for a given performance degradation. This
depends on the relative contribution of the two components to the total power consumption.
For example, Yan, Luo, and Jha [ 231 ] consider the three scenarios, shown in Figure 5.18, for
the relative contribution of dynamic and leakage power in an embedded processor.
In the 70 nm technology, scaling the supply voltage is bound to have a greater effect than
raising V T , for a given performance level—a given frequency—since dynamic power dominates
in this technology. In contrast, in the 35 nm technology, increasing V T isthemoreprofitable
route. Not only the balance of dynamic and leakage power shifts across technologies, or among
different implementations in the same technology, but also changes dynamically as a function
of temperature which has a profound effect on leakage. This aspect, however, has not been
researched adequately.
For a given frequency—a given switching delay—the best possible power savings come
from carefully adjusting both V dd and V T , depending on the balance of dynamic versus leakage