Page 260 - A Practical Guide from Design Planning to Manufacturing

232 Chapter Seven

applications where battery life was a concern would seriously design for
power. Since 2000, power has become a major design concern for all
processors.
Shrinking transistor sizes reduces the power required to switch a tran-
sistor, but increasing frequencies and numbers of transistors have lead
to steadily more total switching power. In addition, reducing transistor
size increases leakage currents, which flow even when the transistor is
switched off. The combination of these two has lead to dramatic increases
in processor power. Increasing transistor density exacerbates the prob-
lem by making processor power per area increase even more quickly than
total processor power.
For portable applications, the impact on battery life has always made
power a concern. For desktop applications, power is limited by the abil-
ity to deliver power into the processor and the need to remove the heat
generated by that power. A high-performance processor might consume
100-W, the same a bright light bulb. However, a 100-W processor is far
more difficult to support than a 100-W light bulb. Smaller transistors have
lower maximum voltages; because the processor is operating at a much
lower voltage, it might draw 100 times more current than the light bulb
at the same power level. Building a computer power supply and mother-
board that can deliver such large currents is expensive, and designing the
wires of the package and processor die to tolerate such currents is a sig-
nificant problem. A 100-W light bulb becomes too hot to touch moments
after being turned on, but the surface area of the processor die might be
100 times less. The same power coming out of a smaller area leads to
higher temperatures; without proper cooling and safeguards modern
processors can literally destroy themselves with their own heat.
The total power of a processor (P total ) consists of the switching or active
power (P active ), which is proportional to the processor frequency, and the
leakage power (P leakage ), which is not.

P total = P active + P leakage

The active power is determined by how often the wires of the proces-
sor change voltage, how large the voltage change is, and what their
capacitance is.

P active = activity × C total × V dd 2 × frequency

In the equation for active power, the activity term measures the per-
cent chance that an average node will switch in a particular cycle. This
is a strong function of what application the processor is currently running.